GROWTH. 47 



different in the two main groups of " Exogens " and 

 "Endogens." 



To the former series belong all our trees and shruhs, 

 the clovers, beetroots, turnips, and the vast majority of 

 plants which have the veins of their leaves disposed in a 

 network. In these plants the woody bundles of which the 

 stem is principally made up consist of " wood cells " and 

 " bast cells," with vessels of various kinds; and on the outer 

 side of each bundle is a thin layer of cambium tissue capable 

 of growth, and in virtue of which the woody bundles in- 

 crease on their outer surface. These woody bundles accu- 

 mulate in wedge-like masses, and these again are arranged 

 in concentric rings around the central cellular pith, thus 

 forming the rings visible on the cut surface of the trunk of 

 a tree, one such ring generally indicating, in these lati- 

 tudes, the growth of one season, or at least of one growing 

 period. 



In Endogens, to which all the cereals, and the grasses 

 and almost all plants in which the veins of the leaf run 

 parallel or nearly so, the woody bundles have their cambium 

 tissue in the centre of each bundle, so that their growth in 

 diameter is limited by the pressure of the older tissues 

 outside, and there are no concentric rings in the stem. 

 Indeed, in this country such plants do not produce a woody 

 stem. 



Growth of Leaves. The growing points of leaves occur 

 in various situations, according to the kind of leaf. Some- 

 times and more generally the direction of principal growth 

 is from within outward that is to say, from the centre 

 outward (centrifugal) ; in other cases, the general tendency 

 is in the opposite direction (centripetal). In addition to 



