3. Female Gametophyte:
The development of female gametophyte in G.
gnemon and G. ula is distinctly tetrasporic. So far as
known Gnetum is the only gymnosperm showing this
type of development of the female gametophyte,
though such a condition can be frequently met with in
a number of angiospermic plants.
At the end of meiosis in the megaspore mother cell, a
four nucleate coenomegaspore is formed. Further
development takes place with free nuclear divisions
within the megaspore.
Dr. Suranjana Sarkar, SNC, Kolkata
4. Dr. Suranjana Sarkar, SNC, Kolkata
A conspicuous vacuole appears in the central portion
of the developing gametophyte and the nuclei lying
in the peripheral layer of cytoplasm undergo further
divisions; these number of divisions vary in different
species.
In addition to a normally developing main
gametophyte, there may be 2-4 accessory
gametophytes (which are smaller in size and contain
a few nuclei only).
These accessory gametophytes are usually placed
above the main gametophyte and ultimately undergo
degeneration.
No archegonium is present within the gametophyte
which is an angiospermic character.
8. Male Gametophyte:
A young pollen grain of Gnetum is usually provided with a
thick spiny exine and a thin intine.
In both the commonly occurring species, G. ula and G.
gnemon, Negi and Madhulata (1957) noted that after first
nuclear division in the microspore or pollen grain, a small
lens-shaped cell is cut off at one end.
This subsequently becomes rounded but neither divides
any further nor it does take part in the development of the
pollen tube. On the other hand, it degenerates in situ, and
is, therefore, regarded as a prothallial cell. The sister
nucleus, however, divides again giving rise to two nuclei,
the tube nucleus and the generative nucleus.
Dr. Suranjana Sarkar, SNC, Kolkata
9. Dr. Suranjana Sarkar, SNC, Kolkata
This generative nucleus soon develops a cytoplasmic
sheath around itself to form a generative cell. Thus, a
mature pollen grain contains a prothallial cell, a
generative cell and a tube nucleus, lying in close
proximity.
At the time of germination of the pollen grain, which
takes place inside the pollen chamber, the exine is
cast off and the intine is pushed out in the form of
pollen tube. The tube nucleus is the first to enter the
pollen tube and is followed later on by the generative
cell. The generative cell finally divides to form two
male cells.
11. Fertilization in Gnetum:
After the entry of pollen tube into the female
gametophyte, whose lower end has become partly
cellular, one or more nuclei (3-8) at its upper end also
become delimited by cell-walls.
These groups of cells are usually present in the
neighbourhood of the pollen tube.
Out of each such group, one or rarely two cells
behave as egg cells, which often become surrounded
by one or two layers of minute and degenerated cells.
In all probability these inconspicuous cells provide
nourishment to the egg and become used up at the
time of fertilization.
Dr. Suranjana Sarkar, SNC, Kolkata
12. Dr. Suranjana Sarkar, SNC, Kolkata
Only a male nucleus enters the egg, while its sheath is left
outside. In cases where two eggs are present in the
vicinity of the pollen-tube, double fertilization may result.
Gnetum is remarkable among gymnosperms in the fact
that a typical gymnospermic endosperm, which is formed
prior to fertilization, is lacking here.
Madhulata (1960) reports in G. gnemon that while eggs
are being differentiated, the female gametophyte remains
in a free nuclear condition, and only when zygotes are
produced in the micropylar region of the ovule, the first
walls are laid down in the chalazal region producing in an
upward direction.
Very rarely the cell-wall formation may take place
simultaneously throughout the gametophyte.
Sometimes walls may be laid down even when eggs are
not differentiated within the gametophyte.
14. The New Sporophyte of Gnetum:
• After fertilization, the zygote develops into the embryo. The
details of the embryo development, however, are not clear
and much confusion exists, which may be attributed to
variations existing among the different species of the
genus.
• In G. gnemon, Madhulata (1960) has shown that a small
tuber-like projection may be developed directly from one
side of the zygote, or in some ovules the zygote divides
forming a two-celled structure, each of which on
germination gives rise to a short tube; frequently, only one
of these two cells germinates.
Dr. Suranjana Sarkar, SNC, Kolkata
15. Dr. Suranjana Sarkar, SNC, Kolkata
• In rare cases, instead of a single tube, two or three
tubes are developed from a zygote. The particular tube
which receives the zygote nucleus remains functional
while the others become abortive. The tube containing
the nucleus grows in a downward direction, pushes
inside the endosperm, grows through intercellular
spaces and finally gives rise to a number of elongated
uninucleate cells by the formation of septa.
•
This is the primary suspensor tube or suspensor tube
or pro-embryo tube. Usually, a small protuberance
appears either above or below a septum. This, later on,
becomes elongated forming a more or less tube-like
structure, into which passes a nucleus. Thus, the
primary suspensor tube usually becomes much
branched.
16. Dr. Suranjana Sarkar, SNC, Kolkata
• After the formation of the primary suspensor tube is completed, a
small cell is cut off at its tip which first divides transversely and then
longitudinally to give rise to a tetrad of cells. From these apical cells
by further divisions there appears a more or less globose mass of
cells which constitute the embryonal cells.
• Sometimes the primary suspensor itself undergoes divisions. Of the
group of embryonal cells, those which are terminal in position
develop into an embryo, while those which are situated behind the
embryo become considerably elongated and divide further to give
rise to a long secondary suspensor. This secondary suspensor helps
in pushing the embryo deeper within the endosperm for obtaining
better nutrition.
• In some species, like G. ula, G. gnemon and others, polyembryony
is frequently met with, which may originate in several ways.
• A mature seed is usually elongated but may be slightly oval also. Its
colour varies from green to red. The endosperm is very conspicuous
and remains enveloped by three envelopes. The cotyledons are two
in number.
18. Relationships of Gnetum:
Gnetum and Other Gymnosperms:
Gnetum shows several resemblances with gymnosperms and has, therefore, been
finally included under this group.
Some of the characteristics common in both Gnetum and other gymnosperms are
under mentioned:
1. Wood having tracheids with bordered pits.
2. No sieve tubes and companion cells are present.
3. Presence of naked ovules.
4. Absence of fruit formation because of the absence of ovary.
5. Anemophilous type of pollination.
6. Development of prothallial cell.
7. Cleavage polyembryony.
8. Resemblance of the vascular supply of the peduncle of the cone of Cycadeoidea
wielandii with that of a single flower of Gnetum.
9. Resemblance of the structure of basal part of the ovule in Gnetum and
Bennettites.
Dr. Suranjana Sarkar, SNC, Kolkata
19. Gnetum and Angiosperms:
A key position to Gnetum has been assigned by scientists while
discussing the origin of angiosperms. Both Gnetales and
angiosperms originated from a common stalk called “Hemi-
angiosperm”.
Thompson (1916) opined that the ancestors of both Gnetum and
angiosperms were close relatives. Some other workers have gone up
to the extent in stating that Gnetum actually belongs to
angiosperms. Hagerup (1934) has shown a close relationship
between Gnetales and Piperaceae.
In a beautiful monograph on Gnetum, Maheshwari and Vasil (1961)
have stated that “Gnetum remains largely a phylogenetic
puzzle. It is gymnospermous, but possesses some strong
angiospermic features”.
Dr. Suranjana Sarkar, SNC, Kolkata
20. Some of the resemblances between Gnetum and angiosperms are
under mentioned:
1. The general habit of the sporophyte of many species of Gnetum
resembles with angiosperms.
2. Reticulate venation in the leaves of Gnetum is an angiospermic
character.
3. Presence of vessels in xylem is again an angiospermic character.
4. Clear tunica and corpus configuration of shoot apices is a character of
both Gnetum and angiosperms.
5. Strobili of Gnetum resemble much more with angiosperms than any
of the gymnosperms
6. Micropylar tube of Gnetales can be compared with the style of the
angiosperms because both perform more or less similar functions.
7. Tetrasporic development of the female gametophyte is again a
character which brings Gnetum close to angiosperms.
8. Absence of archegonia again brings Gnetum and angiosperms much
closer.
9. Dicotyledonous nature of the embryo of Gnetum brings it quite close
to the dicotyledons.
Dr. Suranjana Sarkar, SNC, Kolkata
