380 RELATION TO ENVIRONMENT. 



themselves are often provided with a woody or hairy covering 

 to retard transpiration. When the epidermis becomes more 

 efficient the hairy covering often falls away. 



In the case of naked buds protection is afforded in other ways : 

 by the protection of hairy covering, by physiological adaptation of 

 the tissue, or in many cases by the late appearance of the shoot 

 in spring after the very dry April and May winds have ceased. 



741. Bud-scars, and how to tell the age of the plant. In gen 

 eral the bud-scales when they fall away in the spring leave scars 

 termed scale-scars, and the whole aggregate of scale-scars makes 

 up the bud-scar. The position of the buds of previous winters is, 

 therefore, marked. It becomes an easy matter to determine the 

 age of a branch, since all that is necessary is to follow back from 

 one bud-scar to another, the portion of the stem between repre- 

 senting, except in rare cases, one year's growth. 



A woody plant grows in height only by the formation of new 

 sections of stem added to the apex or side of similar sections 

 produced the previous season, never, as is commonly supposed, 

 by the further elongation of the previous year's growth. Hence a 

 branch once formed upon a tree is fixed as regards its distance 

 from the ground. The apparent rise of the branches away from 

 the ground in forest trees is an illusion caused by the dying away 

 of the lower branches. 



742. Definite and indefinite growth. With the opening of 

 the buds in spring, growth begins. In some cases, when all the 

 members for the season were formed, but still minute, within the 

 bud, such growth consists solely in the expansion of parts already 

 formed; in others only a few members are thus present to ex- 

 pand, while new ones are produced by the growing point as the 

 season progresses. In most cases growth is completed by the 

 middle of July, soon after which buds are formed for next year's 

 growth. Such a method of growth is termed definite. 



In a few woody plants, as, for example, sumach, locust, and 

 raspberry, growth continues until late in the autumn. In such 

 cases the most recently formed nodes and internodes are unable 

 to become sufficiently "hardened" before winter sets in, and 



