284 TEXTBOOK OF BOTANY 



few plants, such as sycamore and beech, the stone cells are the only 

 sclerenchyma cells in the phloem. 



Vascular rays are present m most plants having a vascular cambium. 

 That part of the ray extending from the cambium into the phloem is the 

 phloem ray, and it varies considerably in size, shape, and appearance in 

 different species of plants. 



In stems of most trees and shrubs the epidermis, cortex, and pericycle 

 are present for only a few years. The tissues outside the cork cambium 

 crradually die, and ultimately a new cork cambium develops from paren- 

 chyma cells in the phloem. The cylinder of cork formed from this cam- 

 bium prevents diffusion of water to tissues external to it. The whole 

 outer bark soon dies, cracks, and sloughs off. 



In the birch and cork oak there are conspicuous cylinders of cork that 

 resemble annual rings of the xylem, but in most species such rings are 

 not evident. These cylinders are due to alternating layers of thin-walled 

 and thick- walled cells. The thin-walled cells separate easily, and this ac- 

 counts for the papery sheets of birch bark and the scaly bark of the syca- 

 more. External masses of cork in such trees as cork oak accumulate in 

 layers several centimeters in thickness and are of great commercial 

 importance. 



Stems of pumpkin, squash, tomato, potato, and several other kinds of 

 plants differ from the usual by having phloem both external and internal 

 to the xylem. 



The xylem. The xylem of a woody dicot is a complex tissue composed 

 chiefly of vessels, or water-conducting tubes; tracheids; ivood fibers; 

 xylem parenchyma cells, and xylem ray cells. A vessel may be several 

 millimeters, several feet, or even yards in length. The length of a vessel 

 depends upon the number of end walls that disintegrate in the row of 

 cells from which it is formed. Old vessels may be closed by tyloses, out- 

 growths from adjacent xylem parenchyma ( Fig. 105A ) . 



In the xylem of conifers the vessels and sometimes the libers are ab- 

 sent; the chief cells are tracheids and xylem ray cells. The tracheids are 

 both water-conducting and wood cells. They are thick-walled and spin- 

 dle-shaped, and have thin areas or pits in the walls ( Fig. 105B ) . 



Certain features of the xylem are more easily understood in relation 

 to the growth of the cambium. 



The vascular cambium. In order to see the cambium of stems clearly, 

 one must have thin sections cut from growing stems of dicots or conifers. 

 Between the easily recognized xylem and phloem in cross sections of 



