PTKRIDOPHYTES 



135 



r*^ 



Fig. 303. — Spores of Sela- 



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 poropJiylls. 



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 



megaspores produce female gametophytes. ginella: mature microspore 

 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 

 will be considered under seed plants. 



Male gametophyte. — The male ga- 

 metophyte produced by the microspore 

 is a very simple structure, never out- 

 growing the spore, and therefore en- 

 jacket cells shown surrounding the casec | by the old Spore wall. The tWO 

 groupof sperm mother cells); the dis- he ^ . f ^ 



organized vegetative cell is also shown v - v - ,, ° / 



(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- ,. ,_ . „,, . , ... 



c .u cell (tin. xoz). This single vegetative 



taming free sperms escaping from the v © o 3/ o 



microspore wall. — After Miss Lyon, cell is the sole representative of the 



Figs. 304-307. — Male gametophyte 

 of Selaginella : 304, mature microspore ; 

 303, first division, showing antheridium 

 initial (large cell) and vegetative cell 

 (small lenticular cell against the wall); 

 306, the antheridium (four sterile 



