ro ee 
= - 
>a 
. ‘ 
SPERMATOPHYTA 409 
Megasporangium and female prothallus. In the cycads the 
megasporangium consists of the nucellus and a single integument 
inclosing the nucellus, except that at the apex there is a small 
opening, the micropyle (Fig. 461). Within the 
megasporangium there is produced a mega- 
spore mother cell which divides to form a row 
of three or four megaspores (Fig. 463). All 
except one of these degenerate and disappear, 
while the functional mega- 
spore germinates within 
the megasporangium (Fig. 
461) and produces a large, 
oval female prothallus (Fig. 
464). ‘This prothallus is re- 
tained permanently within 
the megasporangium. As 
Fig. 458. Under the prothallus matures, it 
surface of micro- 

produces archegonia, which 
proweeny oF Cyeas” are usually in a group be- 
many microsporan- low the micropyle (Tig. 
gia. (x 14) 464). The archegonium 
of the cycads consists of 
two neck cells and a large egg cell (lig. 465). 
The neck-canal cells found in bryophytes and 
pteridophytes are lacking, while the ventral- 
canal cell is represented by an evanescent 
nucleus which, with the surrounding cyto- 
plasm, disorganizes soon after its formation 
(Fig. 466). This evanescent nucleus appears y. 459. E een 
to have no function, but represents the sur-  jyicrosporophylls of 
vival of an ancestral characteristic. Zamia floridana. (x1) 
Male prothallus and fertilization. Before 
being shed the microspore (pollen grain) germinates and pro- 
duces a male gametophyte, or prothallus, within the microspore 
(Fig. 468). This gametophyte consists of a sterile prothallal cell, 
a generative cell, and a tube cell. In this three-celled condition 
rumphi, showing 

