136 MORPHOLOGY 



vegetative tissue of more primitive gametophytes. The antheridium 

 initial produces an antheridium with the usual jacket of sterile cells 

 investing sperm mother cells (fig. 306). At maturity the jacket cells 

 break down and the mother cells (with their sperms) are free in 

 the general cavity of the microspore (fig. 307). The male gameto- 

 phyte, therefore, is reduced to one vegetative cell and one anthe- 

 ridium ; and encased by the old microspore wall it is carried to the 

 megasporangium, in which the female gametophjrtes are developing. 

 There the male gametophyte bursts through the microspore coat (fig. 

 307). The sperms are very small, with more or less spirally coiled 

 bodies and two terminal cilia. Selaginella thus shares with Lycopodium 

 and Phylloglossum the character of producing biciliate sperms, a type 

 characteristic of bryophytes, and in strong contrast with the sperms 

 produced by other pteridophytes. 



Female gametophyte. — The female gametophyte is much more ex- 

 tensive than the male gametophyte, but the greater part of it is in- 

 vested by the old megaspore wall (fig. 

 308). The nucleus of the megaspore 

 begins a series of divisions that continue 

 until a large number of free nuclei are 

 produced. This free nuclear division 

 occurs chiefly in the apical (pointed) end 

 of the megaspore, and results in a layer 

 of nuclei, which later become invested by 

 walls. Subsequent divisions result in a 

 cushion of cells at the apex of the mega- 



r Fig- 308. -Female gametophyte ^j^jj^ ^^^ ^ ^. ^j ^^^ ^ 



of Selaginella: the apical cushion ^ '_ o j o 



of ceUs having broken through the spore is free from cells, acting as a great 



heavy megaspore wall; an arche- food reservoir (fig. 308). The wall of 



-IZ M^°™'' " ""' ^^''' ^^^ megaspore cracks at the apex and 



the apical tissue protrudes, developing a 

 more or less expanded mass of tissue in which archegonia develop 

 (figs. 308, 309). Later, the deeper region of the megaspore becomes 

 filled with a tissue of large cells, and continues to act as a food reservoir 

 for the developing embryo. This early differentiation of the female 

 gametophyte into two distinct regions, one that produces archegonia, 

 and the other nutritive, is a marked feature of the female gametophyte 

 in all heterosporous plants. 

 Fertilization. — The male gametophytes enclosed by the old micro- 



