全 文 ：冠果草的性表达状态及其进化含义
黄双全＊ 宋 旎 汪 泉 唐璐璐 汪小凡
(武汉大学生命科学学院 , 武汉 430072)
摘要: 通常认为雄花两性花同株是植物性表达从两性花进化到雌雄异株的过渡类型之一。慈姑属仅冠果草
(Sagittaria guyanensis H.B.K.subsp.lappula (D.Don)Bojin)的性表达为典型的雄花两性花同株 , 其他种为雌雄同
株。对冠果草野外和实验居群的性配置进行了定量观察。东乡和武夷山居群雄花的比例分别为 2.48%和 0.96%。
实验表明 ,栽培条件影响冠果草的性表达:在营养较好的条件下 ,产生花序大且多 ,雄花的比例显著提高(26.78%);
在营养不利时则相反 ,冠果草个体小 , 产生花序少 ,雄花数量少(4.91%)。雄花的花粉产量是两性花的 4.1 倍。两
者的花粉活力和在柱头上的萌发能力没有明显差别。单花期为 4 ～ 5 h , 同一天的开花时间上 ,雄花比两性花早约
0.5 h。尽管单个个体雄花比两性花少 ,但在居群水平上雄花与两性花的比例呈现大幅度波动。但是雄花并未作为
花粉供体存在 ,野外和实验居群都极少有昆虫访花。大量统计表明 ,不同居群仅 2.1%～ 3.2%的果实发育不全 , 性
比对结实率没有显著影响。1/ 4左右的柱头因未授粉而不发育 , 影响冠果草结实的主要因素是花粉限制而不是资
源限制。两性花套袋的结实率与自然传粉相当。冠果草在目前生境基本是通过两性花进行自花授粉 , 雄花存在的
适应意义是提供了潜在的补充花粉和异花传粉机制。其他雌雄异株的种拥有多样的传粉者可能是该属植物性分
化的重要动力。
关键词: 冠果草;性表达;雄花两性花同株;进化
中图分类号:Q944.43 文献标识码:A 文章编号:0577-7496(2000)11-1108-07
Sex Expression and the Evolutionary Advantages of Male Flowers in an
Andromonoecious Species , Sagittaria guyanensis
subsp.lappula (Alismataceae)
HUANG Shuang_Quan＊ , SONG Ni , WANG Quan , TANG Lu_Lu , WANG Xiao_Fan
(College of Life Sciences , Wuhan University , Wuhan 430072 , China)
Abstract: Sex expression in Sagittaria guyanensis H.B.K.subsp.lappula(D.Don)Bojin is typically an-
dromonoecious while the other species in the same genus are basically monoecious.The evolutionary advantages
of male flowers and hermaphrodite flowers in S .guyanensis subsp.lappula were assessed by measuring sex
allocation and pollen movements in two wild populations of the species.Two cultivated populations served as
controls.The percentage of male flowers was very low in the two wild populations in Dongxiang , Jiangxi
Province and Wuyishan , Fujian Province , viz., 2.48%and 0.96% respectively.In the two cultivated popu-
lations , male flower percentage significantly increased when the soil was of higher nutrient content.This indi-
cates that the allocation to male versus female reproduction might change in response to environmental factors.
Pollen production per male flower was 4.1 times higher than that of a hermaphrodite flower.The floral shape
and size of male and hermaphrodite flowers were similar.No difference was observed between these dimorpho-
logical flowers in pollen germination rate in vitro and in the speed of pollen tube growth in vivo.Anthesis was
only 4-5 h.Male flowers usually opened 0.5 h earlier than hermaphrodite flowers.An unexpected finding
was that no pollen from the male flowers was found on the stigmas of the hermaphrodite flowers , in spite of the
occasional visits by insects to both types of flowers in both wild and control populations.A consistent pattern of
fruit development was found to exist in open pollinated flowers as well as in flowers that had been bagged.The
sex ratios did not have significant influence on fruit set.Approximately 25% of the pistils in a gynoecium
failed to develop into fruits because no pollen was deposited on them , indicating that the fruit set of this
Received:2000_04_03 Accepted:2000_07_10
Foundation items:The National Natural S cience Foundation of China(30070054);The Self_confidence Science and Technology Foundation of Wuhan University
for Young Doctors(207990336).
＊ E_mail address:
植 物 学 报 2000 , 42(11):1108-1114
Acta Botanica Sinica
andromonoecious plant is mainly affected by pollen limitation rather than resource limitation.Reproduction in
S .guyanensis subsp.lappula in the habitats was dependent on self_pollination in hermaphrodite flowers.The
male flowers in this species might be a potential source of additional pollination and may facilitate cross_polli-
nation.The fact that the flowers of monoecious species in Sagittaria pollinated by a wide diversity of insect
visitors may contribute enormously to the diversification of sex expression in this genus.
Key words: Sagittaria guyanensis subsp.lappula;sex expression;andromonoecy;evolution
In contrast to higher animals , higher plants have
much more variations in sex expression.The different
combinations of three sexual conditions of flowers , namely
male flowers , female flowers and hermaphrodite flowers ,
all exist in angiosperms[ 1] .The sex differentiation of
plants attributed to the result of the optimum genetic re-
combination has been recognized over a long time.At the
point of the theory of sex allocation , however , the diverse
sex expressions can be attributed to the different allocation
patterns of resources between the two genders in order to
achieve the optimum reproductive success as female par-
ents or male parents[ 2 , 3] .This hypothesis has continued
in more and more studies.In other words , flowers of any
particular sexual condition in plant individuals can not be
preserved by natural selection , unless they are more effi-
cient in performing some reproductive functions than other
sexual flowers[ 4 , 5] .Hence , different sex expressions dif-
fer in evolutionary advantage
[ 5] .Andromonoecy is a kind
of sex expression in which both hermaphrodite flowers and
male flowers are produced in an individual plant.It exists
in various groups that perform wind_mediated or animal
pollination , which accounts for 1.7%of angiosperms.In
certain environments , the investment of resources of two
sexual parents , male and female , can be regulated during
the three phases of flower development , viz., flower bud
differentiation phase , bloom phase and post_pollination
phase.Since andromonoecy can allocate the resources of
parents during all the three phases , this makes it ideal for
studying sex expression
[ 6 , 7] .Some analysis has already
indicated that andromonoecy is not evolved as an out-
breeding mechanism because hermaphrodite flowers can
perform self_pollination if self_compatible;otherwise , the
explanation that male flowers accelerate outbreeding is not
reasonable[ 5 , 8] .Most investigations ascertain that the ex-
istence of male flowers is an economic way to achieve male
function or to enhance reproductive success of female par-
ents through hermaphrodites.Male flowers play either as
pollen donor
[ 9 , 10]
or increasing pollinator attraction
[ 11] .
This , however , does not explain why both andromonoecy
and monoecy exist in nature , since monoecious plants can
carry out this function as well.
In order to solve this problem , it is necessary to
choose closely related plant groups to investigate the adap-
tive value of the two sex expressions of andromonoecy and
monoecy. In addition , in the evolution from
hermaphroditism to monoecy , andromonoecy is generally
considered to be an intermediate stage.The evolutionary
mechanism involved has not been elucidated.There are
about 30 species in genus Sagittaria of which only S.
guyanensis is typically andromonoecious[ 12] while the sex
expression of the other species is monoecious
[ 13] .The sex
expression of monoecious plants of the genus Sagittaria
has been studied[ 14] .In this study , we quantitatively ob-
served the variation of sex expression in S.guyanensis
under both natural and experimental conditions in order to
probe into the evolutionary significance of the existence of
male flowers.At the same time , we related our findings
to the precious work in the field to infer the possible alter-
native evolutionary pressure in the evolution of the sex ex-
pression of the genus Sagittaria .
1 Materials and Methods
1.1 Materials
Sagittaria guyanensis H.B.K.subsp.lappula (D.
Don)Bojin is a small floating herb of Alismataceae ,
which is mainly distributed in the region south of Yangtze
River in China.Its flowering season is from May to
November.The plant is in the habitats of shallow pools
and ditches in sandy soils and is also a common weed in
the well_fertilized rice fields of less_sandy soils.At pre-
sent , owing to the use of herbicides and other weeding
techniques , Sagittaria guyanensis can hardly be found in
its natural environment.In August 1998 and in the period
from July to August 1999 we could not find the species in
Xinyi and Yangchun County of Guangdong Province and
Nanlin County of Guangxi Zhuang Autonomous Region
where its occurrence had previously been recorded.In
September of 1999 , we made observations on pollination
in the field and measured sex allocation and pollen move-
ment in S .guyanenesis in Wuyishan , Fujian Province
and Dongxiang , Jiangxi Province.The cultivated materi-
als in this study were grown from seeds gathered in
Dongxiang in September 1998.The seeds were planted in
pots in the Garden of Wuhan University.In order to
11 期 黄双全等:冠果草的性表达状态及其进化含义(英) 1109
imitate the natural soil conditions closely , one batch of
seedlings was transplanted to sandy soil and another batch
to loam soil upon achieving a height of one to two cen-
timeters.At the same time fifty seedlings each of S .tri-
folia L.and S.pygmaen Miq.were transplanted to pro-
vide a control during observation of pollinator visits.
1.2 Observation of sex expression
From July 15 to October 30 , on a daily basis we ob-
served and recorded floral display and insect visitors of
202 cultivated plants of S .guyanensis.We recorded the
number of male flowers and hermaphrodite flowers in each
inflorescence of every individual daily.Mature seeds from
each type of inflorescence on different individuals were
collected.To assess seed set , we observed the state of
development of all the seeds in each aggregate fruit and
counted the number of seeds per fruit.We recorded the
number of anthers , pollen production of single flowers and
the sizes of 20 pollen grains from each randomly-selected
flower just as the flowers were starting to bloom.The
methods employed in these observations have been previ-
ously described[ 15] .Fourteen hermaphrodite flowers were
selected at random and then bagged.After about ten
days , we counted the proportion of mature seeds in each
fruit.
1.3 Pollen viability and pollen competition on stig-
mas
To compare the viability of pollen from male flowers
with that from hermaphrodite flowers , we transferred the
early releasing pollen grains to solutions of 0 , 1%, 2%,
5%, 10% sucrose respectively with 0.001%boric acid
added as controls to find the suitable germination condi-
tion.The germination rates of pollen grains from male and
hermaphrodite flowers were calculated as the average of 10
flowers.To assess the pollen competition , pollen grains
from the same flower (male flowers versus hermaphrodite
flowers)were hand_pollinated respectively on three parts
that had been previously marked.Then we collected the
pollinated flowers at 10 min , 20 min and 30 min after
pollination and quickly transferred them into standard
FAA solution.We observed the pollen germination and
recorded the length of the pollen tubes in the pistils of the
hand_pollinated flowers as well as some open pollinated
flowers.
2 Results
2.1 Flowering
Anthesis in S .guyanensis lasts only 4-5 h.The
plants were observed to produce 1-9 racemes each mea-
suring 10 -20 cm in length.There were 1 - 5
hermaphrodite flowers at the base of an inflorescence and
0-3 male flowers at the top.Flowers open in a sequence
from the bottom of the inflorescence to the top.The
hermaphrodite flowers always open first.Among the 483
individuals observed indoors and outdoors , only 8 individ-
uals had a hermaphrodite flower and a male flower opened
on the same day.After the hermaphrodite flowers had
closed , 98.3% of the individuals had their male flowers
opening.In a day , male flowers usually opened at 10:30
and closed at 14:30-15:00 , while hermaphrodite flow-
ers unfolded from 11:00 to 14:30-15:00.Based on da-
ta from the observation of 60 male flowers as well as
hermaphrodite flowers , male flowers usually opened 0.5 h
(0.3-1.2 h)earlier than the hermaphrodite flowers.
The number of two types of flowers in two wild populations
and two cultivated controls are shown in Table 1.
The number of male flowers is distinctly less than
that of hermaphrodite flowers in 4 populations(Table 1).
The sex ratio of the population in loam soil with sand is
closer to that of the wild population.There was a prepon-
derance of male flowers in the plants cultivated in the less
sandy loam soil than in those cultivated in sandy soil.
This was due to the fact that the two groups of plants had
been grown from seeds collected from the same locality
(Dongxiang).This indicates that the nutritional status
can influence the sex expression of S.guyanensis when
other conditions are the same.
Table 1 Distribution of male and hermaphrodite flowers in the inflorescences and individuals of Sagittaria guyanensis subsp.lappula in two
wild populations and two cultivated populations
Population1) Individuals Individuals
with male
flowers
Number of
inflorescences
Inflorescences
with male
flowers
No.of male
flowers
No.of
hermaphrodite
f lowers
Percentage
of male
flowers(%)
Dongxiang 134 3 172 3 3 309 0.96
Wuyishan 147 8 158 8 8 314 2.48
Control 1 123 13 335 22 30 581 4.91
Control 2 79 72 319 168 293 801 26.78
1)plants in control 1 were cultivated in loam soil with 50% sand;plants in cont rol 2 were cultivated in loam soil without sand.
1110 植 物 学 报 Acta Botanica Sinica 42 卷
Fig.1. Daily number of male and hermaphrodite flowers and the percentage of hermaphrodite flowers in two cultivated populations of Sagit-
taria guyanensis subsp.lappula.
, hermaphrodite flowers;□, male flowers;— , the percentage of hermaphrodite flowers.
Fig.2. The percentage of hermaphrodite flowers in relation to se-
quence of flower blooming in two cultivated populations of Sagittaria
guyanensis subsp.lappula.
2.2 Floral display
Male flowers opened after hermaphrodite flowers on a
single inflorescence of Sagittaria guyanensis.However
within a population , the proportion of male and
hermaphrodite f lowers that opened simultaneously showed
great variation (Fig.1).
Observations of 551 flowers cultivated in loam soil
with sand and 1 052 flowers in loam soil indicated that the
proportion of the male flowers increased with the sequence
of flowers blossoming on the inflorescence.The first flow-
er was hermaphroditic , so was almost the second flower on
the inflorescence.The proportion of hermaphrodites falled
only of the bigger inflorescences(Fig.2).
2.3 Comparison of pollen function between the male
flower and the hermaphrodite flowers
Both male flowers and hermaphrodite flowers are
bowl_shaped with 3 white membranous petals which some-
times have fuchsia spots on their bases.The average di-
ameter of the corolla in bothmale and hermaphrodite flow-
ers is about 13 mm (Table 2).The pedicel of the male
flower is thinner than that of the hermaphrodite flower.
Each pistil has a single ovule and a pear_shaped stigma.
The anther of a male flower is distinctively larger in size
than that of a hermaphrodite flower.Pollen production per
male flower is 4.1 times higher than that of a
hermaphrodite flower and the average pollen size of male
flowers is bigger (Table 2).Pollen germination is opti-
mum in the solution of 2% sucrose+0.01%boric acid.
11 期 黄双全等:冠果草的性表达状态及其进化含义(英) 1111
Table 2 Comparison of pollen production , pollen size , floral size and pollen germination between male and hermaphrodite flowers of Sagit-
taria guyanensis subsp.lappula
Sample of
f lowers
No.of
flowers
No.of
pollen grains per
flower(±SE)
Pollen diameter
(±SE)
(μm)
Mean pollen
germination
rate(%)
No.of
stamens
Width of
f lower
(mm)
Male 10 20 100±6 200 32.62±1.56 81.6±4.5 8.0±1.6 13.1±0.24
Hermaphrodite 26 4 900±1 800 30.93±9.08 80.2±6.2 6.8±0.9 13.2±0.28
Fig.3. Comparison of pollen tube growth in Sagittaria guyanensis
subsp.lappula after hand pollination.◆, pollen from the same
flower;∷◆, pollen from a male flower;□, pollen from another
hermaphrodite flower.
In this solution , the average germination rate of pollen
grains from male flowers was 81.6% and that of
hermaphrodite flowers was 80.2%.There was no signifi-
cant difference in the germination rate of pollen from the
two kinds of flowers.Pollen grains from a hermaphrodite
flower could germinate on the stigmas of the same flower ,
in addition , pollen from another hermaphrodite flower or
from a male flower could also germinate on the stigmas.
Fig.3 shows the growth of pollen tubes in the pistils after
hand pollination.ANOVA analysis indicated no signifi-
cant difference in the speed of pollen tube growth between
pollen from different sources.
2.4 Pollination mechanism and seed rate
The stamens of a hermaphrodite flower split rapidly
30 min before or 0-10 min after the petals opened.As
there are six anthers in the androecium , usually two an-
thers grow together adnate to the base of the petals around
the gynoecium;as the stamen has 7 , 8 , or 9 anthers , no
definite arrangement was observed.The height of the sta-
mens commonly approached the height of the carpels when
they were splitting and some stamens extended beyond the
height of the carpels.Pollen grains were able to fall di-
rectly onto the adjacent stigmas when the anther dehisced.
Hermaphrodite flowers generally opened at 11:00 am ,
and then their folded membranous petals began to spread
out reaching their full size at about 12:30 pm.Thereafter
the petals shrank and the flower began to close until about
15:00 pm.No pollinators were observed to visit the flow-
ers of S .guyanensis in the field or in cultivated popula-
tions.However , many insects , including bees , flies and
butterflies were observed visiting the flowers of S.trifolia
and S.pygmaea that grew at the same sites.Syrphid
flies would occasionally visit the flowers of S .guyanensis
but due to their failure to make repeat visits , they could
not be considered as pollinators.Most stigmas in both
blossoming flowers and closed flowers received pollen
grains.Stigmas of the top of gynoecium and those not lo-
cated close to the anthers received fewer pollen grains.
The fruit of S.guyanensis is an aggregate fruit.We
studied 305 fruits carefully and counted 44 485 achenes.
We recognized three types of achenes according to size
and stage of development.Well_developed mature achenes
were bigger in size than the abnormally developed achenes
that had a slightly enlarged ovary.The size of the unde-
veloped achenes was only as big as its initial carpel with-
out an enlarged ovary.Fruit set in this species is equiva-
lent to the seed set because each pistil bears a single
ovule.A specific arrangement of the undeveloped achenes
on the thalami was observed.The location of the undevel-
oped pistils was common on the top of gynoecium.
What s more , the undeveloped achenes grew in rows from
the top to the bottom as the aggregate fruit was viewed
from the side.The seed set in the natural populations in
Dongxiang and Wuyishan was relatively lower than that in
the cultivated populations.The seed set in the bagged
flowers is also lower than that of open pollinated flowers.
The value of the difference in both cases was , however ,
not statistically significant(P>0.05).The proportion of
the abnormally developed fruits under different conditions
slightly varied from 2.1% to 3.2%as shown in Table 3.
There were 19.7%-30.5% of the pistils remained un-
developed because they were not pollinated.
3 Discussion
3.1 Environment and sex expression
In the whole growth season , the sex ratio of plants
may change with the size of individuals , temperature ,
moisture , nutrition , light cycle , competition and other
factors[ 7] .In an andromonoecious species , Cneorum tric-
occon , the flowers of the same population have quite dif-
ferent sex ratio during different seasons and years.This
indicates that the environmental resources rather than ge-
netic factors probably determine sex expression[ 16] .The
theory of evolution postulates that the sex expression of
1112 植 物 学 报 Acta Botanica Sinica 42 卷
Table 3 Fruit and seed production in three open_pollinated populations and bagged hermaphrodite flowers in Sagittaria guyanensis subsp.
lappula
Sample of
individuals
Total
number of
achenes
No.of
matured
achenes
with seeds
Matured/
total
seeds
(%)
Incompletely
developed/
total seeds
(%)
Undeveloped/
total seeds
(%)
Dongxiang 93 10 902 7 228 66.3 3.2 30.5
Wuyishan 66 5 716 3 983 69.7 2.1 28.2
Control 2 132 24 005 18 620 77.6 2.8 19.7
Bagged 14 3 862 2 830 73.3 2.4 24.3
plants tends to enhance the differentiation of male in the
unfavourable environmental conditions , and that it is more
advantageous to produce more females in the favourable
environment[ 17] .In the environment of increased nutrient
content , the proportion of hermaphrodites of Leptosper-
mum scoparium , an andromonoecious shrub occurring in
New Zealand and pollinated by insects , increased rapid-
ly[ 8] .But there are also andromonoecious plants that go
against the theoretical prediction.For instance , the an-
dromonoecious individuals of Isomeris arborea produce
more hermaphrodites when they are severely damaged by
beetles;and when they are less damaged , the plant indi-
viduals turn to producing male flowers to increase the fruit
output
[ 18] .The sex proportion of Gagea chlorantha varies
greatly with the gradient of precipitation.The populations
in desert areas seldom produce male flowers , and the pro-
portion of hermaphrodites is 94 percent[ 19] .The sex ex-
pression of S.guyanensis was also influenced by the en-
vironmental conditions.In the conditions found in paddy
fields where the growth was restrained as well as in nutri-
tionally deficient sandy soil , the plant individuals were
small in size and only hermaphrodite flowers were pro-
duced.In the blossom sequence , the first two flowers
were almost hermaphrodites;flowers commonly presented
themselves in the inflorescence containing over two flow-
ers.From Fig.2 , we can refer that the bigger inflores-
cence , the greater possibility of opening male flowers.In
1984 Chen collected some hermaphrodite individuals of
S .guyanensis , in Nanning of Guangxi Zhuang Au-
tonomous Region , Xinyi and Yangchun of Guangdong
Province , a part of which belongs to this type of
hermaphrodite flowers. He stated that , “ the
hermaphrodite individuals of S .guyanensis are mostly
dwarf and only have one or two annuluses of flowers all of
which are hermaphrodite and can bear fruit”[ 12] .
3.2 Sex expression and pollination dynamics
The reason why the sex expression of andromonoecy
achieves reproductive success may be that male flowers
use less resource investment than female flowers(the male
flowers lack the resource allocation of female part)while
increasing the variety of pollen donors.This increases the
pollen deposition and permits a wider choice of which
fruits to abort.It also enhances the possibility of transfer-
ring pollen to other individuals[ 5] .The pollen of male
flowers of Cneorum tricoccon , an andromonoecious
species , does better in bearing fruit than that of female
ones[ 16] .In the hummingbird_pollinated shrub Besleria
triflora , male flowers disperse substantially less pollen
than hermaphrodite flowers do.However , the measure-
ment of pollen dispersal by labeling with fluorescent dye
indicates pollen receipt increased only when male flowers
were used to enlarge floral display
[ 11] .Anthesis of S.
guyanensis was only about 5 h and an individual plant on-
ly opened one or two flowers at a time.Most male flowers
did not open until hermaphrodite flowers closed in one
plant.The appearance of male flowers in S .guyanensis
cannot be regarded as a mechanism for increasing pollina-
tor visitation at large floral display.Pollen grains of male
flowers were not found on the stigmas when we observed
pollen transfer by labeling with safranine dye.Male flow-
ers seemed impossible to provide pollen for hermaphrodite
flowers because of the lack of pollinator visitation.The
variation of seed set between bagged and open_pollinated
flowers was not significant , which also indicated that the
existence of male flowers did not increase seed set in S.
guyanensis.Our present observation showed that sex ex-
pression of S.guyanensis was different from which has
been reported in other andromonoecious plants.The male
flowers of this species did not display male reproductive
function , neither serving as pollen donors nor increasing
pollinator attraction.It is not certain how natural selection
maintains the sex expression of andromonoecy in S.
guyanensis.
3.3 Evolutionary advantages of male flowers in S.
guyanensis
Since the growth rate of pollen tube in the carpellary
tissue is the same for pollen grains both from male and
from hermaphrodite flowers , fertilization of the ovule is
determined by the time of pollen deposition on stigma.
Pollen production per male flowerwas 4.1 times of that of
11 期 黄双全等:冠果草的性表达状态及其进化含义(英) 1113
hermaphrodite flower.Furthermore , male flowers often
opened 0.5 h earlier than the hermaphrodite flowers in the
blossoming time.Although male flowers were far fewer
than hermaphrodite flowers in an individual , the daily sex
ratios varied largely in the populations of S .guyanensis.
The data presented in Fig.1 show that on some certain
days more male flowers opened than the hermaphrodite
flowers.In S.trifolia , the number of opening male and
female flowers affected the movement of pollen grains in
the population[ 14] .The male flowers of S.guyanensis
had the potential mechanism to deliver pollen grains to
hermaphrodite flowers.In spite of some anthers of
hermaphrodite flowers releasing pollen grains before flower
opening , there were still some stigmas that were not polli-
nated.These were the stigmas located at the top of the
gynoecium farther away from the anthers.The percentage
of abnormally developed fruits whose stigmas were polli-
nated ranged from 2.1% to 3.2% in different popula-
tions.This indicates that seed set in S .guyanensis is
weakly affected by resource limitation.A high proportion
of stigmas(19.7%-30.5%)did not develop because
they had not received pollen grains.The seed set in S .
guyanensis may be mainly affected by pollen limitation.
The existence of male flowers may be a potential mecha-
nism for supplemental pollination and cross_pollination.
Under the present status in their habitats , the appearance
of male flowers in S.guyanensis would seem to be
wastage of resources.However , the maintenance of male
flowers is an investment for the future as a safeguard in
the event of unpredictable changes in the environment.
Acknowledgements: We would like to thank WANG
Hai_Yang , WANG Yu_Guo and SUN Shi_Guo for their as-
sistance in the field work.We are indebted to LI Da_
Peng , CHEN Dan and XIA Jing for helping us in the lab-
oratory.The manuscript was improved by critical com-
ments provided by GUO You_Hao.
References:
[ 1 ] Huang S_Q(黄双全), Guo Y_H(郭友好).New advances
in pollination biology and studies in China.Chin Sci Bull
(科学通报), 2000 , 45:223-237.(in Chinese)
[ 2 ] Bawa K S , Beach J H.Evolution of sexual systems in flow-
ering plants.Ann MO Bot Gard , 1981 , 68:254-274.
[ 3] Wyatt R.Pollinator_plant interactions and the evolution of
breeding system.Real L.Pollination Biology.London:Aca-
demic Press , 1983.51-95.
[ 4] Podolsky R D.Evolution of a flower dimorphism:how effec-
tive is pollen dispersal by male flowers.Ecology , 1993 ,
74:2255-2260.
[ 5] Willson M F.Plant Reproductive Ecology.New York:John
Wiley and Sons , 1983.
[ 6] Lloyd D G.Sexual strategies in plants:Ⅰ .A hypothesis of
serial adjustment of maternal investment during one repro-
ductive season.New Phytol , 1980 , 86:69-79.
[ 7] Solomon B P.Environmentally influenced changes in sex
expression in an andromonoecious plant.Ecology , 1985 ,
66:1321-1332.
[ 8] Primack R B , Lloyd D G.Andromonoecy in the New
Zealand montane shrub manuka , Leptospermum scoparium
(Myrtaceae).Amer J Bot , 1980 , 67:361-368.
[ 9] Medan D , DAmbrogio A C.Reproductive biology of the
andromonoecious shrub Trevoa quinquenervia (Rham-
naceae).Bot J Linn Soc , 1998 , 126:191-206.
[ 10] May P G , Spears Jr E E.Andromonoecy and variation in
phenotypic gender of Passiflora incarnata (Passifloraceae).
Amer J Bot , 1988 , 75:1830-1841.
[ 11] Podolsky R D.Strange floral attractors:pollinator attraction
and the evolution of plant sexual systems.Science , 1992 ,
258:791-793.
[ 12] Chen J_K(陈家宽).Systematic and Evolutionary Botanical
Studies on Chinese Sagittaria.Wuhan:Wuhan University
Press , 1989.(in Chinese)
[ 13] Bojin C.Revision of the genus Sagittaria (Alismataceae).
Memoirs NY Bot Gard , 1955 , 9:179-233.
[ 14] Huang S_Q(黄双全), Jin B_F(靳宝锋), Wang Q_F(王
青锋), Guo Y_H (郭友好).Floral display and pollen
flow in a natural population of Sagittaria trifolia.Acta Bot
Sin(植物学报), 1999 , 41:788-790.(in Chinese)
[ 15] Huang S_Q(黄双全), Guo Y_H (郭友好), Wu Y(吴
艳), Liu Q(刘琴), Zhang F(张帆), Chen J_K(陈家
宽).Floral numerical variation and seed set in natural pop-
ulations of Liriodendron chinense.Acta Bot Sin(植物学
报), 1998 , 40:22-27.(in Chinese)
[ 16] Traveset A.Reproductive ecology of Cneorum tricoccon L
(Cneoraceae) in the Balearic Islands.Bot J Linn Soc ,
1995 , 117:221-232.
[ 17] Charnov E L.The Theory of Sex Allocation.Princeton:
Princeton University Press , 1982.
[ 18] Krupnick G A , Weis A E.Floral herbivore effect on the sex
expression of an andromonoecious plant , Isomeris arborea
(Capparaceae).Plant Ecol , 1998 ,134:151-162.
[ 19] Wolfe L M.Regulation of sex expression in desert and
Mediterranean populations of an andromonoecious plant
(Gagea chlorantha , Liliaceae).Israel J Plant Sci , 1998 ,
46:17-25.
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