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 
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Fic. 134.—Srction oF BLapE or LEaF, SHOWING THE es ne OF 
THE Sponay 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. 188); 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. 184), the stellate cells of the pith of certain rushes 
(fig. 185), 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 
