448 ELEMENTARY BIOLOGY. [CHAP. 



opening of this space 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 stoma, 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 intercellular passages which exist in the sub- 

 stance 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 sur- 

 faces, 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 veins, 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 cylinder. 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. 



