Botanical Journal of the Linnean Society (1995), 119: 65–76. With 6 figures
Reproductive biology in Acacia caven (Mol.) Mol.
(Leguminosae) in the central region of
Argentina
JOSE LUIS BARANELLI, ANDREA A. COCUCCI AND
ANA M. ANTON
Instituto Multidisciplinario de Biologıa Vegetal (IMBIV), Casilla de Correo 495,
5000 Co rdoba, Argentina
Received May 1994, accepted for publication May 1995
Studies on Acacia caven (Mol.) Mol. in central Argentina indicate that the species is polygamous
(andromonoecious), some plants having a high proportion of staminate heads. Though
pollen:ovule ratios of flowers, inflorescences and plants are at a level common for ‘facultative
xenogamic’ systems, controlled pollination shows a marked xenogamy. The ratio between
pollen grains in the polyad and the maximum number of seeds per pod is close to one.
Observations support the idea of the inflorescence as a specialized reproductive unit with
gynoecia functioning as fixed modules in which all or none of its ovules develop into seeds.
Mass flowering prior to the growing season, the absence of other floral resources in the
community, the high degree of fruiting limitations and scarce insect visitation suggest that,
as in other Acacia, the reproductive system of Acacia caven involves a great sacrifice of floral
resources.
© 1995 The Linnean Society of London
ADDITIONAL KEY WORDS:*fruit and seed set – phenology – pollen:ovule ratio – polyads
efficiency.
CONTENTS
Introduction . . . . . . . . . . . . . . . . .
Material and methods . . . . . . . . . . . . . . .
Results
. . . . . . . . . . . . . . . . . .
Inflorescence and flowers . . . . . . . . . . . . .
Distribution of stamens and carpels within flowers, cephalia and individuals
Ratios of polyads and pollen grains to ovules and seeds
. . . . .
Phenology
. . . . . . . . . . . . . . . .
Discussion
. . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . .
References
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INTRODUCTION
The genus Acacia comprises about 900 species in the tropical and subtropical
regions of the world. In Argentina, 21 species have been reported, some of
which occur as far south as 30° (Cialdella, 1984).
Taxonomic and palynological features have been described for this genus
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65
© 1995 The Linnean Society of London
66
J. L. BARANELLI ET AL.
in Argentina (Caccavari, 1976; Cialdella, 1984), but there are few studies on
floral biology. Most studies on this topic have been carried out on African
and Australian species (Bernhardt, Kenrick & Knox, 1984; Bernhardt &
Walker, 1985; Knox & Kenrick, 1983; Sedgley et al., 1992; Tybirk, 1993).
Australia is considered the main centre for speciation and evolution of the
genus on account of the number of species and the polymorphism found
there (Bernhardt et al., 1984). Few studies on the floral biology of neotropical
species have been carried out (Moncur, Moran & Grant, 1991; Peralta,
Rodrı́guez & Arroyo, 1992). These include studies on Acacia caven (Mol.)
Mol., the only native Acacia in Chile which also grows in Argentina.
The purpose of this study was to compare information on the floral biology
of A. caven in Chile with information from Córdoba, Argentina, where this
species is a conspicuous component of the native flora. The dominant type
of pollination as well as the reproductive efficiency of polyads was determined
by checking the ratio between the number of pollen grains and number of
ovules per flower. Flowering and fruiting under natural conditions were also
analysed.
MATERIAL AND METHODS
Cialdella (1984) mentioned four varieties for this species and Aronson
(1992) six. Both authors cited two varieties for the area of this study: Acacia
caven var. caven and A. caven var. dehiscens Cialdella. The characters used to
separate these varieties (leaflet and peduncle length, stipules, inflorescence,
fruit dehiscence) could not be used to identify our population, since contrasting
characters such as dehiscent and indehiscent fruit are very often found on a
single tree. Therefore, we shall refer to it simply as Acacia caven in the widest
sense.
The population of A. caven studied occupies 1500 m2, the site being 8 km
south of the city of Alta Gracia in the province of Córdoba, Argentina.
Elevation is 680 m above sea level. Vegetation in the area is typical of the
Chaco-Espinal transition forest. Of the tree and shrub species, the most
frequent one is A. caven followed by Celtis tala Planchon, A. aroma Hook. et
Arn., Caesalpinia dinia rhombifolia Hook. et Arn., Aloysia gratissima (Gill. et
Hook.) Troncoso and Geoffroea decorticans (Hook. et Arn.) Burk. The herbaceous
layer is dominated by grasses such as Stipa, Cynodon and Paspalum species,
and by Nierembergia hippomanica Miers and Oxalis cordobensis Savign. Olea
europaea L., Eucalyptus sp. and Melia azedrach L. are among the introduced
trees. The plot studied has not been grazed since the beginning of the 1980s.
Thirty specimens of Acacia caven, each over 1 m in height, were selected
in December 1986. To estimate the number of inflorescences in each tree,
the crown was divided into four roughly equal parts and the cephalia
(capitate inflorescence) were counted in one of them. The number of cephalia
in the rest of the crown and in the whole tree was estimated from this
count. This estimate was found to give a count with less than 1% error.
To study the dynamics of flowering and fruiting, the reproductive cycle
was followed from 15 July 1987 to 15 April 1988. The beginning of flowering
in each tree was estimated as a relative percentage of cephalia at full anthesis.
The following parameters were studied in flowers and inflorescences:
REPRODUCTIVE BIOLOGY IN ACACIA CAVEN
67
(1) The overall ratio of perfect flowers is referred to as the probability of
a flower to be perfect (R).
(2) The number of flowers per cephalium, number of stamens per flower
(s), polyads per anther (p), pollen grains per polyad (g), and ovules per
ovary (o) were determined.
(3) Pollen grains were counted microscopically in flowers fixed in FAAand stained with safranin. The ovule number per ovary was counted under
a stereomicroscope.
(4) The probable number of ovules for any flower (counting staminate and
perfect) was calculated as O o×R.
(5) The number of pollen grains per flower was calculated as P s×p×g.
(6) Pollen ovule ratios were calculated for flower, of any type (perfect plus
staminate) (P:O) and for perfect flowers (P:o).
(7) The ratio of pollen grains in a polyad to ovules in an ovary is given
by g:o.
In order to determine the nature of the breeding system, five branches
equally distributed within the crown and bearing flowers prior to anthesis
were covered with acetate bags. These bags, 147×57 mm, contain a copper
wire frame and allow exchange of water vapour and air but exclude
pollinators and eliminate wind (Bernhardt et al., 1984). This technique was
used to analyse spontaneous self-pollination (autogamous and geitonogamous)
among cephalia within the same bag. A single cephalium was also bagged
on each tree to determine the degree of spontaneous self-pollination within
it. In addition, a total of 104 inflorescences on four trees were bagged and
hand pollinated, 88 self- and 16 cross-pollinated within the population. All
possible crosses between the four individuals were undertaken.
The number of fruits produced by each infructescence (fruiting cephalium)
and by the whole plant were counted. The number of seeds per fruit was
analysed in fruits at various stages of development, and the number of fruits
and seeds affected by parasites was also noted.
Cephalia were fixed in FAA and a voucher specimen was deposited in
CORD (Anton & Baranelli 154). Meteorological data were kindly supplied
by the National Meteorological Service of Argentina.
RESULTS
Inflorescence and flowers
Inflorescence
1–7 cephalia were borne on short shoots from the previous season. Each
head was up to 12 mm in diameter with pubescent peduncles up to 15 mm
long; the number of inflorescences on each plant was very variable (2–297
in 30 plants examined); the number of flowers on each inflorescence (Table
1) was 36.5129.47 (n 47).
Flowers
Yellowish, due mainly to the long-exserted stamens; calyx gamosepalous,
5–6 lobed, translucent, 1.9620.13 mm long (n 31); stamens 6.7521.13 mm
J. L. BARANELLI ET AL.
68
TABLE 1. Number of flowers per inflorescence
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
Number of flowers
Frequency of inflorescences
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
20–25
2
26–30
8
31–35
15
36–40
10
41–45
5
46–50
4
51–55
1
56–60
3
–––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
(n 50) long; anthers quadrangular 63.1227.05 per flower (n 50), eglandular,
with four polyads per theca; dehiscence longitudinal.
The eight polyads in each anther are visible as soon as anthesis begins.
The presence of furrows in the polyads makes counting of pollen grains for
each polyad difficult, but approximately 32 pollen grains were assumed to
occur in each, as reported by Caccavari (1976). Pollen grains are pyramidal,
with the apex at the proximal pole and the base at the distal pole.
The ovary is superior, ovoid, almost sessile, uni-carpellary, unilocular, manyovuled and has a slender style. The moist, cup-like stigma is inconspicuous; the
ovary contains an average of 30.2628.44 ovules (n 175) (Fig. 1).
1.4
No. of ovules, seeds, pollen grains
1.2
1.0
0.8
0.6
0.4
0.2
0.0
5
10
15 20 25 30 35
Frequency of each class
40
Figure 1. Frequency histogram showing ovules per ovary (Ž) and seeds per pod (); number
of pollen grains per polyad (r).
REPRODUCTIVE BIOLOGY IN ACACIA CAVEN
69
TABLE 2. Percentage of staminate to perfect flowers in nine plants
(n 823 flowers)
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
Actual number of perfect and staminate
flowers in a pool of 1 to 3 cephalia
Percent staminate
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
107:2
1.8
110:17
13
42:10
17
48:10
18
13:87
34
67:42
38
62:48
44
14:68
83
0:126
100
Total: 463:410
Total: 56
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
Anthesis
In flower buds the styles are folded in a zigzag fashion. They unfold
between 8:00 am and 12:00 am when the flowers are still closed. When the
flowers open the stamens unfold. All flowers in a cephalium open during
the first day. Fully opened flowers last 3 to 4 days; then the stamens wither
while the styles remain straight. After a further 2 to 3 days, the flowers fall
if the fruits are not formed and only the bracts remain until the peduncle
falls off.
Distribution of stamens and carpels within flowers, cephalia and individuals
From 138 inflorescences studied 68.8% had perfect flowers, 18.8% were
mixed (staminate and perfect flowers in various proportions) and 12.3% were
purely staminate.
Staminate flowers did not have vestigial gynoecia. All sampled cephalia
from two individuals had no gynoecia; however, the individuals did possess
small numbers of gynoecia, since one produced 3 fruits, out of 41 cephalia,
and the other 34 fruits out of 192 cephalia.
In trees with mixed cephalia, proportions between staminate and perfect
flowers range from 1.8 to 100% as shown in Table 2.
Total numbers of perfect and staminate flowers in each of four cephalia
from two trees were as follows:
24 |+4 { 28 (14% { fl.)
24 |+6 { 30 (20% { fl.)
45 |+15 { 60 (25% { fl.)
22 |+27 { 49 (55% { fl.)
The proportion of perfect flowers among all flowers was 65% (R 0.65).
Pods are lignified and dark brown when ripe. Their mean dimensions are
7.3921.08 cm×2.0920.41 cm (n 50). Seeds are arranged in two series
and are surrounded by a whitish spongy tissue (endocarp) which may either
disintegrate or, in the case of dehiscent fruit, remain in the fruit. The average
number of seeds per pod was 28.6326.16 (n 40) in 1986:87 and 26.5624.47
(n 108) in 1987:88 (mean: 27.1125.04, n 148, Fig. 2).
J. L. BARANELLI ET AL.
70
60
No. of seeds per pod
50
40
30
20
10
0
5
10
15 20 25 30
Number of fruits
35
40
Figure 2. Seeds per pod in two seasons. 1986–87 (); 1987–88 (Ž).
The number of pods produced per infructescence is shown in Table 3.
Both dehiscent and indehiscent fruit could be found on the same tree.
Considerable variation was found in the number of fruits and cephalia on
each plant (Fig. 3), but the ratio between the parameters (number of
cephalia:number of fruits) was seldom less than 2.52 (Fig. 3). This number
is much lower than the expected reproductive efficiency, given the large
number of flowers and cephalia. In fact, according to the mean number of
perfect flowers for each cephalium ((36.529.47) 0.65 27.7326.16) a plant
with 300 cephalia could potentially produce 711921848 pods (Fig. 3, curve
e). However, the observed efficiency in the most productive tree would
give 120 fruits out of 300 cephalia (Fig. 3, curve ), that is 1.69% of the
potential.
Even if it is assumed that, due to physiological and space constraints,
fruiting limitations are necessary at cephalium level, the efficiency at the
level of the individual and population is still low. For example, if fruiting
TABLE 3. Number of pods per infructescence
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
Number of pods per infructescence
Frequency
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
1
489
2
65
3
5
4
1
5
1
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
—
REPRODUCTIVE BIOLOGY IN ACACIA CAVEN
71
160
y=x
y = 23.73 x
140
No. of fruits per plant
120
y = 0.39 x
100
80
60
40
20
0
50
100
150
200
250
300
350
No. of inflorescences per plant
Figure 3. Fruiting success; dots: plots of actual number of fruits to inflorescences of 30
individual plants; lines represent three levels of fruiting success. Observed success level of
the plant that produced the most inflorescences and fruits (); potential fruiting success if
each inflorescence produces one fruit (r); potential success if each flower produces one fruit
(e).
was limited to one pod per inflorescence (300 cephalia give 300 pods, Fig.
3, curve r) the most successful tree utilized 39.73% of that limited capacity
and on average only 23.55% was utilized.
Isolating cephalia from open pollination completely prevented fruiting.
From 608 bagged heads, c. 142 fruits would be expected after open pollination.
Ratios of polyads and pollen grains to ovules and seeds
According to the expressed formulae:
(1) Average number of ovules (O) in flowers of any type
O 30.26×0.65
19.67
(2) Number of pollen grains (P) for each flower
P (63.1227.05)×8×32
16 158.7221804
(3) Pollen ovule ratios for:
flowers of any type
P
821.49291.71
O
perfect flowers only
P
534.00259.62
o
pollen grains in polyads to ovules in ovary
g
1.0623.79
O
J. L. BARANELLI ET AL.
72
40
35
Percentage of pods
30
25
20
15
10
5
0
0
5
10
15 20
Number of seeds
25
Figure 4. Percentage of seeds per pod destroyed by insect infection.
A strong correlation is evident between the number of pollen grains in
the polyad, the number of ovules in each ovary and the number of seeds
in each fruit (Fig. 1).
The difference between the number of ovules in each carpel and seeds in
each fruit is not significant (t 3.98, d.f. 321, P³0.001). This means that
practically every ovule in an ovary produces seed.
The effects of insect parasites were observed in 92% of the fruits. Most of
the pods had less than five affected seeds but in a few there were as many
as 20 (Fig. 4).
Phenology
Cephalium primordia began to be visible on 29 July in a single tree; other
trees showed bud primordia soon after. Each tree attains the full complement
of heads prior to the flowers opening. In the population bud formation lasts
about 60 days. Flowers started opening on 10 August. The number of plants
and cephalia at anthesis increased until 12 October and decreased abruptly
after peak anthesis (Fig. 5).
The date of first flowering of individual trees varied significantly within
the population. Since flowering ends suddenly in the population at about the
same date, a strong negative correlation is apparent between date of
commencement and duration of flowering in single trees (Fig. 6).
REPRODUCTIVE BIOLOGY IN ACACIA CAVEN
100
73
1000
90
800
70
60
600
50
40
Daily rainfall (mm)
Percentage of inflorescences and plants
80
400
30
20
200
10
0
60
120
180
240
Days from 1 June 1987
300
0
360
Figure 5. Percentage of trees in flower (r) and inflorescences in anthesis () compared with
cumulative precipitation during a one year period (June 1, 1987 to May 31, 1988).
The end of flowering can be related to the incidence of rainfall. There
was a drastic decline in the number of cephalia in anthesis and the proportion
of plants with flowers after the first important rainfall following the winter
drought (Fig. 5). The first rain of more than 6 mm was registered on
September 23. This date coincides with the beginning of the vegetative
period in Acacia caven.
Fruit ripening, which covers the period from when fruit first becomes
visible to when it falls or dehisces, lasts for 60 days. The first fruits were
found in November 1. Dehiscence began on December 30 and lasted for
50 days in the population studied. Although the development of these phases
differed among individuals in the population they were uniform for a given
individual, without apparent differences within the canopy.
J. L. BARANELLI ET AL.
74
100
y = 80.61 –1.01
n = 30
r = 0.94
Days from start of flowering
80
60
40
20
0
20
40
60
80
Duration of flowering
Figure 6. Scatter diagram showing beginning of flowering and its duration in 30 individual
trees.
DISCUSSION
With respect to flower structure, the number of stamens in this population
was about 40% higher than that reported by Peralta et al. (1992) and slightly
higher (10%) than that reported by Aronson (1992). The significance of these
differences could not be tested. The number of flowers in each inflorescence,
stamen length and number of ovules in each ovary coincide with the
observations of those authors.
Fruiting occurred in only a fraction of the flowers and inflorescences,
23.55% of inflorescences and 0.99% of the perfect flowers producing fruit.
These values, however, are significantly higher than those reported by Peralta
et al. (1992) who explained such fruiting limitation as resulting from the low
visiting rate by pollen vectors. Our ongoing observations on visiting rate are
not complete. However, insect numbers are also low and in any case not
sufficiently high to account for a difference of 30 to 40 times. Factors other
than pollinators may be operating such as an internal mechanism which
limits fruiting.
Low pod numbers, and consequently great sacrifice of floral resources,
characterize the reproductive system. Even in cephalia with many perfect
flowers only one or few ovaries produce pods, a situation common in many
mimosoids including Acacia. Since simultaneous pollination of the closely
arranged flowers would be expected, a regulatory mechanism probably
favours the predominance of one or a few flowers over others in the same
inflorescence.
An incompatibility system is suggested in which an incompatible pollination
event leads to flower elimination. Such a strategy also fits observed structural
REPRODUCTIVE BIOLOGY IN ACACIA CAVEN
75
features. There is a remarkable adaptation between polyad and stigma, in
that only one polyad can lodge in each stigma and the ratio between pollen
grains in a polyad and the ovule and seed number in an ovary is close to
one. The gynoecium can be thus understood as a reproduction module
according to which all or none of the ovules develop into seeds.
The concept of the Acacia inflorescence as a pollination unit or pseudanthium
(Arroyo, 1981) also receives physiological support from this interpretation.
The numerous gynoecia would act in cooperation as the pistil of a single
true flower. The ratio between pollen grains in the polyad and the maximum
number of seeds per pod is close to one. The majority of the Acacia species
observed from Africa and Australia have ratios close to one, although the
relation between pollen grains and ovules per ovary may vary between 0.8
and 4 (Kenrick & Knox, 1982).
The ratio between polyads and ovules suggests that this species behaves
as ‘facultative xenogamic’ according to Cruden (1977). However, if the real
number of pollen grains in each ovule is considered, the ratio becomes
much lower; this situation was also contemplated and discussed by Cruden
(1977). The isolation of flowers from pollination agents also supports the idea
of a marked xenogamy. Self incompatibility has been registered for A. caven
(Peralta et al., 1992) and other Acacia species (Bernhardt et al., 1984; Kenrick
& Knox, 1989). A high percentage of seed loss by insect predation has also
been reported for A. aroma Hook. et Arn. (Aizen, 1991).
Apparent protogyny exists, but the emerged stigma may not be receptive,
as in Prosopis (Genise et al., 1990).
The population studied is polygamous (andromonoecy). Peralta et al. (1992)
report, as we also do, the existence of plants in which plants produced some
pods so that dioecy is not complete. It is not known if this is also the case
for the Chilean population.
Phenological observation on most African and Australian Acacia species
show flowering peaks within the rainy season (Sedgley et al., 1992; Tybirk,
1993). A. caven is quite different from these species and from other Acacia
species native in our region in that the flowering peak takes place at the
end of the dry season just before the first important rains. Only one African
species A. albida Del. [Syn. Faidherbia albida (Del.) A. Chev.] shows a similar
phenological pattern (Tybirk, 1993).
As in the Chilean population (Peralta et al., 1992), no floral nectar sources
are evident in the community during flowering of A. caven. Insect activity is
also very scarce during this period. Occasionally, solitary bees and rather
dystrophic coleopterans were observed on the inflorescences.
The question of the lack of nectar in the community could be explained
using the above interpretation if pollination by deceit is also part of the
reproductive strategy. Naive bees could be used as pollen vectors by this
precocious, mass flowering, and nectarless plant.
Inflorescences are born on short shoots of the last year, quite different
from other species (e.g. A. aroma) which bear inflorescences on the leafy
shoots of the same year. Thus, in A. caven flowering can take place prior to
the vegetative period. This morphologically fixed pattern may point to a
generalized reproductive strategy in A. caven throughout its range, viz. mass
flowering at the end of the dry season.
76
J. L. BARANELLI ET AL.
ACKNOWLEDGMENTS
We would like to thank Henry Connor (Christchurch, New Zealand), Peter
Brandhan (Kew) and an anonymous reviewer for comments, criticisms and
discussion of this work and the Consejo de Investigaciones de la Provincia
de Córdoba (CONICOR) for financial support. AMA and AAC are members
of the Carrera del Investigador (CONICET).
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