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VEGETABLE PHYSIOLOGY 



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 

 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- 



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

 THE SPONGY MESOPHYLL ABUTTING ON THE LOWER EPIDERMIS. 



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. 133) ; after a little 

 while we find many of them becoming elongated, and 

 ultimately prosenchymatous. 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 by permanent change of form. We must not 

 assume that every increase of bulk is necessarily growth ; 



