THE EVOLUTION OF THE PLANT BODY 43 



cambium ring beneath the bark. The skeletal tissues which char- 

 acterize woody plants are discussed more in detail in Chapter 15, 

 and their economic importance is the subject matter of Part 

 Three in its entirety. 



Growth of roots and stems takes place at the tips, where 

 special growing points are made up of cells which are continu- 

 ously dividing to form new cells, the latter soon becoming dif- 

 ferentiated into the tissues which make up the full-grown root or 



Fig, 21. — The demarcation between (summer and spring) wood of successive 

 seasons results in annual "rings." 



Stem. Growth in diameter is the result of the activity of the cam- 

 bium cells. In many plants living in climates with alternating 

 warm and cold, or wet and dry, seasons, the cambium cells do 

 not grow continuously. During the warm, or wet, season condi- 

 tions are such that they actively divide and form new phloem 

 cells on their outer surface and new xylem cells on their inner 

 face. As the season progresses, fewer and fewer vessels are formed 

 in the xylem or wood, and more fibers. Thus the "spring wood" 

 is lighter colored and more porous, as compared with the denser 

 and less porous "summer wood" (fig. 21). During the winter, or 



