60 ELEMENTARY BIOLOGY. [CHAP. 



follow the course of the so-called veins which are visible 

 upon its surface; ducts being continued into their ultimate 

 ramifications. 



The rootlets present an outer coat of epidermis, enclosing 

 parenchyma traversed by a central vascular bundle. They 

 increase in length by the division and subdivision of the 

 cells at the growing point, but this point is not situated at 

 the very surface of the rootlet, as the growing point at the 

 extremity of the rhizome is, but is covered by a cap of cells. 



When the spores are sown upon damp earth, or a tile, 

 or a slip of glass, and kept thoroughly moist and warm, 

 they germinate. Each gives rise to a tubular, hypha-like 

 prolongation, which developes a similar process, the primi- 

 tive rootlet, close to the spore. The hypha-like prolongation, 

 at first, undergoes transverse division, so that it becomes 

 converted into a series of cells. Then, the cells at its free 

 end divide longitudinally, as well as transversely, and thus 

 give rise to a flat expansion, which gradually assumes a 

 bilobed form, and becomes thickened, in some parts, by 

 division of its cells in a direction perpendicular to its 

 surface. The protoplasm of these cells developes chlorophyll 

 granules, whereby the bilobed disk acquires a green colour ; 

 while numerous simple radicle fibres are given off from 

 its under surface, and attach the little plant, which is 

 termed a prothallus or protkallium^ to the surface on which 

 it grows. 



The prothallus attains no higher development than this, 

 and does not directly grow into a fern such as that in which 

 the spores took their origin ; but, after a time, rounded or 

 ovoidal elevations are developed, by the outgrowth and 

 division of the cells which form its under aspect. Some of 

 these are antheridia. The protoplasm of each of the cells 

 contained in their interior is converted into an antherozooid, 



