Bot. J. Linn. SOC.,68: 173-208, With 2 0 figures
April 1974
Studies on the leaf anatomy of Euphorbia
11. Venation patterns *
LALITA SEHGALt and G. S. PALIWAL
Plant Anatomy Laboratory, Department of Botany,
University of Delhi, Delhi- 7
Accepted f o r publication March 1973
Leaf venation patterns of 150 species of Euphorbia are presented and their value as a diagnostic
feature in the genus assessed. Based o n th e gross venation patterns, t h e species have been
grouped into uni-, bi-, tri-veined and special categories. The majority of the species studied
belonged t o the tri-veined category, in which ornamentation of the veins and the course of
traces in the lamina proved useful additional characters.
Features such as the size of the areoles, the number of vein endings, and their further
ramifications and composition in each areole varied in the same leaf or in different leaves of a
given species. Furthermore, n o direct correlation could be established between the areole size
and t h e numbers of vein endings and tips per areole.
Forty species possess a parenchymatous vein sheath. Globular chloroplasts showed u p
conspicuously in the sheath cells of a few species. Dilated tracheidal elements, localized o n the
venules or at th e tips of vein endings, are characteristic of t h e xerophytic species. In some
instances, correlation was apparent between plants grouped according to their venation patterns
and their habit.
CONTENTS
. . . . . . . . . . . . . . . . .
Introduction
Aim and scope
. . . . . . . . . . . . . . . .
Materials and methods
. . . . . . . . . . . . . .
Terminology
. . . . . . . . . . . . . . . . .
Observations
. . . . . . . . . . . . . . . . .
Venation types
. . . . . . . . . . . . . .
Uni-veined
. . . . . . . . . . . . .
Bi-vein ed
. . . . . . . . . . . . . .
Tri-veined . . . . . . . . . . . . . .
(a) Veins ornamented
. . . . . . . .
(b) Veins unornamented
. . . . . . .
(i) Midrib formed by three strands
. . .
(ii) Midrib organized b y the median strand alone
. . . . . . . . . . . . .
Special types
Discussion
. . . . . . . . . . . . . . . . . .
Acknowledgements
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References
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* Dedicated t o t h e memory of the late Professor A. S . Foster, who laid concrete foundations for
studies on leaf venation patterns.
t N t e Kakker. Present address: Department of Botany, Gargi College, Lajpatnagar IV, New Delhi-24.
173
174
L. SEHGAL AND G . S. PALIWAL
INTRODUCTION
The Euphorbiaceae have been reckoned as one of the most unwieldly
families of the angiosperms, comprising about 300 genera and 5000 species
(Willis, 1966), or as many as 7000 species according to Webster (1967).
Although the family is predominantly tropical, several genera occur in the
temperate regions as well. The family embraces a heterogeneous assemblage of
plants representing diverse habits from herbs, through vines and shrubs, to
trees. The range of morphological variation is so great that it is difficult to
characterize the family, and for that reason many authors have suggested that it
may be polyphyletic in origin (c.f. Croizat, 193713; Metcalfe & Chalk, 1950;
Webster, 1967). The majority of the genera may, however, be recognized by
the presence of unisexual flowers, a floral disk and a trimerous gynoecium.
Focussing attention on the tribe Euphorbieae, one finds that it is
characterized by an inflorescence in the form of a cyathium having numerous
male flowers and a single female flower. There has been a continual
controversy in the literature with regard to the composition of the tribe.
Boissier (1862) included four cyathium-bearing genera in it, viz. Anthostemmu,
Euphorbiu, Pedilunthus and Synudenium. Pax & Hoffman (1931) included
additional genera such as Stenudenium and Monudenium, although these differ
only slightly from Euphorbiu. Some of the later authors have also introduced
Elaeophorbiu, whilst Hutchinson’s (1969) recent account of this tribe contains
several new genera.
Euphorbiu is undoubtedly the largest genus of the Euphorbiaceae, with over
1500 species (Pax & Hoffmann, 1931). According to Willis (1966) it has nearly
2000 species. I t is not only the largest genus of this family but also one of the
largest among angiosperms. It owes its name to King Juba I1 of Mauritania, who
discovered a succulent plant and named it Euphorbiu in honour of his
physician, “Euphorbos”, who was equally fleshy (Pax & Hoffmann, 1931).
Latex is perhaps the most striking product of euphorbias and it is distributed
throughout the plant in a series of tubes derived from either single cells
(Cameron, 1936; Mothes & Meibner, 1963) or articulate laticifers formed by
the fusion of several cells (Milanez & Neto, 1956).
The nomenclature and classification of Euphorbiu are far from satisfactory
and the taxonomists are not in agreement with regard to the composition of
the genus. As early as 1862, Boissier distributed the 600 species of the genus
then known into 26 sections. The number of sections was reduced to six by
Bentham & Hooker (1880), whereas Pax & Hoffmann (1931) increased it to
nine while the number of known species increased to 1500. Subsequently,
Small (1933) included only the shrubby, fleshy, cactus-like species in the genus
Euphorbiu. Croizat (1936, 1937a) questioned the validity of the cyathium as a
criterion for the coherence of an otherwise extremely variable genus (cf. also
Croizat, 1940),whereas Scheref (1940) considered Euphorbiu to display within
itself such a diversity of intergrading characters as to make clear-cut generic
segregation impossible. Perry ( 1943), basing his conclusions on chromosome
number in the genus, agreed with Small in the delimitation of the shrubby,
cactus-like species as a separate genus, and argues for the establishment of a t
least three separate genera.
Ponsinet & Ourisson (1968a, b), who examined the latex for the occurrence
EUPHORBIA-LEAF VENATION PATTERNS
175
of triterpenes and tried t o correlate their results with morphological,
morphogenetical and geographical distribution characters, found additional
support for the system of Pax & Hoffmann.
The tremendous diversity in habit, leaf morphology, chromosome number
and pollen configuration seems to favour the separation of Euphorbia into a
number of smaller genera. The taxonomic history of the Euphorbieae, however,
indicates that the problem does not have an obvious solution. Variations in
features such as the cyathial glands (appendaged or not) and female flowers
(naked or calyculate) do not seem to show strong correlations with major
phyletic groups. Various exceptions to such features make it difficult to single
out diagnostic characters to segregate the taxa.
The final analysis as presented by Webster (1967) is to treat Chamaesyce as a
separate genus, whereas all other taxa are relegated to subgeneric status within
Euphorbia. Thus delimited, Euphorbia sensu stricto is still a large polymorphic
genus consisting of more than 1000 species in seven subgenera, viz. Esula,
Poinsettia, Euphorbia, Agaloma, Eremoph y t o n , Lyciopsis and R hizanthium.
The comparative analysis of the foliar venation, which comprises the main
theme of the present study, has attracted the attention of several morphologists
and anatomists over a long period. A brief resumtr of these works is presented
here to form a basis for further discussion.
As early as 1895, Kerner & Oliver offered a classification of venation
patterns. Subtle variations in leaf venation have long been useful to
palaeobotanists and the early work of von Ettingshausen (1861) is still very
useful in this regard.
In matters of phylogeny, major venation patterns are proving significant. For
example, Foster (1959, 1961) has suggested that the open dichotomous
venation of the ranalean genera Kingdonia and Circaeaster may be primitive
within the angiosperms, and this idea seems worthy of consideration. On the
other hand, Subramanyam & Banerji (1967) encountered a similar situation in
several species of Utricularia, a member of the comparatively advanced family
Lentibulariaceae, and suggest that dichotomous venation might be an adaptive
feature. Solbrig (1960) has also recorded this feature in Raoulia, a member of
the Asteraceae, and considers it to be a derived rather than a primitive
character. Recently, Melville (1969) has traced the evolution of the angiospermous leaf by comparing the venation pattern of living plants from the
countries which formed Gondwanland with their fossil records (cf. also Alvin &
Chaloner, 1970).
In addition, several authors have shown that the careful description of
venation, together with studies of other details of leaf anatomy, can yield
valuable taxonomic dividends (Carlquist, 1959; Dede, 1962; Varghese, 1969).
The works of Pray (1955a, b) suggest that distinctive patterns of minor vein
ontogeny, as well as mature venation, can bear comparative analysis.
AIM AND SCOPE
As indicated above, the genus Euphorbia has been problematical to
taxonomists for a long time, particularly with regard to its infrageneric
classification. Even the application of diverse criteria such as the type of
inflorescence, cytology, embryology, electrophoresis of latex, and the nature of
1 76
L SEHGAL AND G. S. PALIWAL
chemical compounds have provided no satisfactory solution, so that Webster
(1967) rightly remarked that the “major difficulty appears to reside in the lack
of the basic morphological and cytological data” in this genus. The present
work was, therefore, undertaken to find out whether or not the characters of
leaf morphology and anatomy could provide some information in this regard.
The survey has been extended to as large a number of taxa as could be
obtained, fresh or preserved or even as herbarium specimens.
Information has been gathered on the following aspects: (a) vasculaturevenation types, structure of the node, bundle sheath, terminal idioblasts;
(b) epidermis-epidermal cells, stomata, trichomes; (c) laticifers; and (d) tissue
organization of the leaves. The present paper, the second in the series, is,
however, devoted exclusively to the description of the venation patterns.
MATERIALS AND METHODS
Cleared leaves formed the most important object of study. Fresh, fixed or
dried leaves from herbarium specimens have been employed for this purpose
(Paliwal & Kakkar, 1969).
Information about the internal organization of the leaves was gathered from
transverse as well as paradermal sections. Serial sections, passing through the
node as well as the regions just above and below, were employed to trace the
initiation of the vascular supply to the leaves. Paradermal sections of the leaves
were obtained at 6 pm, whereas the transverse sections and those at the nodal
regions were cut at 8 pm. Most of the preparations were stained with safranin
and fast green, although occasionally toluidine blue was also employed.
The venation patterns of the leaves were drawn with the help of a micro-slide
projector or a Kodak photographic enlarger (for extra large leaves). For
measuring the area of the vein-islets, a chequer board was employed. The total
number of small squares covering the vein-islet were counted. In the same
areole, the number of vein endings and terminal tips were also recorded. Care
was taken to measure the smallest as well as the largest areoles, so that a range
could be obtained for each species. Averages of five readings were recorded in
this manner for three leaves, and then the mean value was calculated.
The following 150 species were studied:
1. E. abramsiana Wheeler
2. E. aggregata A. Berger
3. E. akenocarpa Guss.
4. E. algeriensis Boiss.
5 . E. altissima Boiss.
6. E. amygdaloides L.
7. E. antisiphylitica Zucc.
8. E. apocynifolia Small
9. E. ardomontana Boiss.
10. E. arizonica Engelm.
11. E. atlantica Cosson ex Boiss.
12. E. balsamifera Aiton
13. E. berthelotii Bolle ex Boiss.
14. E. bicolor Engelm. & Gray
1 5 . E. bivonae Steud.
16. E. boetica Boiss.
17. E. bombaiensis Santapau
18. E. broteri Daveau
19. E. bubaiina Boiss.
20. E. bulleyana Diels
21. E. buxifolia Lam.
22. E. caducifolia Haines
23. E. campestris Cham. & Schlecht.
24. E. capitellata Engelm.
25. E. capitulata Reichenb.
26. E. capuronii Ursch & Leandri
27. E. caput-medusae L.
28. E. carniolica Jacq.
E UPHORBIA -LEAF VEN AT10 N PATTERNS
29. E. ceratocarpa Tenore
30. E. characias L.
31. E. cinerascens Engelm.
32. E. clarkeana Hook. f.
33. E. clementii Domin
34. E. coccinea Roth
3 5. E. corallioides L.
36. E. cornuta Pers.
37. E. corollata L.
38. E. corrigioloides Boiss.
39. E, cristata Heyne ex Roth
40. E. cyparissias L.
41. E. dsntata Michx.
42. E. dracunculoides Lam.
43. E. dregeana E. Meyer
44.E. drummondii Boiss.
45. E. dulcis L.
46. E. elegans Sprengel
47. E. emodi Hook. f.
48. E. erythroclada Boiss.
49. E. esula L.
50. E. exigua L.
51. E. exstipulata Engelm.
52. E. falcata L.
5 3. E. filijlora Marloth
54. E. fimbriata Heyne
5 5. E. florida Engelm.
56. E. fragifera Jan ex Link
57. E, fruticosa Forsk.
58. E. gedrosiaca K.H. Rech.
59. E. geniculata Ort.
60. E. gerardiana Jacq.
61. E. gfareosa Pallas ex Bieb.
62. E. glaucophylla Poiret
63. E. globosa Sims
64. E. gorgonis A. Berger
65. E. granulata Forsk.
66. E. hamata Sweet
67. E, helioscopia L.
68. E. heterophylla L.
69. E. hirta L.
70. E. humifusa Willd.
71. E. hyberna L.
72. E. hypericifolia L.
7 3. E. hyssopifolia L.
74. E. incisa Engelm.
75. E. indivisa (Millsp.) Tidestrom
76. E. ipecacuanha L.
77. E. isophylla Bornm.
177
78. E. isthmia Tackh.
79. E. kerneri Huter ex A. Kern
80. E. lathyris L.
81. E. leucophylla Benth.
82. E. longistyla Boiss.
83. E. lucida Waldst. & Kit.
84. E. maculata L.
85. E. maddenii Boiss.
86. E. marginata Pursh
87. E. melanadenia Torrey
88. E. micromera Boiss.
89. E. microphylla Heyne ex Roth
90. E. milii Des Moul.
91. E. misera Benth.
92. E. mucronata Clarke
93. E. myrsinites L.
94. E. neriifolia L.
95. E. notoptera Boiss.
96. E. obscura auct.
97. E. orbiculata Kunth var. jawaharii Rajgopal & Panigrahi
98. E. palmeri Engelm.
99. E. palustris L.
100. E. paniculata auct.
101. E. paralias L.
102. E. pediculifera Engelm.
103. E. pentagona Haw.
104. E. peplzs L.
105. E. peplus L.
106. E. petrophila C.A. Meyer
107. E. pilosa L.
108. E. platyphyllos L.
109. E. polycarpa Benth.
110. E. polychroma Kerner
111.E. polygonifolia L.
112. E. portlandica L.
113. E. preslii Guss.
114. E. prolifera Buch.-Ham. ex D. Don
115. E. prostrata Aiton
116. E. pseudovirgata (Schur) Sbo
117. E. pubescens Vahl
118. E. pugniformis Boiss.
119. E. pulcherrima Willd. ex Klotzsch
120. E. pulvinata Marloth
121. E. pycnostegia Boiss.
122. E. radians Benth.
123. E. revofuta Engelm.
124. E. rigida Rieb.
125. E. robbiae Turrill
178
L. SEHGAL AND G. S. PALIWAL
126. E. rosea Retz.
127. E. rothiana Sprengel
128. E. rupestris C. A. Meyer
129. E. rupicola Boiss.
130. E. salicifolia Host
1 3 1. E. serrula Engelm.
132. E. setiloba Engelm. ex Torr.
1 3 3 . E. sikkimensis Boiss.
134. E. strictospora Engelm.
1 3 5 . E. stracheyi Boiss.
136. E. stricta L.
137. E. supina Raf.
138. E. synadenium Ridley
139. E. taurinensis All.
140. E. thymifolia L.
141. E. tirucalli L.
142. E. togakusensis Hayata
143. E. tomentulosa S. Watson
144. E. trigona Haw.
145. E. uralensis Fischer ex Link
146. E. valdevillosocarpa Arvat &
Nyarady
147. E. verrucosa Lam.
148. E. virgata Waldst. & Kit.
149. E. wallichii Hook. f.
150. E. zornioides Boiss.
TERMINOLOGY
From the standpoint of venation types, the classical studies of von
Ettingshausen (1861) remain the most extensive to date. He proposed a
classification that was perhaps the most comprehensive of that period and has
greatly influenced later treatments (de Bary, 1884; Kerner & Oliver, 1895).
Since he did not study cleared materials but based his conclusions only on
impressions of the vascular systems, he did not always see the total venation.
As a result, some of the types suggested by him are artificial and/or
inaccurately described. The disadvantage in attempting t o utilize von
Ettingshausen’s classification is that there is an almost continuous series of
transitional patterns between the various types, and the significance of his
terminology thus tends to become limited to descriptive purposes only.
Moreover, with the increase in the knowledge of venation, the need for
reviewing his terminology has also been felt.
In the present work on Euphorbia, broad categories have been formulated on
the basis of the number of strands entering the base of the petiole or lamina (in
the sessile leaves). These have been named as uni-, bi-, and tri-veined, as most of
the leaves fall within these broad groups. The terms primary, secondary and
tertiary veins have been used in the traditional sense, indicating vein
relationships and relative sizes, but without any direct morphological implications. Veins departing from the midrib that are smaller than the secondary
veins but stronger than the tertiaries have been described as intermediate veins
or intermediates.
I t has been found most convenient to recognize the venation of the leaf as
major (implying a prominent framework of the vein system) and minor (a more
delicate reticulum) for the purposes of general description. Any attempts to
define major and minor venation precisely on the basis of vein histology, such
as were made by Plymale & Wylie (19441, would limit the usefulness of these
terms. Plymale & Wylie use the term “major” for the primary, secondary and
tertiary veins. Further, in exceptional cases they indicated that the quarternary
veins may also become a part of the major system. They carried out a detailed
survey of a number of species of mesomorphic, north temperate plants and
EUPHORBIA-LEAF
VENATION PATTERNS
179
reported that the major veins can be distinguished from the minor ones by:
(a) exhibiting some secondary growth and (b) the presence of vessels and sieve
tubes with sieve plates and companion cells. The minor veins, on the other
hand, were found to possess no detectable cambial activity and to have
tracheids alone in the xylem and phloem consisting of parenchyma only. Such
a grouping is difficult to apply in view of the variations that have been
recorded.
In some species veins are surrounded by a parenchymatous sheath, such veins
being designated ornamented. Sometimes tracheidal nodules are encountered at
the apices and in the serrae of the leaves.
The small veins usually form networks which vary in size and shape, and
subdivide the area of the mesophyll. The smallest areas or regions bounded by
the thinnest branches of the bundles are called areoles and usually contain
blind vein endings. The degree of branching of these vein endings varies
markedly in the leaves of different species.
In some leaves, dilated tracheidal elements have also been seen at the vein
endings in the areoles. In xerophytic forms these are arranged in large groups,
located especially at the apices and near the margins. Tucker (1964) has called
such elements “hybrid cells” between the tracheids and sclereids, whereas
Foster (1956) refers to them as “tracheiod idioblasts”. In the areoles the vein
endings are variously forked. The tracheids which terminate the ultimate tips
show an increase in cell diameter (often several times wider than those farther
back in the vein) and are extraordinarily variable in shape. The tracheidal
elements, sometimes lying free in the areole, have been described as “free vein
endings”.
OBSERVATIONS
Species of Euphorbia bear leaves of variable configurations: alternate,
opposite or whorled, and they may be persistent, deciduous or caducous and
sessile, subsessile or petiolate. Their size also varies widely from 3 x 2 mm to
95 x 22 mm, and their shape varies from oblong to linear-oblong, obliquely
oblong, linear, lanceolate or oblanceolate, obovate and spathulate. The margins
may be entire, serrate, distantly serrulate, serrulate towards the tip, dentate, or
even crenate. The apices may be acute, obtuse, mucronate, apiculate or
acuminate.
Venation patterns of mature leaves, where available, were used. On the basis
of the number of strands entering the base of the leaf (in sessile leaves) or
petiole (in petiolate leaves), they have been designated as uni-, bi- and
tri-veined. The majority of the species bear leaves belonging to the tri-veined
category, which has been subdivided according to the ornamentation of the
veins, organization of the midrib and further behaviour of the strands within
the lamina. The groups thus erected have been described in order to emphasize
the salient features of the venation patterns of the representative species
together with variations, if any, from the “normal” (representative) type. To
bring out the range of areole size, the illustrations in the various plates have
been arranged in an increasing sequence for each type.
13
L. SEHGAL AND G. S. PALIWAL
180
Venation types
Uni-veined
This condition has been observed in the leaves of only six species, which
have sessile or subsessile leaves of diverse shapes (Table 1).
E. incisa has been chosen as the representative type. Here a single strand
enters the base of the leaf (Fig. 1) and forms the midrib. After travelling a short
distance undivided, it branches laterally profusely. The secondaries arise at an
angle of 30" and branch again. Thus the secondaries, tertiaries and quaternaries
form a network. In E. petrophila some adjacent secondaries join near the
margin and form loops.
Figure 1. E. incisa. A single strand enters the base of the leaf and forms the midrib, with the
secondaries arising at acute angles (x 21).
**
5x4to
3 0 x 16
5 5 x 10
1 1 X 3 t O
12 x 1 to
35 x 5
4xlto
17 x 3
4xlto
5x1
12 x 6 t o
70 x 26
6x4to
25 x 9
3x2
5x4to
20 x 3
8x2to
22 x 9
2.5 x 2 to
20 x 11
size (mm)
entire
entire
margin
acute, mucronate,
obtuse
acute, cuspidate,
obtuse
acute and obtuse
apex
~~~
~
entire
obtuse
acute
entire
linear, lanceolate to
ovate
linear, linear-lanceolate,
oblanceolate
entire
acute
entire
linear
deltoid, spathulate
acute, apiculate
triangular
acute
entire
linear, lanceolate
entire and
serrate; may
have upper
half crenate
oblong; base oblique
serrate
acute, obtuse,
cuspidate
oblong, ovate; lanceolate; mostly serrate; obtuse
few entire
mostly with oblique
base
mostly entire; acute and obtuse
linear to lanceolate,
few serrate
elliptic, ovate,
oblanceolate
lanceolate-ovate. oblong,
base usually oblique
linear to ovate, oblong
linear; lanceolate-oblong
shape
General morphology
Refers to the serial numbers of species, as given under the list of “Materials”, falling under each type.
In these species, due to a heavy grouping of tracheidal elements, the areole size, etc. could not be made out.
4. E. hirra: 1.3.1.2
(21, 32,62,69, 126, 143)
5 . E. granulata: 1.3.1.3
(17, 24, 38, 39, 44,46,47, 65, 72, 89, 97,
115, 137, 140)
6. E. milii: 1.3.2.1.1
(3-5, 8, 11, 13-15. 25, 28, 29, 30, 33, 35, 37,
41,45,49, 57, 59, 68, 71, 76.80, 83, 86,
90-92, 94, 99, 100, 107, 108, 110, 116, 117,
119, 125, 127, 129, 130, 138, 145-148)
7. E. pulvinata: 1.3.2.1.2
(26, 53, 120)
8. E. tirucalli: 1.3.2.1.3
( 2 , 12, 27. 103, 141)
9. E. gorgonis: 1.3.2.2.1
(63.64, 118)
10. E. glareosa: 1.3.2.2.2
(16.42, 51, 52, 58, 60, 61, 77, 78, 93, 114,
124, 128, 139)
11. E. peplus: 1.3.2.2.3
((5, 56,67, 79, 85, 96, 98, 105, 112, 135)
150)
(*23,40, 74, 101, 106, 122)
2. E. polygonifolia: 1.2
(66, 81, 104. 111)
3. E. indivisa: 1.3.1.1
(1, 10, 31, 34, 54, 55, 70, 73, 7 5 , 8 2 , 84, 87,
88.95, 102, 109, 1 1 3 , 123, 1 3 1 , 132, 134.
1. E. incisa: 1.1
TY Pe
(with species reference numbers)
Table 1. Leaf characters in the various “Venation Types” of Euphorbia
**
**
0.09-0.3 2
1.3(5.1) to 2.5(6.3)
l.O(S.6) to 1.9(5.5)
*.
**
0.07-0.22
l.O(l.3) to 1.9(3.9)
0.03-0.21
1.0(3.5) to 4.0(9.2)
0.6(1.46) to 1.7(3.8)
0.04-0.2 2
0.04-0.44
0.6(0.7) to 1.7(4.1)
0.4(0.4) to l S ( 3 . 8 )
O.Z(O.6) to 1.6(6.6)
1.2(3.6) to 2.0(6.7)
Vasculature
Vein endings and tips
(no./areole)
0.04-0.20
0.02-0.16
0.04-0.2 3
0.08a.37
Areole size
(mma )
182
L SEHGAL AND G . S. PALIWAL
Figure 2. Venation patterns, E. incisa type. A. E. cyparisrias, with the smallest areoles in the
group; dilated tracheidal elements are seen at the vein endings; a few vein endings also occur in
the areoles (x 70). B-F. Portions of leaves of E. campestris, E. incisa, E. mdians, E. paralias and
E. petrophila respectively, forming a series showing a gradual increase in size of the areoles, the
number of endings being highest in E. incisa (X 7 0 ) .G . Various types of vein endings occurring
in this group; free vein endings are also seen in “h” ( X 5 2 0 ) .
As will be clear from Fig. 2A-F, the average sizes of the areoles are variable,
E. cyparissias possessing smallest (0.08 mm2) and E. petrophila the largest
(0.37 mm2) (Table 1). There does not seem to be any correlation between the
size of the areole and the number of vein endings and tips. For example,
E. incisa (Fig. 2C) shows the highest number of vein endings and tips in each
areole, although it does not have the largest areole size.
The variable composition of the vein endings and their tips has been
EUPHORBIA-LEAF
VENATION PATTERNS
183
sketched in Fig. 2G. Free vein-endings have been recorded at various stages of
development, those occurring in the areoles of E. cyparissius are shown in
Fig. 2Gh.
Bi-veined
This category includes species with leaves having two strands in the petiole.
The leaves are invariably petiolate and diverse in shape and, although all
the species included have two strands at the base of leaf, they differ in the
further course of these strands.
In E. polygonifolia, the representative of this type, the two traces which
separate from the main vascular supply are seen as two distinct strands in the
petiole.* At the junction of the petiole and the lamina these fuse to form the
midrib. In the lamina the secondaries arise at an angle of about 90" to the
midrib (Fig. 3). Near the margin the intramarginal veins either unite to form
Loops or break up into a network.
The veins are ornamented and the sheath is quite prominent (Fig. 4C,D).
Vein endings occur in the larger areoles but are absent from the smaller ones.
The endings may be branched or unbranched.
In leaves of E. peplis, two traces separate from the main supply and may
travel as such in the petiole or may divide at the base of the petiole itself,
forming four strands (cf. also Fig. 20). The veins are ornamented, a feature that
is not distinct at lower magnifications. The areoles are the smallest in the
group, with only a few vein endings. At some places the venules possess groups
of tracheidal elements, whereas at others they may be made up of only a single
element (Fig. 4A, B).
In E. leucophylla also, the two strands entering the petiole form the midrib.
Secondaries arise laterally and join to form loops near the margin. The veins are
not ornamented, and the areoles are relatively large, each containing several
vein endings (Fig. 4E, F).
Leaves of E. hamata are similarly bi-veined at the base of the petiole and
divide at the base of the lamina. Here, too, the veins are unornamented, but
they have a copious accumulation of tracheidal elements.
Tri-vein ed
The majority of the species have leaves with three veins at the basal region.
These have been broadly divided into two groups: (a) Veins ornamented and
(b) Veins unornamented.
(a) Veins ornamented. Leaves of about 40 species have ornamented veins.
Where the cells of the parenchymatous sheath are distinctly visible under low
power, the veins have been surrounded, in the figures, by a dotted line: where
they are invisible at this magnification, the dotted line is omitted. Depending
* The dual situation observed in some leaves of E. polygonifolia deserves special mention. Whereas the
majority of the leaves were seen to have two vascular strands, some possessed three strands also. The serial
sections passing through the nodal regions also bear out this conclusion. An identical situation has also
been encountered in some leaves of E. peplis (another species of this category), where in some leaves four
instead of two strands were observed (resulting from the division of the two traces before entering the
petiole).
184
L. SEHGAL AND C. S. PALIWAL
Figure 3. E. polygonifolia. Two strands are seen entering the petiole; the secondary branches
arise at more or less right angles to the midrib (x 23).
upon the behaviour of the three strands in the leaf blade, this group can be
further classified into three types.
E. indivisa type (22 species)-The leaves in all of these are petiolate and
approximately haIf of them have an oblique base.
Here three strands enter the petiole but lose their identity very soon and fuse
together. At the junction of the petiole and lamina, branches are directed
towards the sides of the leaf and the remainder continues to the apex as the
midrib. The leaf of E. indivisa has an oblique base, and the vein supplying one
side branches more profusely than the one on the other (Fig. 5 ) . The leaves of
some species, e.g. E. abramsiana, E. fimbriata, E. micromera and E. preslii, have
equilateral leaf-bases in which the branching of both sides is almost identical. In
EUPHORBIA-LEAF VENATION PATTERNS
185
Figure 4. Venation patterns, E. polygonifoliu type. A . E. peplir, organization of the areoles
(x 70). B. E. pep[&, part of A at higher magnification; note the variable nature of ven,ules and
fewer vein endin@ per areole (x 260). C. E. polygonifoliu, range of areole size; a distinct sheath
is present, represented here by stippling (x 70).D. A small portion of C magnified; note the
elongated sheath cells (x 260). E. E. leucoph ylla, dilated tracheidal elements randomly
distributed a t t h e vein endings (x 140).
the leaves of E. froridu and E. revolutu, three strands entering the petiole fuse
together immediately forming the midrib. The secondary branches arise
laterally.
The smallest areoles occur in E. micromeru 0.02 mm2 and the largest in
E. Zongistylu 0.16 mm2 (Fig. 6; Table 1).
The number of vein endings and tips per areole is highly variable. In most of
186
L. SEHGAL AND G. S. PALIWAL
Figure 5 . Leaf of E. indivisa showing the 3-veined condition at the base of the petiole; branches
arise at the junction of the petiole and lamina and supply the sides, the remainder forming the
midrib (x 27).
the species the smaller areoles do not contain any vein endings. The range is
therefore from 0 to 1.6 (or exceptionally 4.0).Both types of vein endingsunbranched and branched ones-are encountered. In E. humifusa, E. notopteru
and E. longistyla (Fig. 6F, H, I) they are much branched, whereas in
E. pediculifera they are mostly unbranched (Fig. 6D). There does not seem to
be any direct correlation between the size of the areole and number of endings
and tips per areole. The smaller areoles possess fewer vein endings and tips,
whereas large-sized areoles usually have a relatively large number of both.
E. hirta type (6 species)-The leaves are petiolate and their sizes and shapes
are not so diverse as in the E. grunuluta type (see Table 1).
EUPHORBIA-LEAF VENATION PATTERNS
187
Figure 6. Venation patterns, E. indivka type. A. E. micromera, with some areoles without vein
endings; note t h e thickness of the venules and the vein endings (x 70). B. E. revolura. a narrow
leaf with small areoles; note the sheath around the veins (x 70). C-E. Small portions of leaves of
E. serrula, E. pediculifera and E. indivisa respectively, with areoles containing fewer vein
endings (x 70). F-I. Portions of leaves of E. humifusa, E. hyssopifolia, E. noroptera and
E. longistyla, showing t h e distinctly organized sheath; in some areoles groups of tracheidal
elements occur a t the tips of the vein endings; note also free vein endings in F and I (x 70).
The three strands which enter the petiole follow an independent course, the
median forming the midrib and the laterals supplying the sides (Fig. 7). The
vein supplying the oblique side branches more profusely. The two forms of
E. hirta (red and green) show distinctly different patterns. The nature of the
cells of the sheath is also markedly different in the two.
The sizes of the leaves and the areoles appear to be inversely proportional to
188
L. SEHGAL AND G. S. PALIWAL
Figure 7. E. hirta. The three strands run independently, the median forming the midrib and the
laterals supplying the sides; since the leaf is oblique at the basal region, the lateral of that side
divides more profusely (x 2 9 ) .
each other to some extent. For instance, in E. tomentulosu, which possesses the
largest leaves, the smallest areoles occur. The number of vein endings and tips
per areole is directly proportional to the areole size (Fig. 8A-D).
E. grunulutu type (12 species)-The leaves are all petiolate, but otherwise
show considerable diversity in size and shape (Table 1). They may be as small
as 3 x 2 mm and as large as 16 x 9 mm. The shapes range from oblong or
lanceolate to ovate or obovate, with oblique bases in some instances.
In these leaves, also, three strands enter the petiole of the leaf, the median
68 I
SNXXLLVd NOILVNBA d V 3 1 - VI8HOHdR9
Figure 8. Venation patterns, E. hirta and E. granulata types. A - D , E. hirra type: A, E. hirta
(red), with areoles of small size and a well defined sheath; tips of the vein endings bear feebly
dilated tracheidal elements (x 70); B, E. buxifolia, showing areoles bearing several vein endings
(x 70); C,E. hirta (green), indicating larger areoles as compared t o the red form; the tips of the
vein endings do not bear dilated tracheidal elements (x 70); D, E. glycophylla, showing the
largest areoles in the group; number of vein endings and tips is also high in this species (x 70).
E-I, E. grantdata. type: E, E. drummondii, with small areoles each bearing vein endings
terminating into several tips, the latter being made u p of dilated tracheidal elements (x 70); F-I:
Portions of leaves of E. emodi, E. cristata, E. erythroclada and E. elegans, respectively; note
the distinct sheath and vein tips free from dilated tracheidal elements (x 70).
190
L. SEHGAL AND G . S. PALIWAL
Figure 9. E. granulafa Of the three strands entering the petiole, the laterals give branches to the
median and thereby contribute towards organization of the midrib; the lamina is densely veined
in this species (x 52).
along with the branches of the laterals, forming the midrib (Fig. 9). Some
species bear leaves with oblique bases and the lateral strand of the longer side
then branches more profusely.
The smallest average areole size is 0.04 mm2 (E. drummondii, Fig. 8E) and
largest 0.22 mm2 (E. elegans, Fig. 81). In almost all the species, the small
areoles do not possess vein endings. The number of vein endings and tips
increase according to the size of the areoles in some cases, as seen in Fig. 8E-I
and Table 1.
(b) Veins unornamented. The species included in this group (in which the
veins are not provided with a sheath) can be placed into two subgroups
EUPHORBIA - LEAF VENATION PATTERNS
191
depending upon whether all the three strands present in the leaves take part in
the formation of the midrib or only the median one.
(i) Midrib formed by three strands. Further delimitation of this subgroup is
based on the behaviour of the strands entering the base of leaf.
E. milii type (47 species)-A study of Table 1 will reveal the diversity in leaf
size and shape. The species included in this type bear sessile leaves, except a
few such as E. geniculata and E. heterophylla.
Here the three strands entering the base of the leaf travel independently for a
short distance in the midrib region (Fig. 10). Later they merge to form the
midrib itself. Secondaries arise on both the sides of the midrib and travel
Figure 10. E. milii The three strands travel apart in the midrib region for a short distance, come
closer and organize the midrib; secondary branches are produced laterally at acute angles; note
also the loops produced along the margins (x 16).
TVMITVd ”3 ‘3aNV 7V3H3S 1
Figure 11. Venation patterns, E. milii type. A. E. esula, with the smallest areoles in the group
(x 70). B-F. Portions of leaves of E. heterophylla. F. ipecacuanha, E. palustris, E. verrucosa and
E. salicifolia respectively, presenting a series of progressively larger areoles, the number of tips
per areole being the highest in E. verrucosa (x 70). G . E. rothiana. with large areoles containing
very few vein endings (x 70). H, I. E. milii and E. misera respectively, with almost equal
average-sized areoles, each bearing several vein endings terminated by tips of conspicuously
grouped tracheidal elements; note also the randomly distributed free vein endings in H (x 70).
towards the margin. They join the adjacent brinches, prior to reaching the
margin, in the form of loops. Sometimes the secondaries also divide on the
way, and the branches thus produced join with adjacent branches to form the
loops. E. Iucida and E. neriifolia, which are included under this type, differ in
that the lateral traces divide in the cortical region of the stem or branch. In
view of this, these species have also been referred to in the “anomalous/
transitional” group.
EUPHORBIA-LEAF VENATION PATTERNS
193
Figure 12. E. pulvinotu. The strands travel close together in the midrib region, secondary
branches arise at acute angles and a heavy tracheidal accumulation is seen throughout the
lamina (x 22).
The tertiaries and quaternaries form networks as well as areoles. The smallest
average areole size is 0.04 mm2 and the largest 0.44 mm2. The vein endings are
profusely branched with several tips (Fig. 11).Of the 47 species, several such as
E. milii and E. misera bear prominent groups of dilated tracheidal elements of
various shapes a t the vein endings (Fig. 11C, H, I), whereas the rest show a
comparatively simple vein organization as in E. esula, E. heterophylla,
E. palustris, E. verrucosa, E. salicifolia and E. rothiana (Fig. 1l A , B, D-G).
Distinct loops have also been seen in species such as E. pulcherrima, E. milii
and E. ceratocarpa.
E. pulvinata type ( 3 species)-The leaves are linear to linear-lanceolate in
194
L. SEHCAL AND G. S. PALIWAL
outline. The three strands entering the base of the leaf travel very close
together in the midrib region (Fig. 12). On the way they give out secondaries
which produce tertiaries, and thus a network results. In E. pulvinata and
E. filiflora the areoles are larger near the midrib and smaller towards the margin
(Fig. 13A, C), whereas in E. capuronii (Fig. 13B) they are considerably larger
than in the other two and not differentiated in size.
The tips of the vein endings possess dilated tracheidal elements here as well.
In E. capuronii they are arranged in groups at the apex and margins, as well as
\
~
E
I
\
i
I
Figure 13. A-I. Venation patterns, E. pulvinata, E. tirucali and E. gorgonis types. Portions of
cleared leaf blades respectively, showing the distribution and organization of dilated tracheidal
elements a t the venules and tips of vein endings, in the areoles, at the margins, and even lying
freely. A-C, E. pulvinata type: A, E. filiflora; B, E. capuronii; C , E. pulvinata. D-F, E. tirucalli
type: D, E. tirucalli; E, E. caput-medusae; F,E. balsamifera. G-I, E. gorgonis type: G , E. gorgonis; H, E. globosa; I, E. pugniformis. (all x 7 0 ) .
EUPHORBIA-LEAF VENATION PATTERNS
195
at the vein endings and venules. In E. filiflora such a grouping of tracheidal
elements is seen very prominently at the tip and sides of the triangular apex as
well as the margins. At many places tracheidal elements are also seen lying free
in the areoles (Fig. 13C).
E. tirucalli type ( 5 species)-All species have linear leaves.
The three strands entering the base of the leaf travel independently in a
parallel fashion in the region of the midrib, for about 1/3 to 1/2 of the lamina.
Later they come closer and fuse to form the midrib (Fig. 14).Secondaries
undergo further branching and produce areoles. At the margins and in the
Figure 14. E. tirucalli. The three strands travel apart in the midrib region for about the half
length of the lamina; a heavy accumulation of tracheidal elements is seen within the areoles and
along the margins (x 27.5).
14
196
L. SEHGAL AND G . S. PALIW'AL
I
Figure 15. E. gorgonh Of the three strands supplying the leaf the median alone organizes the
midrib; the laterals soon lose their identity due to a copious accumulation of tracheidal
elements (x 70).
areoles numerous groups of dilated tracheidal elements are seen at the vein
endings.
The areole size is variable and because of the heavy accumulation of dilated
tracheidal elements a t the vein endings, it has not been possible to determine
precisely the areole size and number of vein endings and tips in all of them
individually (Fig. 13D-F). In leaves of E. caput-medusae the dilated tracheidal
elements are densely massed a t the margins (Fig. 13E).
(ii) Midrib organized b y the median strand alone. In the leaves of this group,
the midrib is formed by the median strand alone whereas the laterals supply the
EUPHORBIA - LEA F VENATION PATTERNS
197
sides. Depending upon the behaviour of the strands in the lamina and the
shapes of the leaves, this group can also be divided into three types.
E. gorgonis type ( 3 species)-The leaves are linear or lanceolate to ovate. The
median strand forms the midrib and travels up to the apex. The laterals bear
copious groups of dilated tracheidal elements across the margin and in the
areoles (Fig. 15). In E. globosa the branches of the laterals travel vertically
upwards and possess dilated tracheidal elements. In these leaves, also, the size
of the areole and number of vein endings per areole could not be determined
owing to heavy tracheidal accumulation and profuse branching of the veins
(Fig. 13G-I).
E. glareosa type (14 species)-The leaves vary from linear to lanceolate or
oblanceolate, but show hardly any diversity in the margin and apex (Table 1).
Of the three strands entering the base of the leaf, the median travels
undivided up to the apex. The laterals start dividing soon after entering and
their branches travel vertically upwards almost parallel to the midrib (Fig. 16).
The branches towards the margin gradually fade out, whereas those next t o the
midrib move vertically upward for about 3/4of the lamina and then fade out
after forming loops with the secondary branches of the midrib.
In some other instances the laterals may branch only to a limited extent.
Sometimes, as in E. rigida, variations have been recorded in the vascularization
of some of the leaves. Here a very thin branch of one of the lateral strands has
been seen to join the median to organize the midrib.
An examination of Table 1 shows the diversity in areole size and number of
endings and tips per areole. The smallest areole recorded is 0.05 mm2 and
largest 0.22 mm ’. In Fig. 17 the range of areole sizes and vein endings in this
type have been sketched. An average small-sized areole contains a large number
of vein endings than do the larger ones. Thus, in this type the size of the areole
is inversely proportional to the number of vein endings and tips.
E. peplus type (10 species)-The leaves are mostly oblanceolate t o spathulate
and deltoid in outline.
The median strand forms the midrib and the laterals are almost parallel to
the margins. The laterals branch soon after entering (Fig. 18) and the branches
fade out after joining other lateral or midrib branches in the form of loops.
Sometimes well-marked compartments are also produced in this way.
The secondary branches arise approximately at an angle of 45’ and join their
neighbours in the form of loops near the margins. Among the species bearing
deltoid leaves, for example E. obscura, E. palmeri and E. portulandica, the
apical portion of the leaf is triangular. The lateral veins travel parallel to the
margin and branch after entering the leaf base. They may join amongst
themselves or with the branches of secondaries to form compartments. Hence
the compartments or loops are seen more distinctly along the margins in the
apical region. The size of the areoles and the number of vein endings and tips
per areole are highly variable, as seen in Table 1 and Fig. 19.
Special types
It should be recorded that it has not been possible to place a few species in
any definite group. These are listed in Table 2 and some of them exhibit special
(anomalous/transitional) leaf vasculature. Certain aspects of their vasculature
have been analysed below.
size (mm)
21 x 4
10 x 6
37x7
95 x 22
15 x 12
1 5 x 10
5x3
7x1
14 x 4
12 x 4
30 x 5
70 x 26
8x2
23 x 10
9x5
60 x 14
6x3
15x5
Species
1. E. antisiph ylitica
linear lanceolate
obova te
linear lanceolate
oblong
obovate
ovate tapering apex
reniform to rounded
linear
lanceolate
oblanceolate
elliptic
obovate to spathulate
base auriculate
elliptic
obovate
linear lanceolate
oblong
linear lanceolate
shape
entire
entire
entire
entire
entire
dentate
entire
entire
serrate
entire
entire
entire
entire
serrate near apex
entire
entire
entire
entire
margin
mucronate
obtuse
acute
obtuse
rounded
tapering
rounded
acute
acute
acute
acute
cuspidate
obtuse
obtuse
acute
acute
rounded
acute
apex
0.08
0.30
0.04
0.34
0.16
0.65
1.6(4.2)
1.4(1.4)
1.4(4.0)
1.4(5.5)
1.8(5.4)
1.2(2.4)
0.4(0.4)
1.2(3.8)
1.0(4.0)
2(10.4)
0.06
0.09
0.15
0.07
1.2(2.0)
8
1.2 (9.4)
8
1.6(3.2)
Vasculature
vein endings and tips
(no. per areole)
0.08
0.43
0.14
0.09
areole size
(mm2)
In these species, due to the dense massing of tracheidal elements the areole size, etc. could not be made out. ’
2. E. ardomontana
3. E. broteri
4. E. bubalina
5 . E. caducifolia
6. E. cornuta
7. E. dregeana
8. E. exigua
9 . E. exstipulata
10. E. glareosa
11.E. lucida
12. E. neriifolia
1 3 . E. peplis
14. E..pycnostegia
15. E. stricta
16. E. togakusensis
17. E. trigona
18. E. wallichii
No.
Morphological characters of leaves
Table 2. Species which could not be assigned t o any definite group (special cases 1.4)
tl
Z
>
r
E UPHORBIA - LEAF VENATION PATTERNS
199
Figure 16. E. glareosu. It is only below the dotted line that the three-stranded situation is clear;
the median strand alone forms the midrib, whereas the laterals branch off immediately; the
latter travel vertically upwards almost parallel t o the midrib and fade o u t successively. Note
especially the feeble vascular supply of the lamina. (x 23 ).
In the available leaves of E. ardomontana, E. bubalina, E. caducifolia,
E. exigua, E. stricta, E. togakinensis, E. trigona and E. wallichii the course of
the strands could not be clearly determined. Consequently, it has not been
possible to assign them to any of the types erected. In E. ardomontana there is
no areole formation, which is a rather peculiar feature. Leaves of E. caducifolia
bear groups of dilated tracheidal elements, a condition that is also recorded in
the leaves of other xerophytic species. E. trigona differs from the existing types
in the profuse branching of the lateral strands and the copious accumulation of
tracheidal elements (Fig. 20).
IVhUTVd ’S ‘3 a N V 7 V 3 H B S ‘7
Figure 17. E. ghreosa type. Detailed venation patterns in leaves of some of the species of
Euphorbia belonging t o this group to show the range of areole size and the number of vein
endings and tips in them. A. E. rupestris. with small areoles each bearing several tips; the tips of
the vein endings possess dilated tracheidal groups (x 7 0 ) . B-C. E. boetica and E. prolifera,
respectively, with areoles of almost equal average size, the number of vein endings per areole
being higher in the latter (x 70).D-F. Portions of leaves of E. dmcunculoides, E. exstipuhtu and
E. irthrnia, respectively, possessing variously-shaped areoles (x 70).G-I. Portions of leaves of
E. isophylla, E. glareosa and E. taurinensir, respectively; the average size of the areoles is highest
in E. taurinensis but the numbers of vein endings and tips per areole are smaller in this species.
At places dilated tracheidal elements also occur (x 70).
001
E UPHORBIA -LEAF VENATION PATTERNS
201
Figure 18. E. peplus, a characteristically spathulate leaf with its median strand forming the
midrib; the lathrals, which are comparatively weaker, travel on the sides almost parallel to the
margins (x 20).
E. exstipulata leaves show venation of two different patterns. Some
conforming to the E. glareosa type, whereas others exhibit a transitional
condition. In the latter, three strands travel independently for up to a third of
the length of the lamina, and later slender branches from the lateral strands join
.the median one. This pattern thus appears to be typologically intermediate
between the ones where all the three strands take part in the formation of the
midrib and those in which only the median forms the midrib. An identical
situation occurs in the leaves of E. antisiphylitica.
Variable characters were also observed in the leaves of a few species. Some of
these have features by which they can be included in particular groups, others
202
L. !SEHGAL AND G. S. PALIWAL
Figure 19. Venation patterns, E. peplus type. A-C. Portions of cleared leaves of E. obscura,
E. maddenii and E. stracheyi, respectively; note the profusely branched vein endings ( x 70).
D-F. E. palmeri, E. helioscopia and E. portlandica, respectively, present a series showing a
gradual increase in the areole size, the number of endings and their tips per areole being the
highest in E. helioscopia (X 7 0 ) . G-I. Portions of leaves of E. amygdaloides, E. peplus, and
E. fragifera, respectively, t o show thac the last one has the largest areoles and E. peplus has
fewer vein endings per areole. (x 70).
EUPHOR BIA -LEAF
VENATION PATTERNS
203
Figure 20. A few of the illustrative venation patterns placed under the anomalous “special”
category.
are anomalous in the course of differentiation of the vascular elements. Such
species have, therefore, been listed in their respective type as well as under the
anomalous category.
A few species, such as E. broteri, E. cornutu and E. pjmzostegiu, have been
observed to have more than three strands in their leaves. However, because
material was insufficient, their very proxima1 regions could not be studied and
hence it became difficult to ascertain the actual number of strands. It must be
reiterated here that failure t o examine the basal portions of the leaves may
sometimes lead to mistaken notions, such as the presence of a many-stranded
condition in E. glareosa, which is actually three-stranded (Figs 16 and 20). An
204
L SEHGAL AND G. S. PALIWAL
identical state has also been recorded in E. lucida, E. neriifolia (increase of
strands from three to five) and E. peplis (four strands produced due to division
of the usual two). In view of this, these three species also are listed under both
their respective types and the special category.
DISCUSSION
A survey of the literature on leaf anatomy reveals that the data obtained
from it have been employed amply for the elucidation of taxonomic and
phylogenetic relationships. The commonly used characters are venation
patterns, structure of the epidermis (stomata, trichomes, cuticle), laticifers, and
general anatomy of the leaf, petiole and node.
Two trends become apparent from this analysis: anatomical features of the
leaf have been employed either singly or in conjunction with the characters
drawn from disciplines other than morphology and anatomy. Examples of the
former are found in the contributions of Morley (1953) on the revision of the
genus Mouriri, Hagerup (1953) on Ericaceae, Cutler (1965) on Thurnia and
Tomlinson (1956, 1959, 1961) on several monocotyledonous families. The
usefulness of the leaf anatomy in Gramineae is suggested by the work of Prat
(1932) and Stebbins (1956). The studies of Brady et al. (1964) and Simola
(1968) on leguminous taxa indicate the taxonomic value of floral and foliar
vasculature. More recently, Paliwal & Kakkar (1970) even supported the
erection of a separate family, Garryaceae, on this basis.
On the other hand Erdtman & Metcalfe (1963), Forman (1966) and Paliwal
(1 966) couple the leaf features with information obtained from pollen
morphology, cytology and embryology in drawing phylogenetic conclusions.
Various aspects that have emerged from these :udies on the morphology
and anatomy of leaves of 150 species of Euphorbia have been assessed,
particularly in relation to the diversity of habit and habitat and earlier
conclusions with regard to the classification of this interesting taxon.
A perusal of the literature reveals that some of the more important lacunae
which still exist in our knowledge of the mature venation patterns among
angiosperms are:
(a) To what extent do climatic factors influence the various features
associated with veins?
(b) Which of the venation features are more important from the systematic
viewpoint and to what degree?
(c) What is the plasticity of venation characters within various species of a
genus or different genera of a family?
In the light of the present study it may be pointed out that in Euphorbia
there is a definite correlation between the presence of accumulated groups of
tracheidal elements and xerophytism; and a sheath around the veins is present
in the leaves of herbaceous species with a prostrate or ascending habit. Further,
the features of leaf venation to which taxonomic significance may be ascribed
are: the number of strands entering a leaf, the presence or absence of a sheath
around the veins, the organization of the midrib, and the behaviour of the
strands in the lamina, The plasticity of venation characters is shown by the
variation in (i) areole size and (ii) number of vein endings and tips per areole, as
EUPHORBIA-LEAF
VENATION PATTERNS
205
well as in (iii) the organization of the terminal vein endings. The bearing of
these features on the classification of Euphorbia and some other related aspects
are discussed below.
Depending upon the number of strands entering the petiole or base of the
leaf, the 150 species of Euphorbia covered under the present study have been
broadly grouped under uni-, bi-, tri-veined, and special categories. The majority
of the species are tri-veined. Further groups in this category have been
recognized on the presence or absence of a sheath around the veins (Table 3).
Table 3 . Classification of Euphorbia species on the basis of venation patterns
Category 1.1 - UNI-VEINED
Category 1 . 2 - BI-VEINED
Category I .3 - TR I- VEINED
Group 1.3.1
VEINS ORNAMENTED
Type 1.3.1.1
E. indiviru
Type 1.3.1.2
E. hirta
Type 1.3.1.3
E. granulata
Group 1.3.2 - VEINS UNORNAMENTED
Subgroup 1.3.2.1 - Threestranded midrib
Type 1.3.2.1.1 E. milii
Type 1.3.2.1.2 E. pulvinata
Type 1.3.2.1.3 E. tirucalli
Subgroup 1 . 3 . 2 . 2 - Singlestranded midrib
Type 1.3.2.2.1 E. gorgonis
Type 1 . 3 . 2 . 2 . 2 E. glareom
Type 1.3.2.2.3 E. peplus
Category 1 . 4 - SPECIAL. (anomalous or transitional)
-
The groups erected on the basis of ornamentation (sheath) have been further
divided into subgroups and “types”, taking into account the course of vascular
strands within the lamina. In the group where veins are ornamented, strands
may become joined together or may retain their identity in the petiole. In the
former situation, branches are produced at the junction of the petiole and
lamina and supply the sides of the leaf, whereas the remaining part forms the
midrib (E. indivisa type). In the latter, on the other hand, either the median
strand alone forms the midrib (E. hirta type) or the branches of the laterals
may also participate in its organization (E. granulata type).
In the group bearing leaves with unornamented veins two trends are
encountered: (i) all three strands may take part in the formation of the midrib
or (ii) only the median froms the midrib and the laterals navel on the sides.
Depending upon their subsequent behaviour these have also been classified into
various types.
Of the species investigated, some could not be assigned to any definite type
due t o the peculiar branching behaviour of the strands and have, therefore,
been treated under the “special” category. Under this heading are also included
leaves of a few species showing patterns which appear to be transitional
between two subgroups. For example, the venation of E. antisiphylitaca and
E. exstipulata may be regarded transitional between the subgroups of the
unornamented tri-veined category.
206
L. SEHGAL AND G. S. PALIWAL
Within the types, variations have been recorded in the size of the areoles and
the numbers of veinendings and their tips per areole. The areole size was found
to differ within a limited range even in the same leaf.
Levin (1929) attached great systematic importance to vein-islet areas, as a
result of his study of the genera Barosma, Cassia, Digitalis and Erythroxylon.
According to him the vein-islet number varies within narrow limits and the
numbers for different species are sufficiently constant for use as valuable
specific characters. Later, Hall & Melville (1951, 1954) opined that the number
of veinlet terminations, either alone or in conjunction with other histological
characters, is of taxonomic value particularly in genera with only a small
number of species.
In the present investigation it became apparent that the size of the areole
cannot be of great taxonomic value, especially when the number of species is
large, since the ranges of areole size (vein islet number per unit area) for several
species tend to overlap considerably and in certain instances are practically
identical. For example, in the E. niilii type (Table 11, in E. atlantica, E. bivonae,
E. capitulata, E. characias and E. platyphyllos the average areole size is
8.09 mm2. However, it does seem possible to separate groups of species by the
magnitude of the range of their vein islet areas. For instance, in members of the
E. indivisa type the average areole size range is 0.01 t o 0.18 mm2, whereas in
the E. granulata type it is from 0.04 t o 0.22 mm2 and in the E. peplus type it
varies from 0.09 t o 0.40 mm2.
In general, no direct correlation could be established between the size of an
areole and the number of vein endings and their tips per areole. In ornamented
types, however, the number of vein endings increases in relation to the areole
size (Tables 1 to 3), whereas in the E. glareosa type the areole size and number
of endings per areole are inversely proportional to each other.
ACKNOWLEDGEMENTS
We are grateful to Professor B . M. Johri both for suggesting the problem and
for constant inspiration. Our sincere thanks are also due to Dr R. N. Kapil for
kindly reading through the manuscript critically, to Dr S . C. Gupta for
encouragement in various ways, and to Dr K. M. M. Dakshini for offering
several helpful comments. Our deep appreciation is offered to the late Professor
A. S . Foster, formerly of the Department of Botany, University of California,
Berkeley, for kindly lending us a copy of the Ph.D. thesis of Dr Thomas
R. Pray for our perusal. We wish to record our sincere gratitude for the
cooperation received from the various institutions and individuals, by way of
supply of the material.
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