258 THE SECOND BOOK OF BOTANY. 



formed on the under surface of the prothallus, but 

 most abundantly near the centre, among the rootlets 

 (Fig. 352). They are composed of one or two cells, 

 developed from the lower face of one of the cells of 

 the prothallus. Within these cells another is formed, 

 in which soon appear a number of minute vesicles, 

 called sperm-cells. When mature, the top of this 

 antheridial cell falls off, like a lid, and the sperm- 

 cells escape. Each of these, when ejected from the 

 antheridium, emits an anthrozoid, a minute, ciliated 

 body, which has the power of spontaneous move- 

 ment (Fig. 414). It is by the passage of these an- 

 therozoids down the canal of the pistillidia that the 

 corpuscle of the embryo-sac is fertilized. From the 

 embryo thus produced, the young fern is developed, 

 which, at maturity, produces sporanges and spores. 



EXERCISE LXXIX. 

 The Movements of Plants. 



It is usually considered that one of the prime dis- 

 tinctions between animals and plants is, that the for- 

 mer have the power of spontaneous motion, while 

 the latter do not. But plants do manifest this faculty 

 in various ways, and in a quite remarkable degree. 

 It is seen in the very simplest forms of plant-life. 

 These are the Algse, the lowest class in the vegetable 

 kingdom, to which sea-weeds and fresh-water confer- 

 vae belong. The mode of reproduction of the algae 

 is obscure ; but, in certain fresh-water kinds, it takes 



