STEMS 



27 



the limits of two successive years' growth are marked by 

 bud scale scars of terminal buds. In this way we may 

 determine the age of a twig. 



Close observation of the twig will reveal a number of 

 whitish spots on the bark. These are lenticels (Fig. 11), 

 structures on the stem composed of a mass of loosely fitting 

 cells which permit the diffusion of gases inward and outward. 

 Except for the lenticels, the bark prevents the free passage 

 of air, and also the loss of water from underlying "stem parts. 





r' ^-' »--' cs- c. 



i3 



Fig. II. — Section of the lenticel of elder. (After Strasburger.) From A 

 Text-book of Botany by Coulter, Barnes, and Cowles. Copyright, by the 

 American Book Company, Publishers. 



How Does a Stem Grow in Length? — A bud is a young 

 shoot. A lengthwise section of a leaf bud shows a cone- 

 shaped growing point (young stem) upon which is a number 

 of young leaves. These leaves come off at regular intervals, 

 following identically the same arrangement as they do in 

 the adult twig. The growing point, then, consists of a 

 number of very much shortened internodes. Growth in 

 length of the shoot consists in the el6ngation of these in- 



