GROWTH. 49 



and " bast cells/ 7 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 increase on their outer surface. These 

 woody bundles accumulate 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 latitudes, 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 cam- 

 bium 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 pro- 

 duce a woody stem. 



Growth of Leaves, The growing points of leaves oc- 

 cur in various situations, according to the kind of leaf. 

 Sometimes 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 these growing points at definite spots, where 

 new cells are always forming during the active period, 

 new growth may occur in isolated spots by the formation 

 of growing cells in the midst of or between others that 

 have lost their faculty of growth, and thus growth in the 

 substance of the plant may take place by intercalation as 

 well as at the extremities. 



To repeat, then, true growth consists in the formation 

 of new protoplasm from the old, and in the division of 

 the protoplasm into new cells. This division takes place 

 especially and primarily, so far as growth in length is 



