Botanical Studies (2008) 49: 253-260.

*

Corresponding author: E-mail: why@scbg.ac.cn; Tel: +86-

20-37252981; Fax: +86-20-37252981.

INTRODUCTION

Biological invasion is a pervasive and costly environ-

mental problem (Vitousek et al., 1996; Kennedy et al.,

2002; Perrings et al., 2005; Pimentel et al., 2005). Invasion

is defined as the establishment of a species after human-

mediated movement beyond its natural range or natural

zone of potential dispersal and it is distinct from coloniza-

tion that is often viewed as natural range expansion (Lee,

2002). Biological invasions are like natural experiments,

but their processes are far more rapid than those in coloni -

zation (Sakai et al., 2001). For an introduced plant species

to become invasive, it must be able to reproduce, even

in initially small populations (van Kleunen and Johnson,

2005), and its rate of spread is influenced by the mode of

reproduction, the reproductive system and potential for

recombination, particularly if continuous adaptation is a

prerequisite for the invasion process (Sakai et al., 2001).

Mikania micrantha H.B.K. (Asteraceae) is a fast-grow -

ing perennial creeping vine native to Central and South

America (Wirjahar, 1976; Holm et al., 1977). It has many-

branched stems and reproduces easily through both sexual

and vegetative reproduction (Swarmy and Ramakrishnan,

1987; Zhang et al., 2004). It entered South China after

1910, and since the 1980s it has spread and invaded widely

(Zhang et al., 2004). In recent years, this notorious weed

has caused severe damage to many ecosystems and local

economies in Guangdong Province, China and elsewhere

in the world (Deng et al., 2004; Yang et al, 2005; Lian et

al., 2006; Song et al., 2007). Therefore, it has been listed

as one of the 100 worst invasive alien species on earth

(Lowe et al., 2001) and as one of the top 10 worst weeds

in the world (Holm et al., 1977).

Secondary pollen presentation is the developmental

relocation of pollen from the anthers onto another floral

organ which then functions as the pollen presenting organ

for pollination (Howell et al., 1993). It is a reproductive

strategy promoting outbreeding and it occurs in five mono -

cotyledon and 20 dicotyledon families and many publica-

tions discuss it in the angiosperms in general (Howell et

al., 1993; Ladd and Donaldson, 1993; Yeo, 1993; Ladd,

1994). Secondary pollen presentation is a widespread phe-

nomenon that characterizes the family Asteraceae nearly

Secondary pollen presentation and style morphology in

the invasive weed Mikania micrantha in South China

Lan HONG

1, 2

, Hao SHEN

1

, Wanhui YE

1,

*, Honglin CAO

1

, and Zhangming WANG

1

1

South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, P.R. China

2

Graduate School of the Chinese Academy of Sciences, Beijing 100049, P.R. China

(Received March 8, 2007; Accepted April 3, 2008)

ABSTRACT.

Mikania micrantha H.B.K. is a successful invasive weed in many parts of the world. Its

reproductive biology, specifically, floral functional morphology, growth and behavior of the style during

anthesis, and style morphology, was studied in an open M. micrantha population in South China during the

flowering seasons of 2004 and 2005. Floral biology was studied in detail by examining florets at different

developmental stages under a dissecting microscope and a scanning electron microscope. Stigma receptivity

and pollen viability was determined by MTT [3-(4,5-dimethylthiazol-yl)-2,5-diphenyl-2H-tetrazolium bromide]

staining technique. The results show that M. micrantha is protandrous and has a secondary pollen presentation

system which characterizes the family Asteraceae. Typically, the flowering period is 6 days and can be divided

into six floral stages (A, B, C, D, E and F) based on style morphology and behaviour. At the beginning of

anthesis, the style bends to break and enter the tube formed by five fused anthers. Later, the style protrudes

the anther tube and moves the pollen out from the anther. During anthesis, two partially overlapping phases,

functionally male phase and functionally female phase, can be distinguished by MTT tests: the former is from

stage B to D, and the latter is stage E. The style has two style branches in its terminal part. The sweeping

hairs, which act as pollen presenter, are located on the tips and on the upper parts of the style branches form-

ing developed stylar appendages. The stigmatic papillae are separated into two ventro-marginal bands along

each style branch in symmetric arrangement. The bending behavior of the style and the sweeping hairs play an

important role in presenting pollen to pollinator.

Keywords: Invasive plant; Mikania micrantha; Secondary pollen presentation; Sweeping hairs.

REPRODUCTIVE BIOLOGY

254

Botanical Studies, Vol. 49, 2008

throughout, and it is usually the terminal style that acts as

the pollen presenter with active pollen placement (Howell

et al., 1993; Ladd, 1994). In Asteraceae, the pollen pre-

senter is sweeping hairs that cover the stigmas, and it can

just be the style branches for the species that lack sweep-

ing hairs (Ladd, 1994). Pollen is presented on the terminal

section of the modified style where it is actively loaded

onto the distal portion of the style as it elongates through

a connate ring of anthers (Howell et al., 1993). Within the

family Asteraceae, based on the arrangement of the sweep-

ing hairs, different types of secondary pollen presentation

have been found: pump mechanism, brushing mechanism,

transitions between both that were found in the Asteroi-

deae to which subfamily the Eupartorieae belong, and a

special one in the basal subfamily Barnadesioideae (Leins

and Erbar, 1990; Yeo, 1993; Erbar and Leins, 1995, 2000;

Leins and Erbar, 2006).

Mikania micrantha has very small, compact florets in

its inflorescences, and its flower morphology and pollen

presentation are nearly unknown. To provide information

on the basic reproductive biology of M. micrantha, we

initiated this study using laboratory and field approaches

in an open population at the field station of South China

Botanical Garden during the flowering seasons of 2004

and 2005 to investigate: (1) floral functional morphology;

(2) growth and behaviour of style during anthesis; and (3)

style morphology.

MATERIALS AND METHODS

Study site

The study was conducted at the field station of South

China Botanical Garden during the flowering seasons

of 2004 and 2005. It is in the suburb of Dongguan City,

Guangdong Province, China. Dongguan (22¢X39¡¦-23¢X

09¡¦ N, 113¢X31¡¦-114¢X15¡¦E) is in a subtropical area and is

located south of the Tropic of Cancer. It has a lower south

subtropical marine monsoon climate. Mikania micrantha

mostly occurs in open habitats in South China, so at the

station we selected a typical population for our studies in

an open field where an artificial Magnolia denudata forest

was cut 10 years ago, and there were almost no other

plants except M. micrantha.

Flowering phenology

Flowering phenology was observed for 12 M. micrantha

individuals per season from September to February in

2004 and 2005 flowering seasons. On each plant, 10

capitula buds of similar size were marked and observed at

0800, 1200, 1500 and 1800 h every day until the flowers

senesced to collect data on the timing and duration of

flowering and fructifying.

Floral biology

We collected at least 80 samples from flower buds

to flowers at anthesis from different capitula in 10 M.

micrantha plants in each season and observed them under

a dissecting microscope (SE-CTV, Olympus, Japan). The

flowers of M. micrantha are co-sexual and we examined

the spatial and temporal arrangement of male and female

sexual parts within the flowers. We used the information

obtained to define six floral morphological stages (A, B,

C, D, E and F) based on style morphology and behaviour

(Figure 1A-F) and use these stages to classify the flowers

we observed during the flowering seasons.

Pollen viability and stigma receptivity

.

In each

flowering season, at least 50 fresh flowers with dehiscing

anthers in different floral stages (Figure 1A-F) were ran-

domly collected from 10 individual M. micrantha plants.

The pollen samples were removed from the anthers and

then immersed in a drop of 20 g L

-1

3-(4,5-dimethylthia-

zol-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT, Sig-

ma M-2128) solution and examined under a microscope

(Rodriguez-Riano and Dafni, 2000). Dark blue staining in-

dicates the pollen is viable. Pollen viability was expressed

as mean percentage of stained pollen grains of more than

six samples. In each sample, we calculated the percentage

number of dark pollens to total number of pollen grains

in each of seven randomly selected visual fields under the

microscope. The number of dark pollens in each sample

was recorded as the average number in the seven visual

fields. Due to the secondary pollen presentation in M.

micrantha (Figure 1A-F), only pollens in anthers were

examined to avoid including exotic pollens. The same

test was used to determine stigma receptivity (Rodriguez-

Riano and Dafni, 2000).

Fine-scale style and pollen morphology

. A scan-

ning electron microscopy (SEM) was used to examine the

style morphology. For each floral stage, 20 pistils of fresh

flowers were dissected and fixed in 4% glutaraldehyde in

0.1 mol L

-1

phosphate buffer at pH 7.4. The fixed samples

were rinsed three times in the same buffer and fixed again

in it with 1% osmium tetroxide added. They were then

dehydrated in a graded alcohol series (30% - 50% - 70%

- 80% - 90% - 100%) and immediately freeze-dried using

a freeze-drying device JFD-310 apparatus (JEOL Ltd.,

Tokyo, Japan). Then, they were mounted directly on metal

stubs using double-sided adhesive tape and sputter-coated

with platinum using JFC-1600 auto fine coater (JEOL Ltd.,

Tokyo, Japan). Morphology of the stigmas was observed

with a JSM-6360LV SEM (JEOL Ltd., Tokyo, Japan) at

15 KV. For each floral stage, pollen grains were also ran-

domly selected and treated as above, and then 20 were

observed under a SEM to study the pollen morphology

(shape, size, and exine).

RESULTS

Flowering phenology

Flowering and fructifying of M. micrantha occurred

from September to February in both study seasons. The

peak bloom occurred between mid November and mid

December. Fruiting started as early as the end of October,

and almost all fruits had dropped in early February.

HONG et al. ¡X Secondary pollen presentation and style morphology in

Mikania micrantha

255

Floral biology

Floral morphology

.

Mikania micrantha is hermaphro-

ditic and its corymbose inflorescence consists of a 4-flow -

ered tight capitula. Its individual florets are 5.87¡Ó0.46 mm

in length and are actinomorphic. The capitulum is 6.6¡Ó0.50

mm in length and 1.65¡Ó0.63 mm in diameter. Five anthers

attach each other and form a tube, and the stigma and the

style grow through the centre of the tube. The flowers pro -

duce nectar and pollen. There is a green and ringed ovary-

roof nectary which encircles the base of the style (Figure

1A

2

).

Floral stages

. The flowers open at any time of day,

but the majority do so in the morning. Typically, the flow -

ering period is 6 days and can be divided into six floral

stages (A, B, C, D, E and F) based on style morphology

and behaviour (Figure 1). In general, within each capitu-

lum, the central florets flower before the surrounding ones.

During the 24 h before anthesis (stage A), the growing

filament raises the anther tube up to the same level as the

style (Figure 1A). During this time of growth, the anthers

dehisce inwardly and discharge pollen grains inside the

anther tube onto the sweeping hairs of the style. The style

branches are aligned and the stigma is not yet receptive.

During anthesis, two partially overlapping phases can

be distinguished: (1) Male phase: Between about 6.00-8.00

h of day 1, the flower opens and the tip part (see Figure

1C) of the style breaks through the anther tube formed

by five fused anthers, and its lower part (see Figure 1C)

bends inside the corolla tube (stage B, Figure 1B). The

growing style then unthreads through the tube and loosens

the pollen grains which then adhere to the sweeping hairs

of the style branches. The green nectary begins to secrete

and accumulate nectar within corolla tube at stage B and

continues. About 1.5 h later, the lower part of the style

straightens, the style begins to protrude out of the anther

tube (Figure 1C) and the pollen grains are removed from

the anthers by the sweeping hairs (stage C). At this stage,

insect visitors were observed. In the afternoon of day 1,

Figure 1. A-F, Floral stages of Mikania micrantha showing secondary pollen presentation. A

1

, Early floral stage showing flower is

beginning to open; A

2

, Dissected flower in A

1

showing nectary (nc), anther (an) and style (st); B

1

, Flower just opened (the arrow indi-

cates the anther tube); B

2

, Dissected flower in B

1

showing the bending part of the style (arrow); B

3

, Dissected anthers in B

1

showing

the bending part of the style is pushing through the anther tube (arrow); C

1

, The style is out of the anther tube; C

2

, Dissected flower

in C

1

(the arrow shows the empty anthers with few pollen grains); D, Style branches (sb) separate, curved and slightly inflexed, while

the anthers (an) are nearly empty; E

1

, The style branches (sb) expose the stigmatic surfaces (ss) and the anthers (an) are dry, brown

and withering; E

2

, Test result of stigma receptivity showing the stigmatic surface appears dark blue when immersed in a drop of MTT

solution under a dissecting microscope; (F) Senescence stage showing the two style branches bending towards the centre of the floret.

Functionally male phase is from stage B to D, and functionally female phase is stage E. All bars=0.5 mm except E

2

=200 £gm.

256

Botanical Studies, Vol. 49, 2008

the style grows longer until the tip has completely emerged

from the anther tube and presented more pollen attached

on the sweeping hairs of the style. At the end of stage C,

the florets are completely open, and the anthers have re-

leased almost all the pollen. The style and style branches

present the yellow, clumped pollen grains to the inside of

the flower while the style branches are still joined. Insect

visitors appear more frequently than before and remove

pollen. By day 2, the style branches separate with a slight

inward inflexation (Figure 1D) but their lower parts with

stigmatic area located on the inner surface and covered

with receptive papilla cells are still joined (stage D). At the

late male phase, early on the third day after flower opened,

tests of stigma receptivity gave negative results. The flo-

ral scent is present during daylight hours, and it becomes

stronger from stage B to D.

(2) Female phase: From day 3 to 5, the style grows

to its full length, its two branches completely open, two

layers of receptive papilla cells are exposed, and anthers

become dry and brown and begin to wither (stage E,

Figure 1E). Nectar secretion and scent emission continue

and frequent insect visits occur. By day 6, flowers enter the

senescence stage (stage F, Figure 1F) when the two style

branches bend towards the center of the floret. From day 7

on, fertilized flowers continue to grow and the green ovary

develops into a plump and black seed, while unfertilized

ovules shrivel and turn yellow.

Pollen viability

. Pollen viability decreases with the

progress of floral stages (Figure 2). At stage A, pollen

has the highest viability at 95.0%, which then decreases

to 86.3% at stage B. At stage C, pollen viability further

decreases to about 78.3%. At the late male phase (stage D),

early on the third day after flower opened, pollen viability

is about 53.8%, while at the early female phase (stage E),

late on the third day, viability of the few remaining pollen

in the anthers is about 34.4%. In the senescence stage

(stage F) the pollen viability decreases to only 9.5%.

Pollen characteristics

.

The pollen grains appeared

globular or sub-globular under SEM (Figure 3A) and they

are 3-colporate. The size of pollen grain is 19.95¡Ó1.96

£gm in the polar axis and 14.67¡Ó0.86 £gm in the longest

equatorial axis. The ornamentation of exine is echinate,

and the length is 1.85¡Ó0.33 £gm.

Style morphology

The style has two style branches in its terminal part

(Figure 3B), and the morphology of one style branch is

shown in Figure 3C. Sweeping hairs were concentrated on

the tips (Figure 3D) and on the upper parts (Figure 3E) of

the style branches forming developed stylar appendages

that are sterile structures extended above stigmatic areas.

Generally, the sweeping hairs gradually increase in length

basipetally, and they act as pollen presenter which is

responsible for removing pollen out of the anther tube

while the terminal section of the style branches grows

through and presenting them. The stigmatic surface is

covered with columnar papilla cells that are very close

to each other. The stigmatic papillae are separated into

two ventro-marginal bands along each style branch in

symmetric arrangement (Figure 3F). MTT test showed that

the stigmatic area was receptive (Figure 1E

2

).

DISCUSSION

Secondary pollen presentation system is widespread

in angiosperms (Howell et al., 1993). It characterizes the

family Asteraceae (Howell et al., 1993), and we found

that M. micrantha is no exception. Our observations show

that M. micrantha is protandrous, and pollen is shed from

the anthers onto the sweeping hairs of the style in the bud

stage (Figure 1A) when the style branches are joined and

the stigmatic surfaces are not receptive despite that the

pollen has the highest viability. This is consistent with

the observations on some other species of Asteraceae

(Porras and Alvarez, 1999; Roitman, 1999; Cerana, 2004;

Grombone-Guaratini et al., 2004). Our results show that

the sweeping hairs are both located around the tip of the

style branch and reaching below the branching of the

style. Thus, M. micrantha presumably has attained a com-

bination of a pump pollen presentation mechanism and

a brushing mechanism, based on the arrangement of the

sweeping hairs for different types of secondary pollen

presentation (Yeo, 1993; Erbar and Leins, 1995; Leins and

Erbar, 1990; 2006).

We observed during early stages (stages B-C) of

anthesis of M. micrantha that part of the style is bended.

This bending makes it possible for the style tip to break

and enter the anther tube sidewise. With the growing of

the style, the bending part straightens. This unbending

process may generate a pushing force which would not

only help the style to protrude out of the anther tube but

also facilitate the friction between the sweeping hairs of

Figure 2. Pollen viabilities in different floral stages (data are

presented as means¡Ó1SE). See Figure 1 for the descriptions of

the floral stages from A to F.

HONG et al. ¡X Secondary pollen presentation and style morphology in

Mikania micrantha

257

the style and the anthers. The pollen grains lodged in the

anthers were loosened and adhered to the sweeping hairs

of the style. Then, they were brought out of the anther tube

with the growing of style. Thus, the bending behaviour

of M. micrantha style and the sweeping hairs of the style

branches may play an important role in presenting pollen

to pollinator.

Within each of the style branches, the sweeping hairs

gradually increase in length basipetally. Moreover, due

to the echinate ornamentation of the exine, the pollens

can adhere onto the sweeping hairs. Thus, the sweeping

hairs can easily carry the pollens with them out of the

anther tube. Therefore, these morphological and structural

characteristics of the stylar sweeping hairs and pollen

are co-adapted to bring the pollens out effectively. The

sweeping hairs of M. micrantha are much different from

those of Polygalaceae and Fabaceae (Westerkamp, 1999),

Cyphiaceae (Leins and Erbar, 2003), because different

Figure 3. The morphology of pollen and style in Mikania micrantha under SEM. A, Pollen (bar=5 £gm); B, A style gives off two style

branches (sb) (bar=200 £gm); C, Detail of a style branch showing the sterile appendage covered by sweeping hairs (D, E) and the stig-

matic surface (F) (bar=100 £gm); Arrows with figure numbers indicate the approximate position of anatomical sections illustrated in

corresponding figures; D, Sweeping hairs at the tip part of the style branch (bar=20 £gm); E, Sweeping hairs located on the upper part of

the style branch (bar=20 £gm); F, Receptive stigmatic surface consisting of columnar papilla cells (bar=10 £gm).

258

Botanical Studies, Vol. 49, 2008

species display variation in this mechanism, perhaps

reflecting differences in mating systems (Anderson et al.,

2000; Etcheverry et al., 2003; Leins and Erbar, 2003).

As the style grows out of the anther tube, the outside

of the style branches presents pollen for pollination. The

receptive papillate stigmatic surface is hidden between

two appressed style branches, preventing auto-pollination

during the functionally male phase of the floret (Figure

1B-D). Our previous study has suggested that the pollen

grains should be transported from the sweeping hairs of

one plant to the stigmatic surface of another plant, which

has been proven by our pollination treatments indicating

that M. micrantha needs insect pollination (Hong et al.,

2007). During the functionally female phase (Figure 1E),

the style branches separate completely, and two layers of

receptive papillae on the adaxial surfaces become exposed

and are receptive. Thus, the behavior of style determines

the timing of the functionally female phase of the floret.

In some Asteraceae species, maturation of the flower is

marked by the gradual separation of the style branches un-

til they eventually reflex back onto the presenting surface

and auto-pollinate though many species are sporophyti-

cally self-incompatible (Howell et al., 1993).

Our pollen viability results show that pollen viability

began to gradually decrease after flower opened which

results in a temporal separation between pollen presenta-

tion and pollen viability and diminishes the chance of

self-pollination, but some are still viable after the flower

have entered female phase. This indicates that there is an

overlap between the time when pollen is viable and when

the stigma of the same floret is receptive, and thus self-

pollination cannot be totally excluded. This shows that the

receptive stigma is in general temporally separated from

the floret¡¦s own viable pollen, but cannot completely avoid

the probability of autogamy or geitonogamy. However,

our pollination experiments for M. micrantha show that

seed/ovule ratio was 0.56 for open pollination and 0.0034

and 0.0038 for wind pollination and selfing, respectively

(Hong et al., 2007). These indicate that the stigmas almost

completely accept outcross pollens. Thus, although flower

morphology does not fully prevent self-pollination (and

geitonogamy can easily take place), the level of autogamy

is very low. Therefore, some self-incompatibility mecha-

nism seems operative in this species (Hong et al., 2007).

Protandry is prevalent throughout the Asteraceae. In

self-incompatible species protandry can avoid pollen-

stigma interference (Webb, 1985), but this is not fully

achieved in M. micrantha, since some overlap exists be-

tween pollen delivery and receipt, which is a fact in most

Asteraceae species.

In conclusion, the results indicate that M. micrantha

has a system of secondary pollen presentation. During

anthesis, the bending behaviour of the M. micrantha

style and the sweeping hairs of its two branches help the

growing style to move the pollens out of the anthers ef-

ficiently. This style-developmental morphology and its

potential functions would be of great importance to better

understand the evolutionary history of floral structure and

function in angiosperms. The secondary pollen presenta-

tion system and the fine-scale morphology and structure of

style indicate that there is some adaptive advantage to be

derived from their formation.

Acknowledgements. We thank Dr. Simon Hiscock and

Dr. Adrian Brennan for their constructive comments on

the manuscript. This work was supported by the National

Natural Science Foundation of China (No. 30530160),

the Natural Science Foundation of Guangdong Province,

China (No. 05200701), and a Dean Scholarship of the

Chinese Academy of Sciences, China.

LITERATURE CITED

Anderson, G.J., G. Bernardello, P. Lopez, D.J. Crawford, and

T.F. Stuessy. 2000. Reproductive biology of Wahlenber-

gia (Campanulaceae) endemic to Robinson Crusoe Island

(Chile). Pl. Syst. Evol. 223: 109-123.

Cerana, M.M. 2004. Flower morphology and pollination in Mi-

kania (Asteraceae). Flora 199: 168-177.

Deng, X., W.H. Ye, H.L. Feng, Q.H. Yang, H.L. Cao, K.Y. Xu,

and Y. Zhang. 2004. Gas exchange characteristics of the

invasive s pecies Mikania micrantha and its indigenous

congener M. cordata (Asteraceae) in South China. Bot.

Bull. Acad. Sin. 45: 213-220.

Erbar, C. and P. Leins. 1995. Portioned pollen release and the

syndromes of secondary pollen presentation in the Campan-

ulales-Asterales-complex. Flora 190: 323-338.

Erbar, C. and P. Leins. 2000. Some interesting features in the

capitulum and flower of Arnaldoa macbrideana F erreyra

(As teracea e, Barna desioidea e). Bot. J ahrb. S yst. 122:

517-537.

Etcheverry, A.V., J.J. Protomastro, and C. Westerkamp. 2003.

Delayed autonomous self-pollination in the colonizer Cro-

talaria micans (Fabaceae: P apilionoideae): structural and

functional aspects. Pl. Syst. Evol. 239: 15-28.

Grombone-Guaratini, M.T., V.N. Solferini, and J. Semir. 2004.

Reproductive biology in species of Bidens L. (Asteraceae).

Scientific Agriculture (Piracicaba, Braz.) 61: 185-189.

Holm, L.G., D.L. Plucknett, J.V. Pancho, and J.P. Herberger.

1977. The World¡¦s Worst Weeds: Distribution and Biology.

University Press of Hawaii, Honolulu, USA.

Hong, L., H. Shen, W.H. Ye, H.L. Cao, and Z.M. Wang. 2007.

Self-incompatibility in Mikania micrantha in South China.

Weed Res. 47: 280-283.

Howell, G.J., A.T. Slater, and R.B. Knox. 1993. Secondary pol-

len presentation in angiosperms and its biological signifi -

cance. Aust. J. Bot. 41: 417-438.

Ke nnedy, T.A., S . Na eem, K.M. Howe, J .M.H. Knops , D.

Tilman, and P. Reich. 2002. Biodiversity as a barrier to eco-

logical invasion. Nature 417: 636-638.

Ladd, P.G. 1994. Pollen presenters in the flowering plants: form

and function. Bot. J. Linn. Soc. 115: 165-195.

HONG et al. ¡X Secondary pollen presentation and style morphology in

Mikania micrantha

259

Ladd, P.G. and J.S. Donaldson. 1993. Pollen presenters in the

South African flora. S. Afr. J. Bot. 59: 465-477.

Lee, C.E. 2002. Evolutionary genetics of invas ive species.

Trends Ecol. Evol. 17: 386-391.

Leins, P. and C. Erbar. 1990. On the mechanisms of secondary

pollen presentation in the Campanulales-Asterales-complex.

Bot. Acta 103: 87-92.

Leins, P. and C. Erbar. 2003. The pollen box In Cyphiaceae

(Campanulales). Int. J. Plant Sci. 164 (Suppl.): 321-328.

Leins, P. and C. Erbar. 2006. Secondary pollen presentation syn-

dromes of the Asterales - a phylogenetic perspective. Bot.

Jahrb. Syst. 127: 83-103.

Lian, J .Y., W.H. Ye, H.L. Cao, Z.M. Lai, and S.P. Liu. 2006.

Effects of periodic cutting on the structure of the Mikania

micrantha community. Bot. Stud. 47: 185-190.

Lowe, S., M. Browne, S. Boudjelas, and M. DePoorter. 2001.

100 of the World¡¦s Worst Invasive Alien Species. A selec-

tion from the Global Invasive Species Database. IUCN/SSC

Invasive Species Specialist Group (ISSG), Auckland, New

Zealand.

Perrings, C., K. Dehnen-Schmutz, J. Touza, and M. Williamson.

2005. How to manage biological invasions under globaliza-

tion. Trends Ecol. Evol. 20: 212-215.

Pimentel, D., R. Zuniga, and D. Monison. 2005. Update on the

environmental and economic costs associated with alien-

invasi ve species in the United St ates. Ec ol . E con. 52:

273-288.

P orras, R. and J.M.M. Alvarez. 1999. Breeding system in the

cleistogamous species Centaurea melitensis (Asteraceae).

Cana. J. Bot. 77: 1632-1640.

Rodriguez-Riano, T. and A. Dafni. 2000. A new procedure to as-

sess pollen viability. Sex. Plant Reprod. 12: 241-244.

Roitman, G.G. 1999. Pollination biology of Grindelia covasii

(Asteraceae), a potential crop for arid lands. J. Arid. Envi-

ron. 43: 103-110.

Sakai, A.K., F.W. Allendorf, J.S. Holt, D.M. Lodge, J. Molof-

sky, K.A. With, S. Baughman, R.J. Cabin, J.E. Cohen, N.C.

Ellstrand, D.E . McCauley, P. O¡¦Neil, I.M. Parker, J .N.

Thompson, and S.G. Weller. 2001. The population biology

of invasive species. Annu. Rev. Ecol. Syst. 32: 305-332.

Song, L.Y, G.Y. Ni, B.M. Chen, and S.L. Peng. 2007. Energetic

cost of leaf cons truction in the invasive we ed Mikania

micrantha H.B.K. and its co-occurring species: implications

for invasiveness. Bot. Stud. 48: 331-338.

Swarmy, P.S. and P.S. Ramakrishnan. 1987. Weed potential of

Mikania micrantha H.B.K. and its control in fallows after

shifting agriculture (jhum) in north-east India. Agr. Ecosyst.

Environ. 18: 195-204.

Van Kleunen, M. and S.D. Johnson. 2005. Testing for ecological

and genetic allee effects in the invasive shrub Senna didy-

mobotrya (Fabaceae). Am. J. Bot. 92: 1124-1130.

Vitousek, P.M., C.M. D¡¦antonio, L.L. Loope, and R. Westbrooks.

1996. Biological invasions as global environmental change.

Am. Sci. 84: 468-478.

Webb, C.J. 1985. Protandry, pollination, and self-incompatibility

in Discaria toumatou. New. Zeal. J. Bot. 23: 331-335.

Westerkamp, C. 1999. Keel flowers of the P olygalaceae and

Fabaceae: a functional comparison. Bot. J. Linn. Soc. 129:

207-221.

Wirjahar, S. 1976. Autecological study of Mikania spp. Proceed-

ings of the fifth Asian-Pacific Weed Science Society confer-

ence, Tokyo, Japan, pp. 70-73.

Yang, Q.H., W.H. Ye, X. Deng, H.L. Cao, Y. Zhang, and K.Y.

Xu. 2005. Seed germination eco-physiology of Mikania

micrantha H.B.K. Bot. Bull. Acad. Sin. 46: 293-299.

Yeo, P.F. 1993. Secondary pollen presentation. Form, function

and evolution. Pl. Syst. Evol. Suppl. 6: 1-268.

Zhang, L.Y., W.H. Ye, H.L. Cao, and H.L. Feng. 2004. Mikania

micrantha H.B.K. in China - an overview. Weed Res. 44:

42-49.

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