184 



STEMS 



from the leaf bearing portion of a branch to the regions behind 

 where food manufacture is being abandoned, the following struc- 

 tural features are plainly seen. 



First, there are the leaf scars, left where 

 the leaves fell away, and interesting because 

 of the way they are formed. (Fig. 160.) As 

 the time approaches for leaves to fall, a cork- 

 like layer, known as the absciss layer, forms 

 across the base of the leaf, severing the direct 

 connections of the leaf with the twig and re- 

 maining as a covering over the scar after the 

 leaf falls. The absciss layer closes the open- 

 ings which would otherwise be left by the 

 falling of the leaf, and thereby prevents the 

 entrance of destructive organisms into the 

 twig. It is in connection with leaves which 

 still remain on the tree after the absciss layer 

 is formed that the various autumn colors oc- 

 cur due to changes in the dying leaf tissues. 



Second, there are the lens-shaped dots, 

 known as lenticels, which, although common 

 on the branches of all woody plants, are espe- 

 cially conspicuous on 



the branches of the 

 Qh d g; fa 



(Fig. 161.) The for- 

 mation of lenticels accompanies the forma- 

 tion of bark. In the young twig, where 

 the protective covering is an epidermis, 

 air is supplied to the tissues beneath 

 through the slit-like openings of the sto- 

 mata; but, as the twig becomes older and 

 bark is formed, the stomata are replaced 

 by lenticels. Lenticels are stomata dis- 

 torted and transformed in structure by the 

 development of bark. Just beneath each 

 stoma, instead of cork, there is formed a 

 loose mass of cells, and this loose mass of 

 cells is pushed up into the opening of the stoma, as shown in 

 Figure 162, rupturing the stoma and surrounding cells and thus 



FIG. 160. Twig of 

 the White Walnut, 

 showing leaf scars (a) . 



FIG. 161. Twig of Birch, 

 showing lenticels. 



