THE STEM. 55 



peculiarities. Since each year, in temperate regions, the 

 sterns of dicotyledons add a new zone of wood, it is 

 possible to determine the age of a tree by counting the 

 number of annual rings. Not infrequently the record 

 is obscured by irregular growth, due to drought and 

 other causes, but in general these rings are clearly defined. 



In their mode of growth the stems of gymnosperms 

 agree with those of dicotyledons, but their wood elements 

 are peculiar, the wood being composed mainly 

 of elongated cells called tracheids, the radial 

 sides of which have numerous bordered pits, by means 

 of which they communicate with each other and with the 

 medullary rays. 



The structure of stems corresponds with a number of 

 very important .functions performed by the elements that 

 compose them. Thus the epidermis, afterwards 

 replaced by cork, is protective, as is also the 

 bark, which on the trunks of most trees becomes greatly 

 thickened with advancing age. The medullary rays and 

 other parenchyma cells of wood and bark serve for storage 

 of various food products, and are also employed to a consid- 

 erable extent in conducting them from one part of the plant 

 to another. Bast and wood fibers serve a special purpose 

 as mechanical elements by which the stem is maintained 

 in its position, and enabled to resist forces that tend to 

 strain or fracture it. Finally the vessels and tracheids are 

 chiefly concerned in conducting water containing mineral 

 substances and air from the roots to the upper parts of the 

 plant, while the sieve-tubes of the inner bark store up 

 nitrogeneous food materials, and convey them to the points 

 where they are needed. 



It will, of course, be understood that an adequate 

 account of the physiology of stems cannot possibly be 



