450 GYMNOSPERMS LESS. 



wood. The concentric markings, which are against the 

 grain, are the annual rings (xy 1 , xy' 2 , xy 3 ), and owe their 

 existence to the fact that the wood formed in summer and 

 autumn is denser than that formed in spring, while in winter 

 there is a cessation of wood-production. Thus, by counting 

 the annual rings of the main trunk, the age of the tree may 

 be estimated. The wood, it will be observed, grows from 

 within outwards, a new layer being added each year outside 

 the old. 



The power of indefinite increase in diameter, which is so 

 striking a feature in the pines and their allies, is connected 

 with a peculiarity in the structure and arrangement of the 

 vascular bundles. In the very young condition, i.e., in the 

 terminal bud, the vascular bundles of the stem (Fig. 118, A) 

 are wedge-shaped in transverse section and are arranged in 

 a circle, the apex of each being turned towards the axis of 

 the stem, the base towards its periphery. Actually, of course, 

 as in the fern, the bundles are longitudinal strands with pro- 

 longation into the leaves. 



The arrangement of the tissues in the vascular bundles 

 differs in an important respect from the condition we are 

 familiar with in the fern. Instead of the xylem occupying 

 the centre of the bundle and being surrounded by phloem, 

 the xylem (Fig. 118, A, xy) forms the whole of the in-turned 

 side, i.e., the narrow portion of the wedge in transverse sec- 

 tions, the phloem (phi) the outer portion or broad end of 

 the wedge. In a word, the bundles are not concentric as in 

 the fern, but collateral. Moreover, the phloem and xylem 

 are separated by a layer of small thin-walled cells, called the 

 cambium layer (cb). 



By this arrangement of the vascular bundles the ground- 

 parenchyma of the stem is divisible into three portions, an 

 external layer, the cortex (cor), between the epidermis (ep), 



