410 A TEXTBOOK OF GENERAL BOTANY 

Fria. 460. Cone of microsporophylls 
of Cycas 

Fig. 461. Longitudinal section through 
megasporangium of Cycas rumphii 
The megasporangium consists of the nucel- 
lus surrounded by an integument; within 
the nucellus is a young, rounded, female 
prothallus. (x 20) 
the pollen grains are shed. 
The pollen grains are ear- 
ried by the wind and enter 
the megasporangia through 
the micropyles. Here the 
pollen grain sends out a 
pollen tube which grows 
from the tube cell, becomes 
embedded in the nucellus, 
and, by elongating, pushes 
the prothallial and gen- 
erative cells through the 
nucellar region toward the 
archegonia (Figs. 465, 468). 
The generative cell later gives 
rise to a sterile stalk cell and 
a large body cell (Figs. 465, 
468). The latter produces 
two spermatozcids (Fig. 468). 
These are large ciliated struc- 
tures which have the ability 
to swim. The pollen tube 
approaches an archegonium 
and discharges the spermato- 
zoids, one of which enters 
the archegonium and fuses 
with the egg cell (Fig. 466). 
The ciliated spermato- 
zoids in a highly developed 
land plant show the survival 
of a character acquired by 
aquatic ancestors. 
Seed. The fertilized egg 
develops and produces an 
embryo which remains em- 
bedded in the prothallus 
