PTERIDOPHYTES 
I3S 
spores are microspores, and the large ones are megas pores ; the spo- 
rangia producing microspores are micros porangia, and those producing 
megaspores are megas porangia ; the sporo- 
phylls related to microsporangia are micros po- 
rophylls, and those related to megasporangia 
are megas porophylls. 
This differentiation of spores in size (fig. 
303) is associated with a differentiation in 
function, for upon germination the micro- 
spores produce male gametophytes, while the 
r J FIG. 303. Spores of Sela- 
megaspores produce female gametophytes. gindia: mature mkrospore 
The phenomenon of heterospory, therefore, and megaspore drawn to same 
is associated with the sexual differentiation of 
gametophytes. It is a phenomenon exhibited among pteridophytes 
only by certain groups, but it is universal among seed plants. In 
fact, the appearance of heterospory is the necessary antecedent to the 
formation of a seed. It follows, therefore, that the development of 
heterospory among pteridophytes made 
the great group of seed plants possible. 
In Selaginella there is a remarkable 
approach to the seed condition in the 
fact that the megaspores are not shed, 
but are retained within the megaspo- 
rangium, within which the female 
gametophyte is developed, the egg 
fertilized, and the young sporophyte 
(embryo) formed. Just how far this 
falls short of being the seed condition 
FIGS. 304-307- -Male gametophyte wil1 be considered under seed plants. 
of Selaginella: 304, mature microspore ; Male gametophyte. The male ga- 
305, first division, showing antheridium me tophyte produced by the microspore 
initial (large cell) and vegetative cell . 
(small lenticular cell against the wall); ls a very simple structure, never out- 
306, the antheridium (four sterile growing the spore, and therefore en- 
jacket cells shown surrounding the case( J b y tne Q ^ spore wa H The two 
group of sperm mother cells) ; the dis- . , ,. . . , . 
organized vegetative cell is also shown Cells formed b Y the division of the 
(in black); 307, male gametophyte at protoplast of the microspore are the 
time of fertilization, the antheridium an theridium initial and the vegetative 
(jacket cells have broken down) con- ,,. . _. ... 
tainingfree sperms escaping from the Cel1 (% 3S)- This Single Vegetative 
microspore wall. After Miss LYON. cell is the sole representative of the 
