The fusion of male and female gametes is called fertilization.
In seed plants the male
gametes are brought to the egg containing female gametophyte by a pollen tube
(Strasburger, 1884). The phenomenon si called siphonogamy. A large number of
pollen grains (several times more than the number of ovules) come to germinate over
the stigma. A pollen grain oes not pass down the stigma. Only its pollen tube
does so. The pollen tube eats its way through the
-POLLEN GRAINS solid part of the stigma and style by secreting
-STIGMA
pectinases and hydrolytic enzymes. Pollen tube
travels intercellularly and chemotropically along
-POLLEN TUBES
the concentration gradient of calcium - boron -
inositol sugar complex. A small copper containing
protein chemocyanin is also involved in some cases.
Initial growth of the pollen tube takes place on
-CONDUCTING
TISSUE
expenditure of food present in the pollen grain. Pol-
len tube formation seems to be stimulated by fac-
tors present in the stigmatic secretion. For further
growth the pollen tube obtains its nourishment from
OVARY
the interior of stigma and style. The contents of hte
OVULE
pollen grains shift into pollen tube with the tube or
vegetative nucleus moving to its tip followed by the
ANTIPODALS two gametes. Further growth of the pollen tube
occurs only towards its tip. Depending upon the
•POLLEN TUBE length of the style and passage inside the ovary, the
SECONDARY pollen tube may reach a length of a few millimetres
NUCLEUS (POLAR NUCLE) to 45 cm in Maize.
MALE
GAMETES
POLLEN TUBE TIP Style may be hollow or solid. Hollow style has
a canal lined by special large cells. Solid style has
OBTURATOR
a special tissue of pectinised thick walls known as
transmitting or conducting tissue. The pollen tube
Fig. 2.25. Fertilization in an angiosperm through porogamy. travels along the lining of
canal in hollow style
drawing nourishment from its living cells. In solid
style the pollen tube grows through transmitting tissue by separating their cells
through secretion of pectinases. The food is absorbed by diffusion.
Mode of Entry of Pollen Tube in the Ovule:
In the ovary the growth of the pollen tube is directed by another tissue called obtura-
tor. The pollen tube enters the ovule, either through its micropyle (porogamy, e.g.,
Lily), chalaza (chalazogamy, e.g., Casuarina, Juglans) or the sides after piercing
through the integu- ments or funicle (mesogamy, e.g., Cucurbita, Populus). Porogamy
is the most common.
CHALAZA
MICROPYLE POLLEN TUBE
POLLEN TUBE
INTEGUMENT
(C) MESOGAMY In the ovule the pollen tube is
(A) POROGAMY
(B) CHALAZOGAMY
attracted by secretions of syner-
Fig. 2.26. Three modes of entry of pollen tube into the ovule. gids. Usually the pollen
tube enters
the embryo sac by passing into one
of the two synergids. The impact destroys that synergid. The pollen tube also breaks
open to release its contents. Out of the two male gametes one fuses with egg or oo-
sphere to perform generative fertilization. Generative fertilization is also called syn-
gamy or true
U1 62
TRUEMAN'S ELEMENTARY BIOLOGY +2
fertilization. It gives rise ot a diploid zygote or oospore. Soon after, the vacuole and
plasmodesmal connections of the egg degenerate. It now becomes ready to produce
the embryo.
The nucleus of the second male gamete fuses with the two haploid polar nuclei or
diploid secondary nucleus of the central cell to form a triploid primary endosperm nu-
cleus (PEN). The central cell is now called primary endosperm cell (PEC). This second
fertilization is called vegetative fertilization since as a consequence of it a vegetative
or nutritive tissue is formed to nourish the embryo. Vegetative fertilization is also
called triple fusion since three nuclei get fused, two polar nuclei and one male gamete.
