STRUCTURE OF PLANTS. 49 



the atmosphere, and expires gases and watery vapour, are 

 opened wider or contracted as may be required. The 

 orifices with the crescentic cells are called stomates, and 

 the whole layer in which they occur is the epidermis of the 

 plant (PI. i. Fig. 12). 



In every actively vegetating part of a plant exists a 

 continuous influx of new nutrient matter, which is absorbed 

 by the roots while its superfluous water is evaporated 

 through the stomates. This movement of the sap trans- 

 forms the tracts of cells through which it passes, moving 

 with especial activity in the elongated cells. Most of them 

 become very much thickened, some also lose all at once 

 their fluid contents, and receive air instead ; such are called 

 vessels (air-vessels), and thus are formed, in the mass of 

 cellular tissue, bundles of elongated cells and vessels, called 

 vascular bundles (PI. i, Fig. 13, 6), which to the naked eye 

 look like dense fibres running through the tissue of the 

 plant. In one great division of plants, in the Monoco- 

 tyledons, to which the Grasses, Lilies, Palms, &c., belong, 

 the development of these vascular bundles stops short at a 

 certain stage, and they undergo no further alteration. In 

 another class, on the contrary, in the Dicotyledons, to which 

 belong our forest-trees, kitchen vegetables and many 

 others, there is a continuous development of cells on the 

 outer side of each vascular bundle, which become in turn 

 vascular bundle cells, and so unceasingly increase the thick- 

 ness of the bundles. In consequence of this, the bundles 

 gradually close up together into a firm tissue, into that 

 which in common life we call wood (PI. n. Figs. 8, 9, 10). 



When we seek to discover the relation in which these 

 three parts of the plant stand to the wants of Man, here 

 again we find a threefold distinction. In its common 

 condition, the epidermis is useless, but in perennial plants, 



W r- ->*., 



