STRUCTURE OF STEMS 59 



and often cracked into deep furrows. This is the true bark. It 

 is formed by a layer of cells on the outside of the bast portion called 

 cork producer. The soft " bark " lies underneath the coarse dead 

 corky bark in the old stems. In the spring, the soft bark can be 

 very easily stripped off from the stems, as in the willow, basswood, 

 etc. The tissue where the bark parts from the stem when stripped 

 off in this way is the young and delicate cambium, which, we 

 found in paragraph 98, forms a continuous layer entirely around the 

 stem between the bast and the wood of the bundle. The portion 

 of the stem lying inside of this layer then is the wood, except the 

 central portion or pith.* The wood portion of old trees consists 

 of a whitish outer portion called the sap wood, while the darker 

 inner portion is the heart wood. The heart wood is dead, but 

 in the sap wood there are many living cells and it is here that the 

 rise of water in the tree takes place. No rise of water takes place 

 in the dead heart wood. There are three peculiarities of the woody 

 portion of such a stem which are visible to the eye. First, the 

 slender whitish lines which radiate from or near the pith to the 

 outside. These are the medullary rays or pith rays which (para- 

 graph 96) lie between the nbro-vascular bundles. They consist 

 of parenchyma cells which are alive in the sap wood and usually 

 dead in the heart wood.f The cells of the pith rays are very much 

 elongated radially; they are flattened by the lateral pressure of the 

 bundles and they present the smooth shining surfaces in radially 

 split wood: second, the porosity of the wood, w r hich appears to the 

 eye (unless it is a very hard compact heavy wood) to have numer- 

 ous minute pores: third, the presence of numerous concentric 

 rings, called annual rings. 



100. Growth in thickness and the formation of annual 

 rings. In woody stems the nbro-vascular bundles lie very closely 

 side by side so that the woody part of one bundle practically 

 touches that of the two adjacent ones. They do not quite touch, 



* The pith varies greatly in extent in different trees and shrubs. In 

 some it is very abundant, as in the elder, sumac. It may be continuous, 

 chambered or diaphragmed, etc., in different species. 



t In some trees the pith ray cells remain alive for many years. 



