448 GYMNOSPERMS LESS. 



The trunk is continued downwards by a great primary root, 

 from which secondary roots arise in regular order, and, these 

 branching again and again, there is produced a root-system 

 of immense size and complexity, extending into the soil to a 

 sufficient depth to resist the strain to which the aerial part or 

 the tree is subjected by the wind. 



One remarkable feature about the pines and their allies as 

 compared with the plants previously studied, is their practi- 

 cally unlimited growth. In mosses, ferns, &c., the stem 

 after attaining a certain diameter ceases to grow in thick- 

 ness, so that even in the tallest tree-ferns the stem is always 

 slender. But in pines the trunk, the branches, and the 

 roots continue to increase in thickness for an indefinite 

 period, the trunk in the common Scotch Fir (Pimts 

 sylvestris) attaining a circumference of four or five metres 

 or even more, and the other parts in proportion. The tree 

 may survive for hundreds of years. 



The changes undergone during this remarkable process of 

 growth are best studied, in the first instance, by a series of 

 rough transverse sections of branches of different ages. In a 

 first year's branch the middle is occupied by an axial strand 

 of soft tissue, the pith or medulla (Fig. 118, A and B, med) ; 

 outside this comes a ring of wood (xy), divided into radially 

 arranged wedge-shaped masses; and this in turn is sur- 

 rounded by the bark or cortex (cor\ which can be readily 

 stripped off the wood, and which 'contains numerous resin- 

 canals (r. c) appearing in the section as rounded apertures 

 with drops of resin oozing from them. In a somewhat older 

 branch the layer of wood is seen to have increased greatly 

 in thickness, and has a well-marked concentric and radial 

 striation (c) : the cortex also has thickened though to a less 

 extent, while the pith is unaltered. The bark, moreover, is 

 clearly divisible into an inner light coloured layer, the bast 



