156 



THE INDIVIDUAL ORGANISM 



of the stem. One of the two cells produced at the division remains a 

 part of the cambium; the other, if on the inside, becomes a xylem cell; 

 if on the outside, a phloem cell. Usually several layers of xylem cells 

 are produced for each layer of phloem. 



Besides the ordinary tall cambium cells that produce the vertically 

 elongated cells of the vascular tissues, the cambium contains other cells 

 that are radially elongated. These generally occur in groups, forming 

 vertical bands or streaks in the cambium cylinder one to several cells 

 wide and sometimes as much as several inches high. These groups of 



spring vessels 



bark 



ith 



large 

 wood ray 



large wood rays 



Fyg. 1 1.4. A segment of an oak log. (Redrawn from Sinnott, Botany: Principles and Problems.) 



special cells constitute the ray cambium and give rise to radial rays of 

 parenchyma cells called the vascular rays. 



The wood of the stem is the xylem. In most trees of the temperate 

 zone the cambium is most active in the spring and then produces numer- 

 ous large xylem cells; such spring wood is porous and light and usually 

 contains many vessels large enough to be seen with the unaided eye as 

 minute pores on a cross section of the wood. Later in the season the 

 cambium becomes less active, and the summer wood then formed is 

 made up of much smaller cells with relatively thick walls; it is denser and 



