﻿448 ELEMENTARY BIOLOGY. [CHAP. 



is surrounded by two kidney-shaped 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 stoina, and the two cells are the guard-cells, and, 

 as the stomata are present in immense numbers, there is a 

 free communication between the outer air and the inter- 

 cellular 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 throughout the substance of the plant. 



The vascular bundles break up in the pinnules, and 

 follow the course of the so-called reins, which are visible 

 upon its surface : ducts being continued into their ultimate 

 ramifications. 



The growing point of the stem terminates in a single 

 apical cell, by the divisions and subdivisions of which all the 

 tissues of the stem and leaves are formed. 



Each root presents an outer coat of epidermis, bearing a 

 number of unicellular root-hairs, and enclosing parenchy- 

 matous and sclerenchymatous tissues traversed by a central 

 vascular bundle. The latter contains the same elements as 

 are found in the bundles of the stem, but the xylem and 

 phloem have a different arrangement. The roots, like the 

 stem, develop by means of an apical cell at the growing 

 point, but this is not situated at the extreme end of the 

 organ, as the growing point of the rhizome is, but is covered 

 by a root-cap of protective cells. 



