submerged aquatic plants, it has
a double wall, a thin delicate wall of unaltered cellulose, the
endospore or intine, and a tough outer cuticularized exospore or
extine. The exospore often bears spines or warts, or is variously
sculptured, and the character of the markings is often of value
for the distinction of genera or higher groups. Germination of the
microspore begins before it leaves the pollen-sac. In very few cases
has anything representing prothallial development been observed;
generally a small cell (the antheridial or generative cell) is cut
off, leaving a larger tube-cell. When placed on the stigma, under
favourable circumstances, the pollen-grain puts forth a pollen-tube
which grows down the tissue of the style to the ovary, and makes its
way along the placenta, guided by projections or hairs, to the mouth
of an ovule. The nucleus of the tube-cell has meanwhile passed into
the tube, as does also the generative nucleus which divides to
form two male- or sperm-cells. The male-cells are carried to their
destination in the tip of the pollen-tube.

[v.02 p.0011]

_Pistil and embryo-sac._

The ovary contains one or more ovules borne on a placenta, which is
generally some part of the ovary-wall. The development of the ovule,
which represents the macrosporangium, is very similar to the
process in Gymnosperms; when mature it consists of one or two coats
surrounding the central nucellus, except at the apex where an opening,
the micropyle, is left. The nucellus is a cellular tissue enveloping
one large cell, the embryo-sac or macrospore. The germination of the
macrospore consists in the repeated division of its nucleus to form
two groups of four, one group at each end of the embryo-sac. One
nucleus from each group, the polar nucleus, passes to the centre of
the sac, where the two fuse to form the so-called definitive nucleus.
Of the three cells at the micropylar end of the sac, all naked cells
(the so-called egg-apparatus), one is the egg-cell or oosphere, the
other two, which may be regarded as representing abortive egg-cells
(in rare cases capable of fertilization), are known as synergidae.
The three cells at the opposite end are known as antipodal cells
and become invested with a cell-wall. The gametophyte or prothallial
generation is thus extremely reduced, consisting of but little more
than the male and female sexual cells--the two sperm-cells in the
pollen-tube and the egg-cell (with the synergidae) in th