THE STEM 95 



the leaf-scars on the branches of our trees. The darker, knob- 

 like processes which are seen on the smooth surface of the 

 leaf-scar are the brpken ends of so many vascular bundles, 

 which passed from the stem to nourish the leaf. Each of 

 these bundles can be traced down in the stem, passing several 

 older leaves, until it becomes more and more delicate, and 

 finally splits up between the bundles of the older leaves, its 

 elements passing into those of the lower lying bundles. 



If we examine a branch which has been cut across in the 

 region where it is beginning to get woody, we will notice in 

 the middle of the cut surface a soft parenchymatous tissue, 

 which we call the pith or medulla. This is surrounded by a 

 firm, white, and radially divided ring termed the wood-cylinder, 

 the radially running lines being the medullary rays.^ The 

 wood-cylinder consists of thick-walled elongated cells, which 

 interlock with their pointed ends, and are termed the wood- 

 flbres (libriform fibres). Between these are found the actual 

 wood-vessels. The medullary rays, however, consist of paren- 

 chymatous cells, which are often elongated in the radial direc- 

 tion, so that the passage of water within the medullary ray 

 from the pith to the cortex is retarded by fewer walls than if 

 it were to pass vertically through the ray. These medullary 

 rays, therefore, represent the most convenient passages in a 

 radial or horizontal direction. The wood cylinder is surrounded 

 on the outside by a thin layer of very delicate and easily- 

 damaged cells, the cambium ring, which passes over more 

 gradually into the external layers. These layers terminate 

 externally in those portions of the stem which are still soft in 

 a layer of tabular cells, which fit very closely together, and 

 are usually devoid of chlorophyll. This layer can often be 

 separated in the form of a thin skin, and is termed the epi- 

 dermis. In the older brown portions of the branch, new cells 

 will be found to be formed below the epidermis, which consist 

 of brick-shaped cells, and form the cork layer which constitutes 

 the protective sheath around the soft cortical tissue. But the 

 tissues outside the cambium are still further differentiated. 

 This can best be seen by scraping with the finger-nail a green 



1 See Fig. 2, p. 11. In this longitudinal section through the wood and the 

 inner cortex, the medullary rays are indicated hy the letter m. 



