58 ELEMENTARY BIOLOGY. [vil. 



singularly modified in form, their walls being thrown out 

 into lobes, which interlock with those of adjacent cells. 



Between many of these cells an oval space is left, forming 

 a channel of communication between the interior of the 

 frond and the exterior. The opening of this space is sur- 

 mounted by two reniform cells, the concavities of which 

 are turned towards one another, while their ends are in 

 contact. The opening left between the applied concave 

 faces is a stomate ; and, as the stomates are present in 

 immense numbers, there is a free communication between 

 the outer air and the intercellular passages which exist 

 in the substance of the frond. Those cells of the green 

 parenchyma of the frond which form the inferior half of its 

 thickness, in fact, are irregularly elongated, and frequently 

 produced into several processes, or stellate. They come into 

 contact with adjacent cells only by comparatively small 

 parts of their surfaces, or by the ends of these processes. 

 They thus bound passages between the cells, intercellular 

 passages, which are full of air, and are in communication 

 with similar, but narrower, passages, which extend through- 

 out the substance of the plant. 



The vascular bundles break up in the pinnules, and 

 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, prolonga- 



