172 THE MORPHOLOGY OF PTERIDOPHYTES 



initials abort in the one case (Fig. 27B) and the mega- 

 sporangial initials abort in the other. In both types of 

 sporangium there is an abundance of mucilaginous 'peri- 

 plasmodium', which becomes organized into 'massulae'. In 

 the megasporangium there are four such massulae, in one 

 of which the single megaspore is buried. Fig. 27C illustrates 

 the dehiscence of a megasporangium, the apex of which is 

 cast adrift as a cap over the four massulae. The megaspore 

 then germinates to produce a cap of prothallial tissue, 

 within which several archegonia develop (Fig. 27D). 



When the microsporangium dehisces a variable number of 

 spherical frothy massulae are liberated, each with several 

 microspores near the periphery. Each bears a large number 

 of peculiar anchor-hke 'glochidia' (Fig. 27E). These become 

 entangled with the massulae surrounding a megaspore and, 

 together, they sink to the bottom, where the microspores 

 germinate, without being released from the massulae. The 

 male gametophyte (Fig. 27F) has a single antheridium from 

 which eight antherozoids are liberated. 



The cleavage of the zygote is typical of the leptosporangi- 

 ate ferns, and as soon as the first leaf appears the sporehng 

 rises, carrying the massulae etc. once more to the surface. 



There are about twelve species of Salvinia, several of 

 which occur in Africa. Its horizontal floating stems, up to 

 10 cm long, have a much reduced vascular anatomy and 

 bear leaves in whorls of three (Fig. 27G), two floating and 

 one submerged. Whereas the floating leaves are entire and 

 covered with pecuHar unwettable hairs, the submerged 

 leaves are finely divided into linear segments that bear a 

 striking resemblance to roots (Fig. 27H). However, it is 

 doubtful whether they perform the functions of roots. 

 Growth is rapid and fragmentation occurs easily, with the 

 result that ponds and lakes in tropical regions may rapidly 

 become covered and canals choked. 



The first few sporocarps to be formed in each cluster 

 contain megasporangia, up to twenty-five in each, and the 



