vii.] THE BRACKEN FERN. 59 



walls, and terminating in a point at each end. These 

 sclerenchymatous/#ra- have oblique cleft-like clear spaces, 

 produced by interruptions of the process of thickening in 

 their walls. 



The vascular bundles, the green parenchyma, and the 

 epidermis are continued into each pinnule of the frond. 

 The epidermis retains its ordinary character on the upper 

 side of the pinnule, except that the contours of its com- 

 ponent cells become somewhat more irregular. On the 

 under side, many hairs are developed from it, and the 

 cells become singularly modified in form, their walls being 

 thrown out into lobes, which interlock with those of adja- 

 cent 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 stomata 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 



