PTERIDOPHYTES 



135 



spores are microspores, and the large ones are megas pores ; the spo- 

 rangia producing microspores are micro sporangia, 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 FIG ^ _ Spores of <^_ 

 megaspores produce female gametophytes. gineiia: 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 

 FiGs. 3 o4-3o7.-Malegameto P hy t e 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); 1S a very simple structure, never out- 



306, the antheridium (four sterile growing the spore, and therefore en- 

 jacket cells shown surrounding the case ^ b y the old spore wall. The two 

 S^r^Hst:^ cells formed by. 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- , c N ' m, . . , ,. 



tainingiree sperms escaping from the Cel1 (% 3<>5). This Single Vegetative 



microspore wall. After Miss LYON. cell is the sole representative of the 



