I4C 



MoKI'HOLOGY 



cells contribute to the nutrition of 40-75 mother cells that function. 

 These mother jells become relatively very large and produce large 



spores, the total output of megaspores being 



150-300 (fig. 317). Both kinds of spores 



escape by the decay of the sporangium 



wall. 

 Male gametophyte. — The male gametophyte 



is still more simple than that of Selaginella (figs. 



318-320). It consists of a single vegetative 



cell and a single antheridium, as in Selaginclla ; 



but the sperm mother cells are only four in 



number. This is the lowest number reached 



among pteridophytes, and the nearest approach 



to seed plants, among 



which the sperm mother 



cells are reduced to two. 



The sperms are large, 



spirally coiled, and 



multiciliate (fig. 320), 



such as characterize all 



other pteridophytes ex- 



taining a sperm) are free cept Lycopodiales. It 

 in the antheridial cavity; fe this feature of 7w/« 

 320, sperm. — After Bela- , , 



JEFF> that perhaps presents 



the greatest obstacle to 

 including it among Lycopodiales. 



Female gametophyte. — The female gameto- 

 phyte develops in the same general way as does 

 that of Selaginella, but with some interesting 

 differences. After the free nuclear divisions, 

 followed by the formation of an apical tissue, a « exposed through . 



, in the megasporc wall 



layer of cells develops completely about the (not s hown); 322 (after 

 cavity of the megaspore and then grows centripe- Campbell), archegonium, 

 tally until the megaspore is filled with tissue showing two tiers of neck 



01 cells and an axial row of 



(hg. 321). The growth of this tissue is precisely t h re e cells (neck canal 

 like the growth of the endosperm tissue (female cell, ventral canal cell, 

 gametophyte) in gymnosperms (p. 196 ). The and egg) ' 

 gametophyte of Isoetes does not protrude through the broken megaspore 

 wall and develop tissue outside, as in Selaginella, but is exposed only 



Figs. 3 1 8-320. — Male 

 gametophyte of Isoetes: 318, 

 stage showing the vegetative 

 cell (at bottom and without 

 nucleus) and the antheridium 

 (with three wall cells visible 

 and the central primary sper- 

 matogenous cell) ; 319, older 

 stage, in which the wall cells 

 have broken down and the 

 four mother cells (each con- 



Figs. 321, 322. — Fe- 

 male gametophyte of 

 Isoetes: 321 (after Hof- 

 MEISTER), complete ga- 

 metophyte, bearing an 

 archegonium whose neck 

 . crack 

 in 



