Vegetable Pliysiology. 377 



which run through a peculiar course of development of their 

 own. In our ordinary trees the cambial tissue is not confined to 

 the growing points or buds, but it extends as a layer all over the 

 surface of the stem beneath the bark, forming the seat of the 

 annually -repeated development of wood by which the plants of 

 this order acquire their often so enormous dimensions. 



In the cells left behind by the advance of the formative energy 

 in the growing points, or detached from the main body as the 

 foundations of lateral organs (leaves, &c.), it next becomes a 

 question of the first importance, whether they are destined to form 

 the seats of active physiological processes, or are to form parts 

 of the structures by which the physiologically active tissues are 

 protected or supported, or through the medium of which a com- 

 munication is to be established between the active cells of the 

 different organs and regions. If they are to be devoted to the 

 first office, their physical characters undergo comparatively slight 

 changes : increase of size, and perhaps alteration of form, being 

 the principal affections to which they are subject. Tliey like- 

 wise continue to possess the formative and assimilated matters 

 which existed in the young cells ; and indeed their life is essen- 

 tially devoted to the production and conversion of these sub- 

 stances, to supply the waste taking place in the cambium 

 regions, or in tlie formation of fruits, and to furnish material for 

 the consolidation of the permanent structures of the plant. This 

 "parenchymatous tissue forms the mass of aliuost all the structures 

 most important to agriculturists ; and in root culture and green 

 crops especially, it is the object of cultivation to increase its 

 quantity relatively to the other tissues. The soft, succulent sub- 

 stance of leaves, of herbaceous stems, and of tuberous roots, of 

 pulpy fruits, &c., is mainly composed of suph tissue ; and while 

 in a state of active vegetation, its cells are always loaded with 

 the assimilated matters, rendering these productions highly nutri- 

 tious to animals. 



It is important, however, to notice that these cellular or paren- 

 chymatous tissues are transitory structures. In the natural course 

 of events leaves fall, tuberous roots shrivel up, and succulent 

 fruits fall off and rot. They are developed as temporary nourish- 

 ing agents, or as reservoirs for accumulated nutritive substance ; 

 and nature is too economical to leave the acquired stores in them 

 after the duty is performed. Long before leaves fall off the chief 

 proportion of the assimilated matter is removed from them : the 

 roots of the turnip and beet contain very different qualities of 

 cell-contents in the months before and after their winter rest. And 

 even during the active seasons of growth, if the development, 

 but above all the expansion, of the succulent tissues is stimulated 

 by free supply of heat and moisture, without a proper amount of 



