84 



GENERAL BOTANY 



lateral bud*, 

 at apex of stem 



bud, and that the lower limit of the season's growth on both the 



main and the lateral shoots is marked by a ring of bud-scale scars. 



The brown bark which gradually covers over the early green 



bark of the growing shoot is also seen to be broken by minute 



pores, the lenticels, through which 

 air penetrates from the outside to 

 the internal tissues of the shoot. 

 Carbon dioxide and water vapor are 

 also eliminated from the lenticels as 

 they are from the stomata of leaves. 

 These same structures, together with 

 the wood cylinder, will appear in the 

 sections of the stem to which we will 

 now turn our attention. 



Internal structure. The gross 

 structure of the shoot is represented 

 in Fig. 46 as it appears in a 

 transverse section. The epi- 

 dermis and the brown bark 

 are not distinguishable in 

 gross sections and appear 

 as a single brown layer cov- 

 ering the outside of the 

 entire section. This layer 

 is designated as the brown 

 bark, within which is the 

 green bark, composed of cells 

 which contain green chloro- 

 phyll ; within the green bark 

 there is an inner lighter 

 layer, called the inner bark, or phloem. The wood cylinder is 

 clearly marked, but the cambium, or growing layer, cannot be 

 clearly distinguished from the inner lighter bark. Its position 

 at the junction between the bark and the wood is indicated by 

 a line. The pith occupies the center of the section. Within 

 the wood cylinder two annual rings of wood are represented as 

 having been formed. The wood rays are also shown extending 



Year's growih m length 



FIG. 45. External features of a lilac twig 

 in winter 



In the lilac the terminal bud dies early and 

 is not usually present in mature twigs. The 

 spring growth is produced from the last two 

 lateral buds formed at the end of the twig of 

 the season, as in the figure 



