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 

 FIGS. 318-320. Male to seed p i antS) among 



gametophyte of Isoetes: 318, , . , , i 



stage showing the vegetative which the Sperm mother 



cell (at bottom and without cells are reduced to two. 



nucleus) and the antheridium The sperms are large, 



(with three wall cells visible 



and the central primary sper- Spirally COlled, and 



matogenous cell); 319, older multiciliate (fig. 320), 



stage, in which the wall cells 



have broken down and the 



four mother cells (each con- 



such as characterize all 

 other pteridophytes ex- 



taining a sperm) are free cept Lycopodiales. It 



in the antheridial cavity; j s this f ea ture of Isoetes 



320, sperm. After BELA- 



JEFF> that perhaps presents 



the greatest obstacle to 

 including it among Lycopodiales. 



322 



FIGS. 321, 322. Fe- 

 gametophyte of 



Female gametophyte. The female gameto- male 



phyte develops in the same general way as does Isoetes: 3" ( after HOF - 



r J r MEISTER), complete ga- 



that of Selagmella, but with some interesting met ophyte, bearing an 



differences. After the free nuclear divisions, archegonium whose neck 



followed by the formation of an apical tissue, a f exposed through a crack 



J ' m the megaspore wall 



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



cavity of the megaspore and then grows cen tripe- CAMPBELL), archegonium, 



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



3 cells and an axial row of 



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



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



gametophyte) in gymnosperms (p. 196 ). The an egg) * 

 gametophyte of Isoetes does not protrude through the broken megaspore 

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



