RolanicnlJournal of the Linnean Society, 80:293-3 17. With 12 figures
Juiic. 1980
Morphological, anatomical, cytological
and biochemical aspects of evolution
in East African shrubby species
of Aloe L. (Liliaceae)
D. F. CUTLER, F.L.S.
Anatomy Section, Jodrell Laboratory,
Royal Botanic Gardens, Kew,
Richmond, Surrey TW9 3 D S
P. E. BRANDHAM
Cytogenetics Section, Jodrell Laboratorj,
Royal Botanic Gardens, Kew,
Richmond, Surrey T W9 3 DS
S. CARTER
The Herbarium, Royal Botanic Gardens, Kew,
Richmond, Surrey T W9 3 A B
AND
S. J. HARRIS‘!
Biochemistry Section, Jodrell Laboratory,
Royal Botanic Gardens, Kew,
Richmond, Surrey TW9 3 D S
T\vcl\c previouslv drsci-lhed shrubby species of Aloi L. from Kenya, Congo, Tanzania and I’ganda
\ y e w iiivcstigarc~d u5ing a multidisciplinar). approach. Half of rhr species were found to be
tetraploids ( 2 4 1 2 8 ) ,and because of the rarity ot tetrapolody in the genus it is suggested that these
tlo,ely I-rlated a n d of common origin. Studies of their gross m o r p h o l o p a nd leaf surfarr
\(-ulptu;.ing reveal clinal variation patterm. These patterns indirate the diploid sperirs likelv t o he
\iiiiilai- t o the ancestor o f the tetraploids, and the geographic- region where the c h r ~ n i o ~ o n i c
crl.
in the tomposition of the leaf exudate\ hi-gel\ ri~nfirnithr
(1oiil)litig p i . i h t i l \ - o ( c u ~ ~ ~ -Variations
Pi-c\ciir addrr\\: I k p r t i i i e t i i of Agrirultural Botany, Planr Scienre Laboratories, ~~nive initv
01 Reading,
Whiteknights, Reading, RG6 2AS.
0024-4074/80/040293 + 25/$02.00/0
293
t
2 1980 The Lirinraii Socieri
of I.ondon
D. F. CUTLER ET A L
294
conclusions drawn from the cytological and anatomical observations. A taxonomic revision based o n
the experimental findings changes the status o f Rift Valley forms o f A . kedongemzs to a subspecies o f
A . nyerzenszs and also enlarges the latter to include A . ngobitemzs.
KEY WORDS: - Aloe - East Africa - evolution - leaf cuticle sculpiul-irlg - kaf' cxutlatc
biochemistry- taxonomy- tetraploidy-variability.
CONTENTS
Introduction
. .
Material and methods
Results
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Cytological observations
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Biochemical observations
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General description o f leaf surface features
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Observations o n internal anatomy . . . . . . . . .
Discussion
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The origin o f the tetraploid species and their interrelationships
. . . . . . . . . . . . . .
with diploids
Evolutionary trends within the tetraploid group
. . . . .
Ecological considerations
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Biochemical considerations . . . . . . . . . . .
Taxonomic considerations
. . . . . . . . . . .
Artificial key to species
. . . . . . . . . . . . .
General conclusion-overall evolutionary trends . . . . . .
Acknowledgements
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References
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INTRODUCTION
Previous work has shown that in most members of the tribe Aloineae (family
Liliaceae) the cuticular pattern on the epidermal cells of the mature leaf surface is
uniform within a species (Cutler, 1969, 1972, 1978) and is under precise genetic
control (Cutler & Brandham, 1977; Brandham & Cutler, 1978). Between species
there is a great diversity of pattern (Cutler, 19791, but where there are similarities
between species in this respect a close relationship might be indicated which
could be either evolutionary or ecotypic.
In order to test this idea we looked for a group of aloes that appeared to be
closely related. The species chosen included two which were known to be
tetraploid, a phenomenon which is rare in the genus (Brandham, 1971). These
species, Aloe dawei and A . elgonica, are shrubby and form part of a
morphologically distinct cluster of species occurring together in East Africa.
Other species from this area, beside the shrubby forms, are either rosette types
(more or less stemless) or are arborescent, neither of which habit is likely to be
confused with that of the shrubby species.
To date, 12 species of shrubby aloes have been recognized from Kenya and
from regions immediately bordering that country, but some of these are
extremely similar to each other in gross morphology. Hence they were ideal
subjects for an investigation of leaf surface morphology and the occurrence and
distribution of tetraploidy. The taxonomy of the group was in need of revision,
and we were able to make use of the combined resources of the Herbarium and
the Anatomy, Biochemistry and Cytogenetics Sections of the Jodrell Laboratory
to provide a multidisciplinary approach to the problem.
From the biochemical point of view these plants are also very suitable for
comparative studies since they contain in their leaves a range of compounds,
SHRUBBY ALOE EVOLUTION
29 5
principally chrornenes and the anthroquinone aglycones and glycosides, which
can be separated readily using thin-layer chromatographic techniques. A survey
of the patterns obtained from exudates from cut leaves of representatives of a
large number of species of Aloe has revealed a great diversity, and there are
indications that the patterns within a species are uniform. I t was therefore
considered that valuable information on the affinities within the gl-oup of East
African shrubby aloes could be obtained by a survey of their leaf exudates.
MATERIAL AND METHODS
Living specimens of shrubby aloes were obtained from 37 localities in East
Africa, a total of 329 plants. Most were collected by one o r more of the authors,
but we are grateful to Mr G. Powys for supplying a living specinien of A.fibrosn
from Ngarendare (locality 321, to Mr G. Classen for A . at€.kedongensis from Lake
Hannington (locality 23) and A . aff. elgonica from Busia (locality 161, to Mrs C.
Giddy for A. dawei from near the Congo River (locality 1 7 ) , and to Mrs D. F.
Wilson for A . wilsonii from Mt. Moroto, Uganda (locality 22). The species and
their localities are listed in Table 1, and the position of each locality is shown in
Fig. 1, with the exception of localities 2 , 1 7 and 37 which are each several
hundred kilometres outside the boundary of the map in the directions indicated.
The nomenclature in Table 1 is based on geographical and taxonomic
information given by Reynolds (1966)in his monograph on Aloe, supplemented by
reports of further newly-discovered species by Lavranos & Newton ( 1976) and
Carter & Brandham (19791, but it should be noted that the results of this present
investigation will assign every plant given only an ‘affinity’ status in Table 1 to a
particular taxon (with the exception of the incompletely-known plants from
locality 361, and that the nomenclature finally adopted in this paper necessitates
changes in rank o r reduction to synonymy of some of the taxa in Reyiiolds’
nionograp h.
All of the plants were brought into cultivation at Kew and representative
vouchers have been deposited in the Kew Herbarium. The extensive collection of
dried material of the group in the Herbarium was also utilized for anatomical
and taxonomic investigation and as vouchers for populations from some of the
localities revisited and sampled in the present study.
Cytological methods
Somatic chrosomes were studied in root tips which had been pretreated for
24 h in saturated aqueous alpha-bromonaphthalene at 4OC, fixed in 1 :3 acetic
ethanol, hydrolysed in 1 N HCI at 6OoC for seven minutes and stained in Feulgen.
Slides were made permanent using the liquid CO,/alcoholic dehydration method
of Bowen (19561, and are retained in the collection of the Cytogenetics Section of
the Jodrell Laboratory for references purposes.
Anatomical methods
Methods for the study of the leaf surface were as outlined by Brandham &
Cutler (1978), with minor modifications. Platinum was used instead of gold in
the sputter coater. All specimens were photographed in a Jeol 35 SEM at a
19
296
D. F. CUTLER E T A L
?
Figurr I . Map o f part o f East Africa showing the 37 localities from which shrubby aloes werc
collected. Localities 2, 1 7 and 37 are outside the boundary of the map in thc diru tion\ indicated.
T’tw Rift Vallev, shown by interrupted lines, runs north-south across the map.
SHRUBBY ALOE EVOLUTION
297
Table 1. Origin of material, chromosome number, leal exudate t!pe and iiuinbcr
ot' specimens st udied from each locality. The number preceding each locality
corresponds to a site numbered on Fig. 1. Type localities for some species are
indicated by (TI
Spccies
Locality
Voucher
1 Mbulu (Ti
Reynolds 7523 [KI
21 Moropus (TI
Bi-alidhdili 1727 ( K )
Y O E. o f Cheperaria
19 W. o f c h e p e r a r i a
I 4 Awasi
15 Kisumu
17 Congo
18 Elgon (TI
18 Elgon (TI
16 Busia
4 Kilirna Kiu
3 2 Ngarendare
8 Kedong
8 Kedong
9 "ni\nsha f T )
10 S. of Gilgil
1 I N.W. o f Gilgil
11 N.W. of Gilgil
12 E. of Gilgil
13 Elementeita
23 Hannington
3 Moi-iju (TI
27 Ngohit (TI
27 Ngohit (T)
28 E. of Ngobit
26 Suguroi
26 Suguroi
3 1 N. of Nanyuki
3 0 Rongai
29 Naro Moru (TI
Nvahururu
E.'of Nyahui'uru
Rumuruti
Rumuruti
Rumuruti
Poro
Karisia
El Barta
5 Lukenva
6 Langata
7 Ngong
2 Malindi
22 Moroto iT)
37 Yavello (TI
24
25
33
33
33
34
35
36
2n
Leaf
exudatc
group
N o . of
plaritc
studied
14
28
28
D
A
A
A
CCCC
3
6
1
2
4
-
2X
-
28
28
28
28
29
28
14
ReTnolds 7513 [KI
-
Brandham 1706 iKi
-
Carter & Stannard
747 ( K )
POUTSs n ( K )
-
Rr\iioltl\ 6544 ( K )
Re\nolds 6546 ' K )
-
Re\nolds 6552 K1
-
Classen 288 (EA)
Ball, 17021 ( K i
ReTnolds 6579 iK1
-
Revnolds 6582 ( K i
Brandham 1756 (Ki
-
Rc\nold, 6575 ( K l
Pole-E\anr & Ercnr
1198 ( K )
Brandham 1737 ( K )
Brandham 1739 (K1
Brandham 2002 ( K i
-
-
Re\noldr 7930 ( K i
-
Tweedie 1365 (PREI
Re\nolds 7063 ( K l
14
27
28
28
28
28
29
28
28
28
14
28
29
28
28
29
28
28
28
28
28
28
27
29
28
2X
28
14
14
14
14
14
14
c
c
I
D
-
A
4
-
A
A
A
D
R
-
B
B
-
B
B
B
B
B
B
-
B
R
B
G
G
~
~
F
E
46
12
8
D
AA
10
1
1
1
14
1
7
7
I
5
8
1
1
10
1
2
5
2
3
9
6
7
4
6.5
1
1
5
1
6
9
44
9
3
I
1
magnification of c. 300 with a tilt of 3 0 ° , and the photogra hs reproduced here
are orientated with the long axis of the leaf running up a n a d o w n the page. The
material used is listed in Table 1. The photographs are from a selection of this
material and are fully representative. Descriptions of leaf surface characters
follow the format used in previous papers quoted above.
D. F. CUTLER ET A L
298
Biochemical methods
Exudates from cut bases of mature leaves were collected by standing each leaf
in a beaker containing a little 100%methanol. It was found to be essential to use
freshly cut leaves because the amount of exudate produced from the cut
surface declines after a few minutes and cannot be stimulated again by re-cutting
the detached leaf. Exudate extracts were filtered into storage bottles and coldevaporated if necessary to concentrate them. Thin layer chromatograms were run
on 0 . 2 mm silica gel TLC plates. Several drops of exudate solution were placed
on each origin spot, with drying between each application. Normally about 10
drops were sufficient, but with weak extracts up to 50 drops were needed. Plates
were run with a solvent consisting of 7 parts chloroform: 3 parts absolute
ethanol: 1 part distilled water for about 35 min., until the solvent front had
travelled 70 mm from the origin.
Plates were air-dried and developed by spraying with 0.5%aqueous fast blue
B. They were finally oven-dried at 100°C for 15 min.
Selected spots were removed from unsprayed TLC plates for identification,
shaken in 5 ml of methanol and filtered to remove the silica gel. The filtrates were
analysed in an ultraviolet spectro-photometer and the spectra were matched
against those of'known compounds.
RESULTS
Cytological observations
The chromosomes of all plants studied were found to match the karyotype
which has been observed in every species of the Aloineae to date. The basic
number is x = 7 and comprises four long submetacentric to acrocentric
chromosomes and three short acrocentrics which are about one third of the
length of the long ones (Brandham, 197 1). Table 1 shows that some of the East
African shrubby species of Aloe are diploid with 2n= 14 chromosomes, i.e., A .
babatiensis, A . jbrosa, A . monyensis, A . rabaiasis, A . wilsonii and A . yavellana, in
common with the great majority of species in the genus as a whole. Other species
were found to be tetraploid with 2n=28 chromosomes, i.e., A . dawei and A .
elgonica, confirming earlier records (Brandham, 197 11, also A . cheranganiensis and
every sampled population of A . kedongensis, A . aff. kedongensis, A . ngobitensis, A .
nyeriensis, A . aff. nyeriensis and their intermediates.
N o population was found to contain different levels of ploidy, but in some
tetraploid populations aneu loids were detected, occurring usually at low
frequencies. These aneuploi s were in A . elgonica (12 plants from locality 18
having 2n=29 chromosomes out of 58 plants studied), in A . kedongensis (one
plant with 29 chromosomes out of eight from locality 1 11, in A . aff. nyeriensis (one
plant with 27 and one with 29 chromosomes out of 67 studied from locality 331,
in A . nyeriensis & A . ngobitensis (two plants with 29 chromosomes out of seven
from locality 261, and in A . ngobitensis (one plant with 29 chromosomes out of
eleven from locality 27). Of the above examples the aneuploids of A . elgonica,
together with structural chromosome aberrations in this and other species have
been reported and analysed fully by Brandham &Johnson (1977).
I t should be pointed out here that the record of 2n=14 Aloe ngobitensis
given in an earlier paper (Brandham, 197 1 ) was in error. The plant in question
B
299
SHRUBBY ALOE EVOLUTION
0
1
I
x
-
I
O
,
Rf
0.6
a
I
I
0.9
1
I
B
.ICOLOUR
0.3
OF
'
SPOTS (R.H.S. S T A N D A R D )
INTENSITY
Figure 2. The nine groups of compounds in Aloe leaf exudates as 5 h o W n by chi-oniarog-I-apliic
wparation. Each group reads from the origin X towards the right as the Rf increases. lntrnsitv of'
\pots ic shown by block size. The geographical/taxonomic distribution o f each group is given in
Tabk I .
300
D. F. CUTLER E T AL.
was a then undescribed species from Ngomeni Rock, Kenya, and was later named
as A. ch?ysostachys (Lavranos & Newton, 19761, which is not related to the shrubby
species.
In some of the localities (14, 26, 27) the shrubby species were observed to have
hybridized with non-shrubby diploid species ( A . lateritia and A . secundzJora) to
produce triploids (2n= 2 1) having an intermediate appearance-either a low
shrub branching from the base or a short unbranched stem carrying a rosette
(i.e. having very short internodes between mature leaves) at the top.
A preliminary investigation of meiosis in some of the tetraploid plants has
shown that A . kedongensis from locality 8, also A. dawei, A . elgonica and their F,
hybrid, behave as autotetraploids, each having a high incidence of quadrivalent
formation.
Biochemical observations
Thin-layer chromatograms were run of leaf exudates collected from
representative plants from nearly every locality. Except in a few cases in which
their chemical identity is known we are reporting here only the distribution,
colour, number and relative intensities of the spots. The exudate patterns fell
broadly into nine groups, although it must be pointed out that a slight amount
of variability occurred within each group, this being due to differences between
the genotypes of different individuals within a population. The distribution of
exudate pattern types is given in Table 1 and the nine patterns are illustrated in
Fig. 2. Each pattern is set out according to the colour, the relative intensity and
the Rf value- of each spot, the latter (Relative front value) being the distance
moved by the spot expressed as a factor of the distance moved by the solvent. I t is
widely recognized that Rf values are not always uniform between one run and
another, with a 10%margin of error being acceptable.
Figure 2 shows that of the nine groups, group A- differs from group A only by
one spot being absent at Rf 0.4, although spots common to both groups may be
of different intensities. Group C- differs from group C only by one spot which is
absent at Rf 0.47. Again the two groups may display spot intensity differences, but
it is quite clear that group A- is closely related to group A and Group C- to group
C.
The plants in each exudate group occupy distinct geographical areas, as can be
seen by comparing the data from Table 1 with the distribution map in Fig. 1 .
Group D is southern/south eastern in distribution and includes A . babatiensis
(locality l ) , A.fibrosa (localities 4,321 and A . morijensis (locality 3). Groups C and
C- are west of the Rift Valley ( A . dawei and A . elgonica, localities 14- 18). Groups A
and A- occur in the Rift Valley ( A . kedongensis, localities 8-13, 231, or in a side
valley opening into it ( A . cheranguniensis, localities 19-21). Group B is entirely east
of the Rift Valley ( A . ngobitensis, A . nyeriensis, A . aff. nyeriensis, localities 24-31,
33-36). Group E is represented by a single collection only, a long way north of
the rest ( A .yavellana, locality 37). Group F is also found in only one accession ( A .
wilsonii, locality 22) and group G is found only in A . rabaiensis (localities 5-6).
Work on the identification of the large number of compounds found in the
exudates is still in progress, but up to the present the identity of two of them has
been established. In groups B and E at Rf 0.53 the compound is Homonataloin,
and in group A at Rf 0.4 the compound is Barbaloin.
SHRUBBY ALOE' EVOLUTION
30 1
Figure\ % 5 . Lrafepitierinis of three Rift Valley tetraploid forms, x 300. Fig 3. Localitv IS. Lakr
Elci~rentcita.Fig. 4. Locnlitv 10, S. of Gilgil. Fig. 5 . Localitv9, Lake Naivasha.
General description of leaf surface features
Figures 3-5 show the leaf surface of three specimens of Rift Valley forms of
Aloe kedongensis and will serve as examples of the main features considered to be
of taxonomic importance. Each stoma consists of a pair of guard cells which are
302
D. F. CUTLER ET AL.
Figures 6-9. Leafepidermis offour species, x 300. Fig. 6 Aloejbrosa (diploid) from locality 4, Kilima
Kiu. Fig. 7 . A . rabaiensis (diploid) trom locality 5, Lukenya. Fig. 8. A . cheranganiensis(tetraploid)from
localitv 20, E. of Cheperaria. Fig. 9. A . aff. nyeriemis (tetraploid) from locality 36, El Barta Plain.
deeply sunken and appear very indistinctly in most of the photographs. Their
position is indicated by the dark areas bounded by four modified epidermal cells,
each of which has a distinct raised lobe where it borders the suprastomatal cavity.
They can be called subsidiary cells and the stomata are tetracytic. The degree to
which the lobes overarch the cavity is often but not always significant at the
SHRUBBY ALOE EVOLUTION
303
species level. The outlines of other epidermal cells in this taxon are indicated by
bands with little or no sculpturing. The relative width of such bands is of
importance when species are compared. Individual epidermal cells are more o r
less square to oblong in outline with a slight tendency towards being 5 to 7-sided.
In many species of Aloe the majority of epidermal cells are markedly hexagonal,
e.g. A . rauhii (Brandham & Cutler, 19781, so this rather rectangular form is quite
diagnostic within the East African shrubby aloes.
The sculpturing of the outer cell walls and associated cuticle is of two orders.
First the entire wall may be relatively flat, as in Figs 3-5, o r more convex,
sometimes being conspicuously domed, as in A . rabaiensis (Fig. 7). The second
order of sculpturing consists of micropapillae showing varying degrees of
aggregation o r fusion. A characteristic feature of the Rift Valley A . kedongensis and
its close relatives is that the micropapillae o n the four subsidiary cells are larger
than those on other cells (Figs 3-51.
Wax is present on the leaf surface, either as fine flakes or larger particles, all
plants examined having a wax layer covering the cuticle and most having
additional particles o r flakes. In the preparation of specimens for SEM
examination n o attempt was made to remove any foreign matter in case the wax
particles were disturbed or altered.
Descriptions of the more important features of different populations of
shrubby Aloe illustrated in Figs 3- 10 are given below:
A . Jibrosa (2x1; locality 4 (Fig. 6). Stomata and the majority of' cells are smaller
than in the tetraploid plants. Anticlinal walls indicating the cell outlines show as
conspicuous bands more o r less lacking in sculpturing. Micropapillae are among
the smallest in the group of species under consideration, and even those on the
subsidiary cells are relatively small. The epidermal cell walls are flat to slightly
convex. Wax is inconspicuous.
A . rabaiensis (2x1; locality 5 (Fig. 7). Stomata and the majority of cells arc as large
as o r larger than those of the tetraploid plants. Anticlinal walls are indicated by
relatively narrow, deep channels. Micropapillae are generally absent, but each
epidermal cell has a conspicuous, irregular, more o r less central papilla arising
from a convex wall. Papillae on subsidiary cells tend to be smaller than the rest.
Wax is conspicuous as fine, more o r less upright flakes, except on the tops of the
papillae.
A . cheranganiensis (4x1; locality 20 (Fig. 8). Stomata are relatively small for a
tetraploid species, and have a marked overarching of the lobes from adjacent
epidermal cells. Anticlinal walls are indicated by conspicuous bands with slight
sculpturing. Micropapillae o n epidermal cells are more conspicuous than in Rift
Valley forms of A . kedongensis, and are not markedly smaller than on the
subsidiary cells. Wax is present as upright flakes, and as an amorphous layer
somewhat obscuring the cuticular sculpturing. A . cherunganiensis from Moropus,
locality 21 (Fig. 10) is very similar in most respects, but has even more
amorphous wax, fewer of the upright wax flakes and a slightly less open
suprastomatal cavity.
A . aff. nyeriensis ( 4 ~ )locality
;
36 (Fig 9). Stomata are of average size for the
tetraploid plants and show a limited degree of overarching of the lobes from
lateral epidermal cells. There is a marked depression around the four lobes. The
position of anticlinal walls is marked by narrow, rather irregular grooves.
Micropapillae on the strongly domed epidermal cell walls are relatively large and
304
D. F. CUTLER ET AL.
show little size distinction from those on subsidiary cells. Thick wax layers
obscure the sculpturing. Wax is also present as upright flakes.
In Fig. 10 the photographs are laid out in a scheme which follows the
geographical distribution of the specimens depicted (within the limitations of the
proportions of the page). The numbers refer to the localities plotted on the map
shown in Fig. 1 . A description of the important feature of each follows :
A . dawei (4x1 variant with broad leaves; locality 1 7 (Congo) (Fig. 10). Stomata
are of average size for a tetraploid, but have a wide suprastomatal cavity.
Anticlinal walls are marked by conspicuous bands with very slight sculpturing.
Micropapillae on epidermal cell walls show some lateral coalescence and are
similar in size to those in Rift Valley forms ( A . Redongensis). Micropapillae on
subsidiary cells are slightly larger than the others. The outer walls of the
epidermal cells are more or less flat. Wax is present as fine particles.
A . elgonica (4x1; locality 18 (Elgon)(Fig. 10). The example depicted here is one of
a very variable range, associated with extreme chromosomal instability
(Brandham &Johnson, 1977), but is quite typical of plants from this population
having normal karyotypes. Stomata are overarched by the lateral lobes and their
overall size is slightly smaller than that of most tetraploids from further east.
Anticlinal walls are marked by narrower, somewhat irregular channels between
strongly domed epidermal cells. There is a marked depression adjacent to the
lobes surrounding the stoma. The relatively coarse micropapillae on epidermal
cells are aggregated and their individual identity is further obscured by wax.
Those on subsidiary cells are only slightly more conspicuous than the rest. In
addition to amorphous wax la ers, irregular upright flakes are present.
A . dawei (4x1; locality 15 kisumu) (Fig. 10). This is similar to Rift Valley
tetraploids in most respects, except that the sculpturing on all cells is less
pronounced.
A . morijensis (2x1; locality 3 (Morijo) (Fig. 10). Stomata are smaller than in the
tetraploid plants and are similar in size to those in A.Jibrosa (Fig. 6). Anticlinal
wall positions show as relatively indistinct bands with little or no sculpturing.
Epidermal cell outer walls are more or less flat, with fine micropapillae similar
in size to but less regular than those in A.fibrosa. Micropapillae on subsidiary
cells are conspicuously larger than those just described. Wax is visible as small
particles.
A . kedongensis (4x1; locality 8 (Kedong) (Fig. 10. See also localities 13
(Elementeita), 10 (Gilgil)and 9 (Naivasha),Figs 3-5). Stomata are of average size
for tetraploid plants with a wide suprastomatal cavity. Anticlinal wall positions are
marked by broad bands more or less devoid of sculpturing. These bands are either
lower than or more or less equal in height to the low relief small micropapillae
on epidermal cells. Subsidiary cells have markedly larger micropapillae and
sometimes a depression between the lobes and the micropapillae. Wax flakes are
more conspicuous on some examples than others.
A . aff. Kedongensis (4x1; locality 23 (Hannington) (Fig. 10). This is similar in most
respects to the samples from further south (Figs 3-5 and Fig. 10 (8 Kedong)),with
the exception that the micropapillae are markedly more pronounced than in
these samples.
A . cheranganiensis (4x); locality 21 (Moropus) (Fig. 10). This is similar to the
specimen of this species from locality 20 (Fig. 8). The positions of anticlinal walls
are indicated by somewhat narrower channels and the lateral lobes to the
5HRIIBBY .4LOE EVOLUTION
305
suprastoinatal cavity are strongly overarching. Micropapillae are very
pronounced.
A . aff nyeriensis (4x); locality 24 (Nyahururu Falls) (Fig. 10). The stomata are
similar in size to those of most tetraploids, but have conspicuouslv wider
overarching lobes. Anticlinal wall positions are indistinctly marked by narrow
bands lacking sculpturing. Micropapillae o n epidermal cell outer walls are small
and aggregated and are rendered indistinct by wax layers. Subsidiary cells have
large micropapillae and a depression adjacent to each lobe. Wax flakes are
prominent.
A . aff. nyeriensis (4x1; locality 25 (E. of Nyahururu) (Fig. 10). Stomata are of
average size for tetraploid plants and lateral lobes are slightly overarching. The
position of anticlinal walls is marked by deeper, narrower and somewhat more
irregular grooves than in the Rift Valley representatives ( A . hedongensis).
Micropapillae on slightly domed epidermal cells are well developed, showing
some coalescence. Those on subsidiary cells are similar in size o r slightly larger.
Micropapillae are separated from stomata1 lobes by a narrow depression. Wax
flakes arise from a layer which does not obscure the cuticle sculpturing.
A . aff.. nyerienszs (4x); locality 33 (Rumuruti)(Fig. 10).Stomata are slightly larger
than in the previous specimen with a wider aperture to the suprastomatal cavity.
In other respects this plant is very similar to that from East of Nyahururu. Details
appear more clearly because part of the wax covering had become detached
during preparation.
A . aff. nyeriensis (4x1; locality 35 (Karisia) (Fig. 10). This plant is very siniilar to
that from Rumuruti except that the lobes on the flanks of the suprastoniatal cavity
overarch. Also the channels indicating the position of anticlinal walls are
narrower.
A . ngobitensis (4x); localitv 26 (Suguroi) (Fig. 10). In most respects this leaf
surface is similar to that of plants from E. of Nyahururu.
A . ngobitensis (4x); locality 27 (Ngobit) (Fig. 10). Epidermal sculpturing as in
plants from E. of Nyahururu, but with slightly smaller stomata.
A . nyeriensis (4x); locality 28 (E. of Ngobit) and 30 (Rongai) (Fig. 10). These
plants are very similar to each other and to those from locality 27, although their
sculpturing is partly obscured by the presence of a greater quantity ofwax.
A . nyeriensis ( 4 ~ )locality
;
3 1 (N. of Nanyuki) (Fig. 10).The suprastomatal cavity
is conspicuously wide in these plants and the anticlinal walls are evident as bands
lacking sculpturing. The epidermal cells are wider than in other specimens.
Micropapillation is similar to that in plants from nearby localities (28, 30), but is
more obvious because the wax covering is thin.
The characteristics of leaf epidermises of the final three species in the East
African shrubby group (not illustrated) are described below:
A . habatiensis (2x); locality 1. The epidermis of these plants is very similar to that
of A . morijensis. The epidermal cells are small, 4 to 6 sided, with fine micropapillation. Anticlinal walls are of moderate width, with very little pattern. The cells
bordering the stoma have more pronounced micropapillae and the lobes do not
overarch the suprastomatal cavity. Wax is present as an amorphous layer and as
fine flakes.
A . wilsonii (2x1; localitv 22. This plant has epidermal cells smaller than those of
A . morijensis. The outer wall is slightly domed. Micropapillae are either small and
very poorly developed or completely lacking. Anticlinal walls appear as distinct
306
D. F. CUTLER E T A L
shallow channels between cells. Stornatal lobes are not overarching. Wax cover is
dense, both amorphous and flaky.
A . yauellana (2x1; locality 3 7 . The epidermal cells of these plants have mediumsized micropapillae, those of cells surrounding the stoma being larger. Anticlinal
walls are not easily distinguishable. Stornatal lobes are very overarching and nearly
occlude the suprastomatal cavity. Wax is both amorphous and in fine flakes. I t is
very thick and obscures many finer surface features.
Observations on internal anatomy
Most Aloe species have a cap of thin-walled secretory cells at the phloem pole of
each vascular bundle in the leaf (Cutler, 1972). From these cells come the exudates
whose chromatographic separation is described below. In A . jibrosa and A .
morijenszs fibres are present at the phloem poles and secretory cells appear to be
lacking. This lack corresponds with the biochemical findings that the amount of
exudate produced by these species was by far the lowest in the entire group. In A .
babatiensis the fibres are present in the lower parts of the leaves only, which suggests
an affinity with A.fibrosa and A . moriiensis.
DISCUSSION
In this paper we have reported the results of the first attempt to relate leaf
surface sculpture patterns to macro-morphological, cytological and biochemical
aspects of certain Aloe species in order to revise their classification and indicate
possible evolutionary trends among them.
The origin of the tetraploid species and their interrelationshipswith diploids
In the Aloineae as a whole the extreme uniformity of the karyotype (Brandham,
19 7 1) makes the elucidation of interspecific relationships almost impossible
using criteria such as the comparison of chromosome size and number alone.
Only by making interspecific hybrids and analysing their meiotic behaviour will
any significant progress be made in this respect. However, an exception occurs in
the East African shrubby aloes because of the high incidence of tetraploidy in the
group. The genus overall is almost exclusively diploid, with one well-known
hexaploid species ( A . cilzaris), and a scattering of probably locally-occurring
triploids (Sharma 8c Mallick, 1966; Fedorov, 1969; Brandham, 197 1 ) .
Tetraploidy is found in very few species; A.juvenna (Brandham & Carter, 19791,
A . jacksonii (Brandham, 197 l ) , A. calidophila and a few populations of A . inermis
(Brandham, unpublished). None of these is related to the tetraploid shrubby
species reported in this paper (A. dawei, A. elgonica, A . cheranganiensis, A.
kedongensis, A. ngobitensis and A . nyeriensis). Reference to Fig. 1 and Table 1 will
show that the latter group occupies the localities 8-21 and 23-36, and it can
therefore be seen that these tetraploid species are in a cluster, with the boundary
of the distribution of each species being in contact with that of at least one other
tetraploid. This close geographical link combined with a general morphological
affinity (Reynolds, 1966) and the overall rarity of tetraploidy in the genus
suggests that the tetraploid group is of common origin and that the doubling of
the chromosomes of a common ancestral diploid occurred once only, with
subsequent diversification to achieve the present situation. Supporting evidence
SHRUBBY ALOE EVOLUTION
307
is provided by the finding that many of the tetraploids are cytological
autotetraploids, even including the F, hybrid between A . dawei and A . elgonica,
which confirms their close relationship.
The likelihood of a common ancestry and the close relationship of the
tetraploids is further borne out by the similarity of their leaf surface patterns. It
has been demonstrated previously that these patterns (or sculpturing, to use a
currently more acceptable term) are characteristic for a given species, with closely
related species having similar patterns which are nevertheless distinguishable if
all elements of the sculpturing are taken into account (Cutler, 1969, 1972, 1978;
Cutler & Brandham, 1977; Brandham & Cutler, 1978). Experiments in
breeding, together with comparative studies of plants from the field and from
cultivation, have indicated that the sculpturing is under close genetic control
and, within a clone, is little affected by variation in the environment.
Consequently the use of these patterns, which include features such as epidermal
cell outline, stomata1 details and the sculpturing of the outer cell wall-cuticle
complex, in conjunction with other morphological and chemical data for
taxonomic purposes, is justified.
The similarity of leaf surface sculpturing of the tetraploids at once confirms an
extremely close relationship between them. In some instances the sculpturing of
different species (i.e. A . ngobitensis and A . nyeriensis) is so similar as to indicate that
they might be conspecific.
Shrubby diploid species from the area (A.jibrosa, A . babatiensis, A . morijensis, A .
rabaiensis, A . wilsonii and A . yavellana) were screened anatomically with the object
of determining whether any of them had leaf surface characters resembling those
of the tetraploids. If any of them showed a similarity, this might point to close
affinity and hence give some indication of the type of diploid from which the
tetraploids arose. These studies showed that most of the diploids could not be
considered as ‘ancestral’ because of their distinct differences from the tetraploid
group. Aloe rabaiensis is one of these (Fig. 7). Its leaf surface sculpturing is distinct
and does not resemble that of the tetraploids. I t is also a large plant which would
probably become even larger with autotetraploidy and exceed the size of the
tetraploids. Furthermore, its capitate inflorescence is unique in the group, with
the exception of the geographically isolated A . yavellana. Aloe wilsonii is aIso
isolated, and this species, together with A . yavellana can be eliminated on the
grounds of this isolation. Also their epidermal morphology does not resemble
that of the tetraploids. Aloe babatiensis can also be eliminated, principally because
of its unique pink flowers.
Two other possible ancestral forms remain, A.jbrosa and A . morijensis. Both
show sculpturing similar to that of some of the tetraploids. Both are also very
obviously closely related because they exhibit an anatomical feature unusual in
the genus. Most aloes have a cap to the phloem pole of the vascular bundles in
the leaf which is composed of secretory cells, but A . jbrosa and A . morijensis
contain fibres instead of secretory cells in this position. The decision as to which
of these two species was more closely related to the putative ancestor of the
tetraploids cannot rest on anatomical evidence alone, but when other evidence is
considered there is little doubt. AloeJibrosa is too large a plant to be a likely
ancestor, but A . morijensis is a small plant which is so similar to the Rift Valley
forms of A . kedongensis in all but size (except for the presence of fibres in its leaves)
that a straightforward diploid/autotetraploid relationship seems quite likely.
308
D. F. CUTLER E T A 1
Furthermore, the similarity between the epidermal sculpturing of A . morijensis
(locality 3, Fig. 1) and that of populations of A . kedongensis nearest to it at locality
8 (Fig. 1) is too close to be coincidental (Fig. 10). Therefore we suggest that the
ancestor of the tetraploid group was a diploid form very similar to A. morijensis,
and that doubling of the chromosomes of this ancestor occurred in the Rift
Valley of south-western Kenya to produce the tetraploid forms which then
spread northwards.
Evolutionary trends within the tetraploid group
The tetraploids demonstrate a range of variability on a theme. The basic
sculpturing common to all is that of a micropapillation on the cell walls with the
micropapillae on subsidiary cells of stomata being larger than those on other
cells. The micropapillation becomes obscured to varying degrees by aggregation
or grouping together of micropapillae. An extreme instance of this is seen in A .
elgonica which is both anatomically and cytologically diverse (Brandham 8c
Johnson, 1 9 7 7 ) . Its gross morphology is distinct from that of the other shrubby
tetraploids since it is a relatively dwarf plant with short internodes and reflexed
sinuous leaves having very large marginal spines.
I t was not until the photographs of the leaf surfaces of all the tetraploids and
related diploids were arranged geographically that the significance of the
variations in sculpturing became clear. In order to demonstrate this, Fig. 10 is
composed of selected examples in such an arrangement. The broad-leaved
variant ot Aloe dawei from the far west, near the Congo river and A. dawei from
near Kisumu are included to show the continuity of the basic sculpturing
through those distinct but related tetraploid taxa.
The trends in evolution can be discerned in two directions. The first is from
south to north, starting from A . morijensis at locality 3 and passing northwards up
the Rift Valley through forms of A. kedongensis and ending at locality 2 1 where A .
cheranganiensis is found. The second runs from locality 8, Kedong, eastwards from
the Rift Valley with one branch diverging northwards to localities 35, Karisia,
and 36, El Barta and another branch continuing eastwards to localities 30,
Rongai, and 31, N. Nanyuki at the foot of Mt. Kenya.
From south to north the first evident difference occurs between the diploid A .
morijensis and the tetraploid at locality 8 , Kedong. A distinct increase in the size of
the stomata and epidermal cells can be observed, correlated with the doubling of
the ploidy level. Micropapillae on the general epidermal cells are also larger in
the tetraploid. Continuing northwards, three trends become apparent: (a)
micropapillae on general epidermal cells increase in size until they are nearly as
large as those on subsidiary cells; (b) the areas indicating anticlinal cell wall
positions become narrower; (c) there is a tendency for lateral lobes of the stoma
to overarch the suprastomatal cavity. These trends become even more obvious if
consideration is also given to Figs 3-5, which show the epidermis from some of
the plants omitted from Fig. 10 because of lack of space (localities 9, 10 and 13 in
Fig. 1). Aloe cheranganiensis from locality 20 (Fig. 8) is also in this south-north
series.
We consider that the Rift Valley plants from localities 8 , 9, 10, 13 and 23 are
sufficiently similar to constitute one taxon, together with those from localities 1 1
and 12 (not illustrated). They correspond to the taxon which has been referred to
SHRUBBY ALOE EVOLUTION
309
until now as A . kedongensis but which will be renamed below as A . nyeriensis subsp.
kedongensis.
Plants froin localities 20 and 21, regarded as the morphologically distinct
species A . cheranganiensis, principally because of floral characteristics (Carter 8c
Brandham, 1979) can also be distinguished from the rest of the south-north
sequence in their epidermal characteristics by the relative size of the
micropapillae and a tendency towards a narrowing of the anticlinal walls as seen
from the surface, emphasized by the increasingly domed outer walls. The
overarching of the lateral lobes to ;he suprastomatal cavity may o r may not be an
additional difference of significance. This could be a more variable character
within a species.
From west to east another trend can be seen. The first sign of a discontinuity
occurs in the plants from Nyahururu Falls (locality 24). These have a distinctive
surface and d o not easily fit into the west-east series. Possibly their rather moist
habitat, influenced by spray from the Falls, has a bearing on their appearance.
Certainly Aloe species from the group including A . ciliaris from South Africa,
which also grow in moist o r humid habitats, show inherited ecological
adaptation in leaf surface features (Cutler, 1979). The presence of an abundant
wax covering on these plants is difficult to reconcile with a humid habitat. In this
group of aloes as a whole surface wax abundance seems to be variable, but plants
from more arid regions often have large quantities. A feature common in most
samples is the presence of fine, more o r less upright flakes of wax, although their
density varies.
The habitats of plants from locality 8, the lip of the Rift Valley escarpment at
Kedong and those east of the Rift Valley and north o r north east of the Aberdare
Mountains (localities 2 5 - 3 5 ) are quite similar, open and seasonally dry and hot.
Localities 20 and 21, near Mt. Moropus and 36, the El Barta Plain, are lower
and much hotter and drier, particularly the last. Plants from these drier habitats
have a tendency towards more pronounced sculpturing and overarching of the
lateral lobes to the suprastomatal cavity.
A clear discontinuity is demonstrated between plants from the Rift Valley,
regarded below as A . nyeriensis subsp. kedongensis and those east of Nyahururu
Falls (locality 24). Those from the Falls may be either specialized, because of their
unique habitat, or transitional. All of the eastern specimens have narrower and
less regular channels above the anticlinal walls of the general epidermal cells
(except plants from locality 31). There is also a marked eastwards tendency
towards the formation of a channel between stomata1 lobes and papillae on the
four subsidiary cells surrounding a stoma. This last feature is hintcd at in t h e
Rift Valley forms from Elementeita (locality 131, but is increasingly evident in the
plants from north eastern areas. In addition there is a trend towards an
equalization i n size of micropapillae on subsidiary cells with those on the other
cells. This is obscured to some extent by wax on the surface of plants shown in
Fig. 10, but can be observed easily if the wax is removed. The plant from locality
35 does not show this distinction. I t is very similar to A . cheranganiensis with its
overarched suprastomatal cavity. It is not clear whether it has closer affinities
with plants from the Rift Valley o r belongs to the sequences to the east of the Rift
Valley, but biochemical investigations favour the latter (see below).
Reference has been made to the fact that the plants from north of Nanyuki
(locality 31) have anticlinal walls wider than those of others in the eastern series.
310
D. F. CUTLER ET A L
These plants also have conspicuously wider suprastomatal cavities and less
pronounced micropapillae than the others of this series. Perhaps they are
demonstrating signs of a further incipient discontinuity, although no
macromorphological difference can be seen. On morphological grounds all of
the tetraploid shrubby aloes east of Nyahururu Falls, i.e. all those listed in
Table 1 as Aloe aff. nyeriensis, A. nyeriensis, A. ngobitemis and intermediates, are now
regarded as A . nyeriensis subsp. nyeriensis (see below). This taxon at the moment
includes the isolated specimens from the El Barta Plain (locality 36) although as
flowers of these plants have not yet been seen a final decision on the taxonomy of
this population must be deferred. On the whole, the anatomical evidence
supports the amalgamation of all shrubby tetraploids east of the Rift Valley into
this single taxon. It is to be expected that these outbreeding plants may show a
degree of variability in leaf surface pattern, both clinal and otherwise, as shown
here, but the small overall range of variability is not inconsistent with that which
we have found in some of the South African species.
Ecologacal considerations
The significance of leaf surface sculpturing in aloes is not understood.
Observations made on many species in this genus indicate that some form of
surface roughness may be of importance to the plant, but the exact nature of the
pattern, apart from its scale, does not seem to be so important. The present study
provides little evidence for particular interpretations, but a few points are worth
considering and may prove of interest to physiologists. Firstly, the surface
roughness as indicated by the size of the micropapillae is greatest in species from
the more arid habitats. Secondly, species from such habitats tend to have a
greater proportion of stomata with overarching lobes, so that the suprastomatal
cavity is quite narrow at the outer pore. Thirdly, individual cells are less domed
or convex in surface view when the species normally grows in less harsh
environments. At present one might speculate that surface roughness may be
related to heat exchange properties.
I t is known that surface wax in the configuration found in these plants can
reflect both light and heat. There is sufficient wax on most specimens for their
leaves to show a bloom in the wild and, in most cases, when the plants are grown
under glass. Another relevant observation is that the waxy surface bloom on
leaves of many Aloe species increases considerably in the dry season (Carter &
Brandham, 1979).
Biochemical considerations
From the cytological, anatomical and morphological evidence considered
above it has been shown that the shrubby aloes of East Africa comprise a central
group of tetraploid species with a number of diploid species forming an outer
group to the north, east and south. It has been suggested that the tetraploids are
very closely related to each other, that they have arisen from a form very similar
to one of these diploids (A. morijensis) in the Rift Valley of south-west Kenya, and
that they are undergoing rapid clinal divergence northwards along the Rift Valley
and east and westwards on either side. Evidence has also been put forward to
show that of the diploids A. fibrosu, A. moriiensis and A. babutiensis are closely
SHRUBBY ALOE EVOLUTION
31 I
related, especially the first two, but that none of the remaining diploids, A .
yavellana, A . wilsonii and A . rabaiensis, is closely related to any other species.
Reference to Table 1 will show that these findings are strongly supported by
the analysis of the leaf exudate patterns of the different populations. I t has been
pointed out above that exudate groups A- and C- differ qualitatively from
groups A and C respectively by the absence of only one spot on the
chromatograms (Fig. 21, and are thus closely related.
Exudate analysis confirms the affinity, determined by anatomical evidence, of
the southern diploids A . fibrosa, A . morijensis and A . babatiensis, which are all group
D. I t also indicates the similarity to each other of all of the Rift Valley tetraploids,
which are either group A o r A - . These are plants from localities 8- 13 and 23, and
also A . cheranganiensis from further north (localities 19-2 1). The latter is therefore
probably closely related to the other Rift Valley tetraploids and could well have
evolved during colonization along the Rift Valley floor.
West of the Rift Valley A . dawei and A . elgonica (localities 14-18) fi)rm another
related group, all having exudate patterns C o r C-.
In the tetraploids east of the Rift Valley the discontinuity which has been noted
above on anatomical and morphological evidence to occur at Nyahururu Falls
also appears in the biochemical data, because the exudate pattern changes to
group B, and all tetraploids east of this point are in this group. This supports the
decision (below) to amalgamate all of the tetraploid populations in this area into
a single taxon ( A . nyeriensis subsp. nyeriensis),with the possible exception of the El
Barta Plain population (361, which might show a morphological difference from
the remainder.
The exudate groupings also confirm that A . yavellana (group El, A . wilsonii
(group F), and A . rabaiensis (group G ) are not related to each other, nor to any of
the remainder.
One group which appears in Fig. 2 to be completely different from the rest is
group D, which has only two purple spots on the chromatograms. I t has been
shown above that two species of this group (A.jibrosa and A . morijensis) d o not
contain the secretory cells which produce the exudate in other species, but
contain fibres in the equivalent positions in their leaves. Consequently the
exudate produced by group D species is so weak that it needs to be concentrated
far more than those of the other groups for any spots to appear on the
chromatogram. Therefore, i t is quite possible that, while this group does not
contain any of the compounds exhibited in various combinations by the other
groups, the latter could contain, in their exudates, small quantities of the
compounds present in group D. Highly concentrated exudates from these other
groups run very badly on TLC plates, with much blurring, so it is not possible at
present to reveal small quantities of any group D compounds which might be
present in them.
The biochemical relationship between group D plants and the remainder
seems at first sight to be rather distant since they have no compounds in
common, but the evolutionary relationship between them and the tetraploids
could nevertheless be very close since the biochemical differences have been
shown above to be dependent only on whether the bundle sheaths were capable
o f secretion o r not. As fibres in bundle sheaths are rare in Aloe except perhaps in
leaf bases in some species (e.g. A . babatiensis) it is probable that they are of
secondary occurrence. Thus they need not have been present in the putative
20
3 12
n.
F. CUTLER E T A L
ancestor either of the non-fibrous (i.e. secretory) tetraploids or of the fibrous
(i.e. non-secretory) diploids which are extant (A.fibrosa and A. morijensis). We have
also made unpublished observations of the occurrence of fibres (replacing
secretory cells) in other genera in the Aloineae, notably some Huworthia species.
Taxonomic considerations
Ever since their original publication, three of the tetraploid species of shrubby
Aloe, A. nyeriensis, A. kedongensis and A. ngobitensis have been identifiable with
reliability only by consideration of the origin of the specimen. One of these, A.
ngobitensis, has as its type locality Ngobit Bridge (locality 271, which is only about
24 km from that of A. nyeriensis (locality 29). I t is but a very local form, with
flowers which are more glossy and brighter red than those of the other two
species. The three populations of this species cited by Reynolds (19661, which
occur within the wider distribution of A. nyeriensis, have been examined in the
field. All stations of A. ngobitensis show a wide variation in flower colour which
cannot substantiate a claim to separate specific or even varietal status for the
glossy red form, this being only one extreme of the colour range shown by A.
nyeriensis, the earliest-named species of the three. The other characters used by
Reynolds for its separation, thickness of stem, size of leaf, height, and number of
branches in the inflorescence, all show a similar variation within cited
populations of A. ngobitensis and A. nyeriensis. Furthermore, in at least two of the
A . ngobitensis populations, Suguroi (locality 26) and Ngobit Bridge (locality 27)
there were also, in addition to the range of colour forms of the shrubby species,
two non-shrubby species of Aloe, A. secundtflora and orange- and yellow-flowered
forms of A. lateritia, which hybridized with the shrubby plants to form an
extremely variable range of triploid shrubby plants. This combination of
variability and natural hybridization makes it impossible to maintain A.
ngobitensis as a taxon distinct from A. nyeriensis.
I t is only under close examination that any difference can be detected between A.
nyeriensis and the third species, A. kedongensis. When variation is taken into account
over the whole of their distribution, both have the same flower colour (orangescarlet with yellow tips), the same number of inflorescence branches and the same
size of leaf. However, there are some constant differences between the inflorescences of these two forms which are sufficient to separate them into distinct
taxa. In an area between the eastern escarpment of the Rift Valley and Mt. Kenya,
A. nyerienris is widespread. In the Rift Valley itself plants hitherto named as A.
kedongensis can be separated by their shorter, more deltoid bracts and, more
significantly, their longer pedicels. Plants from Nyahururu Falls, which is a few
kilometres from the edge of the Rift Valley, are less easily defined and have been
shown to be somewhat aberrant because of their unique moist habitat.
Nevertheless their overall characters tend towards A. nyeriensis.
As a result of the data given above it is here proposed that A. ngobitensis cannot
be upheld as distinct because its particular flower colour grades into that of A.
nyeriensis and also because of demonstrated hybridization at two of its three cited
stations. A. kedongensis is upheld, but at sub-specific rather than specific level. Our
revised consideration of the nomenclature and synonymy of A. nyeriensis is as
follows :
SHRUBBY ALOE EVOLUTION
3 1
Aloe nyeriensis Christian, Flowering Plants ofAfrica, 29: t. 1 126 ( 1952).
subsp. nyeriensis
TYPE:Kenya, 24 miles N. Nyeri, 1938, Pole-Evans 6 Erens 1198, cult in
Christian 9 8 4 (Holotype PRE, isotype K ) .
SYNONYM: A . ngobitensis Reynolds,Journal ofSouth African Botany, 19: 6 (1953)
subsp. kedongensis (Reynolds) S. Carter, comb. 8c stat. nov.
B A S I O N Y M : A . kedongensis Reynolds, Journal of South African Botari.~, 1 9 : 4
(1953).
TYPE:Kenya, 2 miles S . of Lake Hotel near L. Naivasha, 19. iv. 1952,
Reynolds 6 5 4 6 (Holotype PRE; isotypes EAH, K).
The following are previously described species of Alo? which are retained in the
light of the present study:
A . babatiensis Christian & Verdoorn, Bothalia, 6;440 (1954); Revriolds, Aloes of
Tropical Africa: 358 (1966).
A. cheranganiensis Carter 8c Brandham, Cactus 6 Succulent Journul .f Gwut Britain,
4 1 : 4 (1979).
A . dawei Berger, Notizblatt des Konigl. botanischen Gartens und Museums zu Berlin,
4 : 246 (1906); Reynolds, Aloes of Tropical Africa: 368 (1966).
A . elgonica Bullock, Kew Bulletin, 1932: 503 (1932); Reynolds, Aloe.\ of Tropical
Africa: 359 (1966).
A.Jibrosa Lavranos k Newton, Cactus 6 SucculentJournal ( U . S . ) ,4 8 : 273 (1976).
A . morqensis Carter k Brandham, Cactus 6 Succulent Journal of Great Britain, 4 I :3
( 1 979).
A . rabaiensis Rendle, Journal of the Linnean Society. Botany, 30: 410 (189.5);
Reynolds, Aloes of Tropical Africa: 366 ( 1966).
A . wilsonii Reynolds, Journal of South African Botany, 22: 137 1956); Reynolds,
Aloes of Tropical Africa: 26 1 ( 1966).
A . yauellana Reynolds, Journal of South African Botany, 20: 28 1954);Reynolds,
Aloes of Tropical Africa: 344 (1966).
ARTIFICIAL KEY TO SPECIES
1.
1 ’.
. . .
Bracts large, at least 14 x 7 mm ; leaf bases fibrous
Bracts much smaller, 10 x 5 mm o r less; leaf bases not fibrous
2.
2‘.
3.
3’.
2
4
Inflorescence branched; flowers pink
. . . . A . babatiensis
Inflorescence simple o r 1-branched; flowers red and
yellow.. . . . . . . . . . . . . . . 3
Plant not more than 1 m high; leaves small, 170 x 30 mm o r
less; perianth to 28 mm . . . . . . . . . . A . morijensis
Plant to 2 m high; leaves to 300 x 50 mm; perianth to 35 mm A.fibrosa
4.
4’.
5.
.
.
Racemes capitate o r sub-capitate .
Racemes cylindric . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
5
6
Leaves green or bronze-tinged; bracts less than 7 x 1.5 m m ;
A . yauellana
pedicels to 10 mm long; perianth to 27 mm . . . .
5‘. Leaves glaucous; bracts to 11x 3 mm; pedicels to 18 mm long;
perianth to 32 mm . . . . . . . . . . . A . rabaiensis
D. F. CUTLER ET A L
314
Plant small, stem less than 800mm, usually decumbent;
leaves ovate to 250 x 90 mm . . . . . . . . A . wilsonii
6’. Plant usually over 1 m high, erect; leaves lanceolate, more
than 250 mm long, or if 250 mm then much less than
90 mm wide. . . . . . . . . . . . . . .
7
6.
7.
7’.
Stems to 1 m high; leaves to 400 x 90 mm, teeth very large to
9 mm long. . . . . . . . . . . . . . . A . elgonica
Stems usually 1-2 m high; leaves more than 6 times longer
than broad; teeth to 4 mm . . . . . . . . . . . . 8
8.
8’.
9.
9’.
Leaves very large, to 600 x 90 mm; perianth dull red,
rarely yellow . . . . . . . . . . . . A . dawei
Leaves up to 550 x 70 mm, usually less; perianth orangescarlet, usually yellow-tipped . . . . . . . . . . 9
Leaves glaucous; inflorescence usually 2-branched; perianth to
29 mm, tipmarkedlyup-curved
. . . . . . A . cheranguniensis
Leaves green; inflorescence usually more than 2-branched;
perianth 35-40 mm, tip not up-curved . . . . . . . . 10
10. Bracts lanceolate, to 10 mm long; pedicels less than 20 mm
. . . . . . . . . . . . .A . nyeriensis subsp. nyeriensis
10’. Bracts deltoid, not exceeding 6 mm long; pedicels more
than 20 mm . . . . . . . A . nyeriensis subsp. kedongensis
GENERAL CONCLUSION-OVERALL
EVOLUTIONARY TRENDS
The results presented in this paper are summarized diagrammatically as a map
showing evolutionary trends in the East African shrubby aloes (Fig. 1 1). Our interpretation of the evolution of these plants is as follows: a small group of related
diploid species occurs in southern Kenya and northern Tanzania ( A . bubutiensis,
A.fibrosa and A . morijensis),also A . rubuiensis which is not related to these. From a
form very close to A . morijensis, chromosome doubling somewhere in the southern
Rift Valley of Kenya produced a tetraploid which is now evolving actively in
several directions. It has spread northwards up the Rift Valley as A . nyeriensis
subsp. kedongensis with a probable link further north to A . cheranguniensis,which is
morphologically distinct but biochemically related.
West of the Rift Valley the tetraploid has differentiated into a lowland form, A .
dawei, which has spread round the north shore of Lake Victoria as far as the
Congo River. Here the plant is a broader-leaved form, but must remain
undescribed for the present because of the lack of suitable type material. Aloe
dawei has also differentiated northwards via an intermediate at Busia (locality 16)
to a highland, more dwarf form, A . elgonicu, to which it is related biochemically.
East of the Rift Valley the tetraploid exhibits a discontinuity on
morphological, biochemical and anatomical grounds, but taxonomically it has
SHRUBBY ALOE EVOLUTIOK
315
Figurc 1 1 . Map d E a s t Alrica showing the distribution o f t h e shrubby A h taxa idrntlficd ill 11119 I ~ I [ I ( % I .
tlic origin of rhc tetraploid group and the probable evolutionar-) ti-ends withill t l ~ n tP I o u p
316
D. F. CUTLER ET AL.
Figure 12. Aloenyeriensis subsp. nyeriemis at its type locality (29) in savanna
to
the west 01 Mt. Kenya
been retained in the same s ecies as the Rift Valley form as A . nyeriensis subsp.
nyeriensis. This taxon i n c h es the type localities for A . nyeriensis (locality 29,
Fig. 12) and A . ngobitensis (locality 271, the latter of which cannot be upheld as
a distinct species. Within this single taxon east of the Rift Valley anatomical
evidence has shown that active clinal differentiation is occurring in an easterly
direction, diverging around the base of Mt. Kenya, and a northerly direction
towards the El Barta plain where there might or might not be a morphological
discontinuity to produce a new species there. Diploid species to the north of the
tetraploid group ( A . wilsonii and A . yawellanu) are distinct from each other and are
probably not closely related to the tetraploids.
dp
ACKNOWLEDGEMENTS
Thanks are due to Miss M. A. T. Johnson and Mrs R. M. 0. Gale for valuable
technical assistance.
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