334 THE PTERIDOPHYTES 



324. The cones of Selaginella. The cones of Selaginella are 

 not as large as those of Lycopodium, but they are much more 

 complex in structure. The sporangia are of two sorts, both 

 developing singly from a group of cells on the stem just above 

 the origin of the spore leaves and later becoming attached to 

 their bases. The sporangia near the lower part of the cone 

 (Fig. 289, B) produce from one to eight very large megaspores, 

 and frequently a group of four (tetrad). The sporangia higher 

 up on the cones (Fig. 289, D) are smaller and develop a great 

 number of minute microspores, also in tetrads. Selaginella has, 

 then, different sporangia for the two forms of spores, microspores 

 and megaspores, which are accordingly called microsporangia 

 and megasporangia. Furthermore, these sporangia are borne 

 upon different spore leaves, which are consequently termed 

 microsporophylls and megasporophylls. It is important to note 

 that the few megaspores which mature are nourished and grow 

 at the expense of neighboring spore mother cells which become 

 disorganized. 



325. The gametophytes of Selaginella. The microspore 

 develops a reduced and degenerate male gametophyte, as in 

 Marsilia (Sec. 313). There is a small sterile cell (prothallial 

 cell) and two groups of sperm cells in a very simple cellular 

 structure probably representing an antheridium (Fig. 290, A). 

 The sperms are two-ciliate (Fig. 290, B). 



The megaspore develops a female gametophyte which is larger 

 than that of Marsilia, but it is the same sort of a reduced struc- 

 ture, dependent upon food stored by the sporophyte within the 

 megaspore. This gametophyte at maturity fills the megaspore, 

 and bursting through the spore wall it presents an exposed sur- 

 face upon which several sunken archegonia are developed (Fig. 

 290, (7). The female gametophyte actually begins its develop- 

 ment before the megaspore has attained its full size in the mega- 

 sporangium. It is thus parasitic upon the sporophyte during its 

 early history, a habit which is universal in the seed plants, but 

 among the pteridophytes it is only found in Selaginella. 



