140 



MORPHOLOGY 



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

 These mother cells 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 Selaginella; 



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; j s this feature o f IsoeteS 

 320, sperm. — After Bela- f ———~-^ ___ 



jj.j,j.^ that perhaps present s 



the greates t obstacle to 

 including it amon g Lycopod iales. 



Feioaie gametophyte. — The female gameto- 

 phyte develops in the same general way as does 



Figs. 318-320. — 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. 32r, 322. — Fe- 

 male gametophyte of 

 Isoetes: 321 (after Hor- 

 . . . meister), complete ga- 



tnat ot jelagwetla, but with some mterestmg metophytc, bearing an 



differences. After the free nuclear divisions, archegonium whose neck 



followed by the formation of an apical tissue, a 



layer of cells develops completely about the 



cavity of the megaspore and then grows centripe- 



tally until the megaspore is filled with tissue 



(fig. 321). The growth of this tissue is precisely three" cells "(neck canal 



like the growth of the endosperm tissue (female <;ell. ventral canal cell, 



gametophyte) in gymnosperms (p. 196 ). The ^'^ ^^^'' 



gametophyte of Isoetes does not protrude through the broken megaspore 



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



is exposed through a crack 

 in the megaspore wall 

 (not shown); 322 (after 

 Campbell), archegonium, 

 showing two tiers of neck 

 cells and an axial row of 



