a 
CHAPTER XVIII 
PHYLUM XII. CYCADOPHYTA 
THE CYCADS 
484, Like the two preceding phyla this one is a mere 
remnant of a much larger group. All told there are only 
about 140 living species belonging to six families, while 
we know of as many more families whose species have 
-become extinct. Enough has been made out as to the 
structure of living and extinct forms to enable us to 
define the Cycad phylum as follows: 
485. Their archegonial gametophytes are so dependent 
that they are enclosed in the megaspore, which is itself 
retained in the sporangium; the antheridial gametophyte 
is minute and free, and its tubular antherid typically 
develops two or more multiciliated sperms; after fer- 
tilization of the egg the megasporangium becomes a 
“‘seed.”? The sporophyte is first enclosed in the seed, 
where it is nourished by the gametophyte, and later it 
escapes by developing roots below, and expanding its 
leaves above; eventually some leaves become sporophylls 
and develop microspores and megaspores. 
486. It is instructive here to compare the higher 
Lycopods with the Cycads. In both there are micro- 
spores and megaspores, and in both the microspores 
always are set free from the sporangium. In both again 
the microspore produces a very small (one- to few-celled) 
gametophyte. However, the antherid of the higher 
Lycopods is a few-celled structure, with many minute, 
biciliated sperms, while in the Cycads the antherid is 
271 
