GROWTH. 49 
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 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 
