DIV. I 



PTERIDOPHYTA 



515 



the venter of the archegonium (Fig. 481). The first leaf of the germ plant is 

 shield-shaped (Fig. 478 C] and floats on the surface of the water. 



The development of Azolla ( 121a ) proceeds in a similar manner, but the sporangia 

 and spores exhibit a number of distinctive peculiarities. The micro- and macro- 

 sporocarps at first develop alike ; in each a single macrosporangium is laid down 

 surrounded by the tubular indusium, and from the stalk of the macrosporangium the 

 microsporangia grow out. In the microsporocarp only the microsporangia develop ; 

 in the macrosporocarp, on the other hand, only the m Jtcrosporangium becomes mature. 



p J* 



FIG. 480. Sodrinianatans. Development 

 of the male prothallium. A, Division 

 of the microspore into three cells 

 7-777 ( x 860) ; B, lateral view ; C, ven- 

 tral view of mature prothallium (x 

 640). Cell 7 has divided into the pro- 

 thallium cells a and p ; the latter is 

 the rhizoid cell; cell 77 into the 

 sterile cells ft, c, and the two cells s 1? 

 each of which has formed two spenna- 

 tozoid mother-cells ; cell 777 into the 

 sterile cells d, e, and the two cells so. 

 The cells s^ and s.x< 2 represent two 

 antheridia ; the cells b, c, d, e, their 

 wall cells. (After BELA.JEFF.) 



FIG. 481. Salnnia natans. Embryo in longitudinal sec- 

 tion ; pr, prothallium ; S, spore-cell ; e, exinium ; p, 

 perispore ; spw, sporangial wall ; ar, archegonium ; 

 embr, embryo ; /, foot ; W], Wo., ^s> the first three 

 leaves ; st, apex of stem, (x 100. After PRIXGSHEIM.) 



The 64 spores of the microsporangia are aggregated into several nearly spheri- 

 cal balls or massulae, formed from the interstitial substance derived from the 

 protoplasm of the tapetal cells. Each massula, enclosing a number of spores, is 

 beset externally with barbed, hook-like outgrowths of the interstitial substance 

 (glochidia). On the rupture of the sporangia the massulae are set free in the 

 water, and are carried to the macrospores, to which they become attached. In 

 the macrosporangium 32 macrospores are laid down, but only one comes to maturity ; 

 in the course of its development it supplants all the other sporogenous cells, and 

 finally the sporangial wall itself becomes flattened against the inner wall of the 

 sporocarp, frequently undergoing at the same time partial dissolution. The 

 macrospore is enveloped by a spongy perispore, whose outer surface exhibits 



