全 文 ：Received 27 Apr. 2004 Accepted 20 Oct. 2004
Supported by the Key Project of Scientific and Technological Innovation of The Chinese Academy of Sciences (KSCX2-SW-108) and
Director Fund from Institute of Botany, The Chinese Academy of Sciences.
* Author for correspondence. Tel: +86 (0)10 62836115; Fax: +86 (0)10 62590843; E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (12): 1434-1442
Developmental Morphology of Obturator and Micropyle and Pathway of Pollen
Tube Growth in Ovary in Phellodendron amurense (Rutaceae)
ZHOU Qing-Yuan, JIN Xiao-Bai, FU De-Zhi*
(Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China)
Abstract: The developmental morphology of the obturator and micropyle and the pathway of the pollen
tubes in the ovarian locule in Phellodendron amurense Rupr. have been investigated by scanning electron
microscopy and light microscopy. The obturator of the ovule is of funicular origin. As the ovule is fully
developed, the obturator becomes considerably large and closely covers the micropyle. When the pistillate
flower reaches the receptive phase, the morphology of the obturator is greatly modified, and its cells on
the surface are radially expanded and appear to be columnar, semi-papillar or papillar. Following fertilization
the obturator reduces in size and degenerates. The pollen tubes need not necessarily traverse the obturator.
Whether the pollen tubes pass the obturator or not depends on the sites at which they enter the ovarian
locule. This observation does not support the idea that the obturator mainly serves the function of
mechanical guidance for the growth of pollen tubes. Our investigations indicate that the structure of the
micropyle changes with the different stages of the reproductive process, and becomes asymmetrical
during pollination. The development of the obturator and the micropyle correlates with that of the female
gametophyte in P. amurense.
Key words: developmental morphology; micropyle; obturator; Phellodendron amurense; pollen tube
pathway
Obturator has been described in some taxa as a modifi-
cation of ovarian tissue, which the pollen tube must traverse
towards the micropyle. It is most prevalent in the
Euphorbiaceae, Rosaceae, and Liliaceae (Tilton and Horner,
1980). The obturator may originate from a variety of sources:
the placenta (Dalmer, 1880; Bor and Bouman, 1974); the
funiculus (Capus, 1878; Tsai et al., 1973); the funiculus and
placenta combined (Tilton and Horner, 1980); and the in-
teguments (Davis, 1966) and arils (Rao, 1959). Obturators
of different species have different morphological characters,
such as a pad or swelling, hairs, filaments or tufts (Tilton
and Horner, 1980).
Various functions have been attributed to obturator in
different species. Tilton and Horner (1980), according to
the evidence that pollen tubes emerging from the style grow
only along the obturator surface in Ornithogalum, regarded
the obturator as a modification of ovarian transmitting tis-
sue and suggested that it may provide nutrients and me-
chanical and chemotropical guidance for growing pollen
tubes. Tilton et al. (1984) proposed that the obturator in
soybeans might control the direction of pollen tube growth
by secreting a chemotactic exudate. In peach, Herrero and
Arbeloa (1989) revealed a very interesting case in which
pollen tubes stopped growing when they arrived at the
obturator from the base of the style. Pollen tube growth
was resumed 5 d later when the obturator secreted an
exudate. Herrero and Hormaza (1996) proposed that the
obturator functions as a drawbridge connecting the ovule
with the ovary at a particular time. Other structures have
been described in some taxa that appear to have a similar
role. In pistachio (a chalazogamous species), the ponticu-
lus develops in the uppermost area of the funiculus be-
tween the style and ovule upon pollen tube arrival. It plays
the role in opening and closing the access of the pollen
tube to the ovule by physically approaching or isolating
the ovule and the style (Martínez-Pallé and Herrero, 1995).
In Zea mays, papillar hairs present at the base of the style
lose their turgidity after fertilization, which prevents the
access of other pollen tubes to the ovule (Heslop-Harrison
et al., 1985). In the grasses, this function appears to be
fulfilled by the embellum (Busri et al., 1993).
The micropyle is formed by one or two integuments,
and it is a little gate which the pollen tube traverses on its
way towards the embryo sac (Herrero, 2001). Tilton (1980)
presented a review of the studies of the micropyle. Light
microscopic and ultrastructural studies have been made in
a few species (Gelin, 1936; Chao, 1971; Yan et al., 1991).
The prefertitization functions of the micropyle may be
ZHOU Qing-Yuan et al.: Developmental Morphology of Obturator and Micropyle and Pathway of Pollen Tube Growth
in Ovary in Phellodendron amurense (Rutaceae) 1435
involved in nutrition and mechanical and chemotropic guid-
ance for pollen tube growth (Franssen-Verheijen and
Willemse, 1993; Sage and Williams, 1995). Yan et al. (1991)
revealed a close relationship among micropyle organization,
orientation of pollen tube growth and synergid
degeneration. The postfertilization functions of micropyle
may be to preclude pathogen invasion and prevent desic-
cation of embryo and endosperm through the formation of
a plug (Tilton, 1980).
While information is available on the morphology and
function of the obturator in different species, information
is lacking on the organogenesis of this structure. In the
same way, little information has been provided on how de-
velopment of the obturator is related to that of the micro-
pyle and female gametophyte. Phellodendron amurense
is a dioecious tree species. In female trees the carpellate
flower has five carpels each bearing one ovule, or rarely
two. The ovule is anatropous, bitegmic and crassinucellate
and contains a large obturator (Zhou et al., 1999). The scope
of the present study covers (1) the investigation of the
developmental morphology of obturators and micropyles;
(2) the examination of the pathway of pollen tube growth in
the ovarian locule; (3) the analysis of how the development
of the obturator is associated with that of the micropyle
and female gametopyte and with pollen tube growth; and
(4) the study on the function of the obturator in P.
amurense.
1 Materials and Methods
1.1 Plant materials
Female flowers of Phellodendron amurense Rupr.
(Rutaceae) at various stages of flower development were
collected in the Beijing Botanical Garden, Institute of
Botany, The Chinese Academy of Sciences. The female
flowers were hand-pollinated after anthesis and were then
collected at the interval of 1 h. The materials were immedi-
ately fixed in FAA (90 mL 50% ethanol, 5 mL glacial acetic
acid, and 5 mL 37% formalin).
1.2 Scanning electron microscopy
The fixed materials were dehydrated in ethanol series.
The ovaries of the female flower were dissected and ob-
served in 95% ethanol with the dissecting microscope, then
transferred through an ethanol iso-amyl acetate series, criti-
cal-point dried, mounted on a metal stub, and sputter coated
with gold palladium. The samples were observed and mi-
crographs taken with Hitachi S-800 scanning electron mi-
croscope (SEM) at 30 kV (Figs. 1–28).
1.3 Light microscopy
The fixed materials were dehydrated in alcohol series,
and embedded in paraffin wax. Serial sections were made at
7–10 mm and stained with Heidenhain’s iron-alum
hematoxylin, counterstained with safranin and fast green.
Photographs were taken under Olympus BH (Figs. 29–32).
2 Results
2.1 The early development of obturator
The obturator of P. amurense is of funicular origin. As
the nucellar primordium begins to curve, the funicular tis-
sue slightly above it proliferates, and an obturator is initi-
ated (Fig.1). Following further curvature of nucellus, the
obturator is carried upward by the growing ovule and be-
comes a prominent bulge on the funicle (Fig.2). But in some
cases, the obturator develops so slowly that it remains in-
conspicuous until the initiation of the inner integument or
even of the outer integument (Fig.3). During the early stages
of integuments development, the funicle is elongated axi-
ally (Fig.4). As the ovule is gradually rotated, the funicle
bends at the obturator (Figs.5, 6). The portions of the fu-
nicle above the obturator take part in the construction of
the obturator. As the two integuments overgrow the nucel-
lus and arch over its apex to form the micropyle, the obtura-
tor extends toward the micropyle to varying degrees (Figs.
7, 8). Soon the obturator covers it completely (Fig.9). The
ovule begins to increase greatly in volume and the micro-
pyle opens 6 d before anthesis. Concomitantly, the obtura-
tor also increases considerably in size. Usually the micro-
pyle opens wide rapidly, so that the obturator cannot en-
tirely cover it until 1 d after the anthesis (Fig.10).
2.2 The maturation and degeneration of obturator
Close to anthesis, the obturator expands radially and
rapidly enlarges. Viewed externally with SEM, its upper
surface becomes rough and irregular; some shallow fur-
rows are usually produced in different widths and direc-
tions (Fig.12). Due to rapid expansion, the obturator sur-
face becomes flat, and the outline of the cells is indistinct
where the ovary wall crushes them. The cell outline other
than in this area becomes distinct. The cells appear tessel-
late and are slightly domed. When the petals spread apart
in anthesis, two long grooves are radially and medially pro-
duced on the lateral sides of some obturators (Fig.11). They
are more or less deepened later. In a few instances, a wide,
dome-like protuberance in the base of the obturator is
formed, which fills the ovular micropyle (Figs.13, 14).
As the female flower enters the receptive stage 2–3 d
after anthesis, the obturator reaches its greatest volume
(Fig.15). Surface furrows develop further and become most
obvious. Cell surfaces are raised and appear dome-like or
even semi-papillar or papillar (Figs.15, 18, 19). In anatomical
Acta Botanica Sinica 植物学报 Vol.46 No.12 20041436
Figs.1–9. Developmental morphology of the obturator of Phellodendron amurense. 1. A funicular obturator begins to be initiated
slightly above the nucellus. Bar = 100 mm. 2. Obturators appear as prominent bulges. Bar = 200 mm. 3. The obturator does not become
obvious until the initiation of the inner integument. Bar = 200 mm. 4. The obturator becomes quite large and the funicle is nearly straight.
Bar = 200 mm. 5, 6. The funicle bends at the position of the obturator following the rotation of ovule. Bar = 200 mm. 7, 8. Two integuments
overgrow nucellus and are forming micropyle. Concomitantly, the obturator extends radially toward the micropyle. 7. Bar = 200 mm. 8.
Bar = 300 mm. 9. Showing the obturator soon after the micropyle is formed. Bar = 400 mm. Abbreviations: nu, nucellus; Ob, obturator.
observations, the surface cells clearly expand and become
columnar. These cells fully enlarge, and a large vacuole is
commonly visible in the distal end upon arrival of a pollen
tube (Fig.32). However, cells in the shallow furrows remain
small and nearly isodiametric (Fig.31), their surfaces being
slightly raised.
ZHOU Qing-Yuan et al.: Developmental Morphology of Obturator and Micropyle and Pathway of Pollen Tube Growth
in Ovary in Phellodendron amurense (Rutaceae) 1437
The placenta next to the obturator often develops a pro-
tuberant structure about 4–5 d prior to anthesis (Fig.10). Its
top develops a relatively small obturator-shaped structure,
and its base extends axially along the locule and becomes a
Figs.10–18. Developmental morphology of the obturator of Phellodendron amurense and pollen tube pathway in ovaries. 10. As the
ovule increases in volume, the micropyle opens and the obturator enlarges in size. The placenta develops a protuberance beside the
obturator. Bar = 300 mm. 11. Deep grooves are produced on the lateral side of the obturator. Bar = 200 mm. 12. The furrows develop on
the obturator surface following approach of receptive phase. Bar = 200 mm. 13, 14. The obturator and micropyle of an ovule, respectively,
showing that the base of obturator protrudes into micropyle at the stage of pollination. Bar = 200 mm. 15. The obturator during the
receptive phase of female flower. Bar = 200 mm. 16. Germinated pollen on the stigma 3–4 h after pollination. Bar = 60 mm. 17. A pollen
tube (arrow) is traversing obturator. Bar = 600 mm. 18. High magnification of the obturator in Fig.17, the ovule underneath it being
removed. Bar = 200 mm. Abbreviation: Ob, obturator.
Acta Botanica Sinica 植物学报 Vol.46 No.12 20041438
long, funicle-like structure. This could be remnant of a sec-
ondary ovule. The development of the obturator is prob-
ably correlated with that of the female gametophyte. The
obturator of the ovule at the 2- or 4-nucleate stage of the
female gametophyte is significantly different from that of
the ovule with mature female gametophyte during recep-
tive stages (Fig.29). The former is relatively small, with in-
distinct surface cell outlines, and the surface cells are not
Figs.19–27. Pollen tube growth in ovaries and developmental morphology of the micropyle. 19. The pollen tube (arrow) does not reach
the obturator, but arrives directly at the lateral side of micropyle. Bar = 200 mm. 20. The pollen tube (arrow) does not arrive at obturator.
Bar = 200 mm. 21, 22. The obturator and micropyle of an ovule after fertilization respectively, showing the protuberance at the obturator
base and the presence of white exudates inside the micropyle. Bar = 200 mm. 23. A large gap between the obturator and micropyle is
produced soon after obturator reduces in size. Bar = 200 mm. 24. An ovule in Fig.9, showing that the micropyle is an extremely narrow
canal. Bar = 200 mm. 25. The micropyle 5 d prior to anthesis. Bar = 200 mm. 26. The micropyle of the corresponding ovule in Fig.15. Bar
= 200 mm. 27. An underdeveloped micropyle during pollination stages. Bar = 200 mm. Abbreviation: Ob, obturator.
ZHOU Qing-Yuan et al.: Developmental Morphology of Obturator and Micropyle and Pathway of Pollen Tube Growth
in Ovary in Phellodendron amurense (Rutaceae) 1439
radially expanded, nor do they show secretory form.
The pollen grains begin to germinate on the stigma 3–4
h after pollination (Fig.16). Pollen tubes grow from the
stigma down to transmitting tissue of the style, reaching
the base of the style about 5 h after pollination, and enter-
ing the ovary. They penetrate below the inner epidermal
cells of the ovary cavity and travel for different distances,
then turn to enter the ovarian locule. The pollen tubes which
come to the locule sites near the obturator at first reach the
obturator and then subsequently crawl along its surface or
a surface furrow towards the micropyle (Figs.18, 31, 32).
The surface furrows through which pollen tubes pass are
more or less deepened. The pollen tube that does not grow
along the surface furrows makes a deep groove due to dis-
solution of the cells below it (Fig.18). The pollen tube that
arrives at the locule site far away from and below the obtu-
rator does not reach the obturator surface but grows di-
rectly toward the micropyle (Figs.19, 20). They generally
arrive at the lateral side of the micropyle proximal to the
funicle.
After the ovule has been fertilized, the obturator changes
significantly. The obturator gradually degenerates and re-
duces in size, and its surface cell outlines become indis-
tinct (Figs.21, 23). The base of the obturator facing the
micropyle protrudes into the micropyle. This protuberance
changes from being very long to short; its surface cells are
papillar. The region of the obturator facing the micropyle
produces radial shallow furrows (Fig.21). The ovule con-
tinues to increase in size after pollination. The growth of
the ovule proper, i.e. the nucellus and integuments, is
Figs.28–32. Developmental morphology of micropyle of Phellodendron amurense and pollen tube growth in ovaries. 28. The
micropyle is being closed after pollination. Bar = 200 mm. 29. Longitudinal section of an underdeveloped obturator in the ovule with 2-
nucleate embryo sac during the receptive stage of the female flower. Bar = 50 mm. 30. Transverse section of an exostome near the time of
pollination. Bar = 40 mm. 31, 32. The transverse sections of obturators when pollen tubes (arrows) are traveling through them. Bar = 40
mm. Abbreviation: Ob, obturator.
Acta Botanica Sinica 植物学报 Vol.46 No.12 20041440
asymmetrical, and the abaxial parts grow more rapidly than
the adaxial ones, resulting in declivity of the micropyle with
the abaxial side higher than the adaxial side (Fig.23). A large
gap develops separating the obturator from the micropyle
soon after the degeneration of the obturator. During the
late stage of ovule development the obturator closely cov-
ers micropyle, and its surface becomes smooth.
2.3 The development of micropyle
The two integuments form a micropyle by the megaspore
tetrad stage. The outer integument on both sides of the
micropyle is appressed soon after the micropyle has been
produced, so that the micropyle is an extremely narrow ca-
nal (Fig.24). The cells of the outer integument forming the
micropyle differ from those of the rest of the outer
integument. The former are parenchymatous cells with con-
spicuous nuclei, and the latter are stained entirely dark.
Subsequently, the ovule starts to increase greatly in vol-
ume and the micropyle opens (Fig.25). The extent of micro-
pyle opening varies greatly in different ovules, and the
endostome opens much less than exostome does. As the
micropyle is widened, the outer integument cells lining the
micropyle are modified (Fig.25). They expand radially and
become dome-like. Following the approaching of the re-
ceptive stage in female flowers, the micropyle comes to the
greatest width (Fig.26), along with the occurrence of the
greatest volume of the obturator. At this stage, the inner
epidermal cells of outer integument in the micropyle region
are fully enlarged and radially expanded, and they appear
to be secretory. These features in the cells directly above
the endostome are more conspicuous among them. This
indicates that the development of the micropyle is
asymmetrical.
Secretion is often visible inside the micropyle after
pollination. It is possibly derived from the obturator prod-
uct (Fig.22). The micropyle of the ovule with an abnormally
developed female gametophyte differs greatly from that of
the ovule with a normal mature female gametophyte during
the floral receptive stage. The outer integument cells lining
the former do not radially expand, nor do they show a secre-
tory phase (Fig.27). After the ovule has been fertilized, the
outer integument cell outline in the micropyle zone gradu-
ally becomes indistinct, the micropyle becomes narrow and
eventually occluded (Fig.28).
3 Discussion
The obturator of P. amurense arises from the funicular
tissue. The obturator of the young ovules develops at vari-
ous speeds. As the ovule is fully developed, the obturator
becomes considerably large and closely covers the
micropyle. Following maturity of the female gametophyte,
the surface morphology and the surface cells will be greatly
modified in the obturator. Its surface furrows occur in dif-
ferent directions, and the surface cells are expanded radi-
ally and become columnar, papillar or semi-papillar, they
appear to be secretory. After fertilization of the ovule, the
surface secretory cells disappear and the base of the obtu-
rator protrudes into the micropyle. Following this, the ob-
turator degenerates.
Previous literature described that pollen tubes entering
the ovarian locule consistently traverse along the obtura-
tors to reach the ovules (Tilton and Horner, 1980; Herrero,
2001). However, in our investigations, the pollen tubes need
not necessarily pass along the obturator surface towards
the micropyle. Whether pollen tubes traverse along obtu-
rators surface or not depends on the sites which they enter
the ovarian locule. The pollen tubes that reach the ovarian
locule sites near the obturator travel on its surface towards
the micropyle. Those which arrive at the locule sites far
away from and below the obturators do not pass it, but
grow directly towards the micropyle.
As mentioned in the introduction section, the obturator
may have various origins and diverse structures in differ-
ent species. It may serve the functions of nutrition, me-
chanical and chemotropical guidance, and control of the
pollen tube kinetics, as well as control of pollen tube num-
bers entering an ovule. Based on our observations in P.
amurense, we suggest that the primary role of the obtura-
tor in this species is not to provide mechanical guidance
for pollen tube growth. Our investigations do not support
the idea of Tilton et al. (1984) that the obturator controls
the direction of pollen tube growth by secreting a chemot-
actic exudate. Some previous scientists proposed that the
chemotropic substances are derived from the embryo sac,
and they are abundant at the micropyle zone (Knox, 1984;
Cheung, 1996; Russell, 1996; Herrero, 2001). Recently,
Higashiyama et al. (2001) showed by laser cell ablation that
two synergid cells adjacent to the egg cell attract the pollen
tube. In general, we propose that the primary function of
obturator is the nutrient one.
When the micropyle is just formed by both integuments,
it is an extremely narrow canal. Concomitantly with the in-
crease in size of the ovule, the micropyle may open greatly.
The extent of the micropyle opening varies from very wide
to narrow. We did not find correlation between the width of
the micropyle and the nature of the obturator. It is assumed
that the obturator does not play a role in regulating the
micropyle. During pollination, the cells of outer integument
lining the micropyle are modified and appear to be secretory.
ZHOU Qing-Yuan et al.: Developmental Morphology of Obturator and Micropyle and Pathway of Pollen Tube Growth
in Ovary in Phellodendron amurense (Rutaceae) 1441
The integument cells proliferate at the micropyle zone and
close the micropyle after fertilization.
Yan et al. (1991) suggested that micropyles are of two
basic types: open and closed, and they probably are re-
lated to the type of the ovule and the style. Based on our
examinations on P. amurense, we contend that the micro-
pyle is not unchangeable in different stages of ovule
development, it may be regularly transformed to provide
optimal conditions for the growth of pollen tubes and the
fertilized ovules in agreement with Willemse’s investiga-
tions on Gasteria verrucosa, in which the micropyle wid-
ens more or less during the receptive period (Willemse,
1999). Yan et al. (1991) revealed that the micropyle in sun-
flower is asymmetrical in the structural and possibly physi-
ological features, and the micropyle is better developed on
the side proximal to the funicle than on the opposite side.
We also noted the asymmetrical nature of the micropyle in
P. amurense. During pollination, the cells of outer integu-
ment lining the micropyle are modified in varying degrees.
Among them, those directly above the endostome are more
obvious. But the question as to how the asymmetrical mi-
cropyle is associated with the pollen tube growth remains
to be investigated.
The development of obturator and micropyle is associ-
ated with that of female gametophyte in P. amurense. We
found that the formation of the well developed obturator
and micropyle coincides with the occurrence of the mature
female gametophyte; if the female gametophyte develop-
ment were arrested or remained at the early stages after
pollination, the micropyle and obturator would be
underdeveloped. Consequently, the ovule would abort. We
postulate that the mature embryo sac probably provides a
chemical signal that triggers the normal development of
obturator and micropyle.
Acknowledgements: The authors thank Dr. María Herrero
of the Unidad de Fruticultura, SIA-DGA, Zaragoza, Spain
for critically reading, giving comments on and making revi-
sion to the manuscript. We also thank Prof. CHEN Zu-Keng,
Dr. ZHANG Shu-Ren and Dr. WANG Yin-Zheng of the In-
stitute of Botany, The Chinese Academy of Sciences for
suggestions.
References:
Bor J, Bouman F. 1974. Development of ovule and integuments
in Euphorbia milii and Codiaeum variegatum. Phytomorpho-
logy, 24: 280-296.
Busri N, Chapman G P, Greenham J. 1993. The embellum: a
newly defined structure in the grass ovule. Sex Plant Reprod,
6: 191- 198.
Capus G. 1878. Anatomie du tissu conducteur. Ann Sci Nat Bot
sér VI, 7: 209-291.
Chao C Y. 1971. A periodic acid-Schiff’s substance related to the
directional growth of pollen tube into embryo sac in Paspalum
ovules. Am J Bot, 58: 649-654.
Cheung A Y. 1996. The pollen tube growth pathway: its molecu-
lar and biochemical contributions and responses to pollination.
Sex Plant Reprod, 9: 330-336.
Davis G L. 1966. Systematic Embryology of the Angiosperms.
New York: John Wiley and Sons.
Dalmer M. 1880. Über die Leitung der Pollenschläuche bei den
Angiospermen. Jena Z Naturwis, 14: 530-566.
Franssen-Verheijen M A W, Willemse M T M. 1993. Micropylar
exudate in Gasteria (Aloaceae) and its possible function in
pollen tube growth. Am J Bot, 80: 253-262.
Gelin O E V. 1936. Zur Embryologie und Zytologie van Berkheya
bergiana Soderb. du B. adlami Hook. Sven Bot Tidskr, 30:
324-328.
Herrero M. 2001. Ovary signals for directional pollen tube growth.
Sex Plant Reprod, 14: 3-7.
Herrero M, Arbeloa A. 1989. Influence of the pistil on pollen tube
kinetics in peach (Prunus persica). Am J Bot, 76: 1441-1447.
Herrero M, Hormaza J I. 1996. Pistil strategies controlling pollen
tube growth. Sex Plant Reprod, 9: 343-347.
Heslop-Harrison Y, Heslop-Harrison J, Reger B J. 1985. The
pollen-stigma interaction in the grasses. 7. Pollen tube guid-
ance and the regulation of pollen tube number in Zea mays L.
Acta Bot Neerl, 34: 193-211.
Higashiyama T, Yabe S, Sasaki N, Nishimura Y, Miyagishima S
Y, Kuroiwa H, Kuroiwa T. 2001. Pollen tube attraction by
the synergid cell. Science, 293: 1480-1483.
Knox R B. 1984. Pollen-pistil interactions. Ann Rev Plant Physiol,
17: 508-608.
Martínez-Pallé E, Herrero M. 1995. The ponticulus: a structure
bridging pollen tube access to the ovule in Pistacia vera. Sex
Plant Reprod, 8: 217-222.
Rao C V. 1959. Contributions to the embryology of Palmae. Ⅰ.
Sabaleae. Proc Nat Inst Sci India, 25: 143-169.
Russell S D. 1996. Attraction and transport of male gametes for
fertilization. Sex Plant Reprod, 9: 337-342.
Sage T L, Williams E G. 1995. Structure, ultrastructure, and his-
tochemistry of the pollen tube pathway in the milkweed As-
clepias exaltata L. Sex Plant Reprod, 8: 257-265.
Tilton V R. 1980. The nucellar epidermis and micropyle of
Ornithogalum caudatum (Liliaceae) with a review of these
structures in other taxa. Can J Bot, 58: 1872-1884.
Tilton V R, Horner H T. 1980. Stigma, style, and obturator of
Ornithogalum caudatum (Liliaceae) and their function in the
reproductive process. Am J Bot, 67: 1113-1131.
Acta Botanica Sinica 植物学报 Vol.46 No.12 20041442
Tilton V R, Wilcox L W, Palmer R G, Albertsen M C. 1984.
Stigma, style, and obturator of soybean, Glycine max (L.)
Merr. (Leguminosae) and their function in the reproductive
process. Am J Bot, 71: 676-686.
Tsai A H, Harney P M, Peterson R L. 1973. Megasporogenesis
and megagametogenesis in Pelargonium × hortorum. Can J
Bot, 51: 607-612.
Willemse M T M. 1999. Pollen coat signals with respect to pistil
activation and ovule penetration in Gasteria verrucosa ( Mill.).
(Managing editor: WANG Wei)
Cresti M, Cai G, Moscatelli A. Fertilization in Higher Plants.
New York: Springer. 145-156.
Yan H, Yang H Y, Jensen W A. 1991. Ultrastructure of the micro-
pyle and its relationship to pollen tube growth and synergid
degeneration in sunflower. Sex Plant Reprod, 4: 166-175.
Zhou Q-Y, Jin X-B, Mu X-J. 1999. Embryological studies on
Phellodendron amurense. Acta Bot Yunnan , 21: 317-322.
(in Chinese with English abstract)