92 



GENERAL BOTANY 



Cork 



Cortex 



Parenchyma 

 cells 



cortex is composed of thin-walled cells which serve a 

 storage function. These cells are living during the early life of 

 a tree, but they are ultimately crushed by the growth in diameter 



of the stem or -branch, and then 

 they form a part of the dead 

 tissue of the bark. 



The phloem is early differen- 

 tiated into two distinct layers in 

 most common trees and shrubs. 

 The outer layer is composed 

 of cells with greatly thickened 

 wails, which are termed phloem 

 fibers, since in long sections these 

 thick-walled strengthening cells 

 are seen to be long, pointed, 

 fibrous cells. These fibers are 

 of special interest, since it is from 

 such cells that the commercial 

 fibers of flax, hemp, etc. are de- 

 rived. Their great strength and 

 flexibility make them invaluable 

 to the plant as well as to man. 

 We shall learn also that they are 

 always favorably located for the 

 greatest efficiency in supporting 

 delicate stems and soft tissues. 

 The inner phloem layer is com- 

 posed of large conducting cells, 

 joined end to end, called sieve 

 tubes, and between them smaller 

 living cells called phloem paren- 

 chyma. The chain of cells which together constitute each sieve 

 tube is so named on account of the fine perforations in the 

 transverse walls of the cells, composing the sieve plates, which are 

 supposed to facilitate the flow of soluble food materials up and 

 down the stems. The small cells between the sieve tubes, being 

 also living cells, serve both for the storage of starch and for the 



Phloem 



Cambium 



Xylem 



Wood fiber 

 1Voo~d parenchyma 



FIG. 52. Microscopic structure of 



the stem of the alder (Alnus) in 



transverse section 



The figure represents the camhium in 

 the fall and winter condition, after the 

 year's growth was completed. Note 

 the regular rows of xylem, phloem, 

 and wood-ray tissue cells, originating 

 from definite portions of the camhium 

 layer 



