THE PINE 



are peculiar to the different tissues of the trunk. These 

 tissues are well shown in a section of a stem cut some dis- 

 tance below the growing point 

 (Fig. 67). In the center of the 

 stem is the pith. Just outside 

 this is a ring of vascular bundles, 

 the spaces between which (look- 

 ing like dark lines in the figure) 

 contain cells much like those of 

 the pith. Each vascular bundle 

 is composed of wood and bast. 



The wood is the part of the 

 bundle that is turned toward 

 the center of the stem ; the bast 

 is the part turned outward. 

 The wood carries the sap from 

 the roots to the upper parts of 

 the plant, and the bast carries 

 manufactured food from the 

 leaves to the other organs. 

 Most of the cells of the vascular 

 bundles, especially those of the 

 wood, have very thick walls ; and the strength and stiffness 

 of the stem are due mainly to the thick-walled cells of the 

 wood. Outside the ring of vascular bundles is the cortex, 

 and outside this is the epidermis, which is composed of a 

 single layer of cells. Here and there in the cortex, groups 

 of cells are breaking down to form resin passages cavities 

 which extend for some distance up and down in the stem, 

 and which contain resin. 



141. Growth in Thickness. The way in which the stem 

 grows thicker is shown by a cross section of a part of the 

 stem that has been growing for several years (Fig. 68). After 

 the different tissues have reached the condition already de- 

 scribed, divisions begin to occur in each vascular bundle in 



stem (winter bud) ; a, bud scales ; 

 b, a young spur branch; c, an 

 older spur branch, with the bases 

 of the foliage leaves that it bore. 



