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SPERMATOPHYTA 
427 
of the seed. The embryo is a cylindrical structure with a num- 
ber of cotyledons. The integument forms the seed coat (Fig. 454). 
Alternation of generations. In the conifers the alternation of 
generations is very similar to that described 
for the cycads. The sporophyte is a large 
plant which produces megaspores and mi- 
crospores. The megaspore gives rise to a 
female gametophyte, or prothallus, bearing 
archegonia, while the microspore produces 
a male gametophyte. The fertilization of 
the egg in the archegonium results in the 
production of a sporophyte. There is, thus, 
as in the Bryophyta and the Pteridophyta, 
an alternation of a sporophytic and a game- 
tophytic generation. As in_ bryophytes, 
pteridophytes, and cycads, the sporophyte 
is characterized by a double number of 
chromosomes, reduction taking place dur- 
ing the division of the spore mother cells. 
The gametophyte has the single number of 
chromosomes, the double number being re- 
stored at fertilization. 
Relationship. The Coniferales appear to 
be descended from an extinct order of 
gymnosperms, the Cordaitales, which either 
were derived from or were closely related 
to the Cycadofilicales. As the Cycadofili- 
cales gave rise to the Cycadales, the Conifer- 
ales are related through the Cycadofilicales 
to the Cycadales. The cycads, as we have 
seen, are much more primitive than the 
conifers. A diagrammatic representation of 

Fic. 483. Pollen tube 
of pine 
Below, near the apex, 
is the tube nucleus. 
Above, and still within 
the spore wall, is the 
stalk cell next to the 
spore wall and, adjoin- 
ing this, the body cell. 
(Redrawn after Miss 
Ferguson) 
the relationships of these orders is clearly shown in Fig. 474. 
Distribution. There is a striking difference in the distribution 
of cycads and of conifers. The former are tropical and subtropi- 
cal, while the latter are found largely in temperate zones. They 
