312 



VEGETABLE PHYSIOLOGY 



Growth is in the strict sense, then, always associated 

 with the formation of new living substance, and is very 

 generally accompanied or immediately followed by additions 

 to the framework of the growing cells or organs. It is in 

 nearly all cases attended by a permanent change of form. 

 This is perhaps not so evident in the case of axial organs 

 as it is in that of leaves and their modifications, though 

 even in them it can be detected to a certain extent. It is 

 much more conspicuous in the case of leaves, for the latter, 

 as they expand from the bud, have usually a different 

 shape from that of the adult ones, and the assumption of 



FIG. 134. SECTION OF BLADE OF LEAF, SHOWING THE IRREGULAR CELLS OF 

 THE SPONGY MESOPHYLL ABUTTING ON THE LOWER EPIDERMIS. 



the mature form is a gradual process, taking place as the 

 age of the leaf increases. 



This change of form can be seen not only in the case 

 of an organ such as a leaf, but also in that of the indi- 

 vidual cells of which a plant consists. In the apical meristem 

 of the root of a flowering plant the cells when first formed 

 are almost cubical (fig. 183) ; after a little while we find 

 many of them becoming elongated, and ultimately prosen- 

 chymatous. Many other cases can be noted, particularly the 

 irregularly shaped cells of the spongy parenchyma of leaves 

 (fig. 134), the stellate cells of the pith of certain rushes 

 (fig. 135), the laticiferous cells of the Spurges, &c. 



Growth may, in the light of the considerations just 

 advanced, be defined as permanent increase of bulk, attended 



