286 INTRODUCTION TO BOTANY 



The well-protected chlorophyll tissue is able to withstand 

 severe cold and drought, and lives through periods that would 

 kill deciduous leaves. The amount of leaf surface is compara- 

 tively small, but pine leaves probably begin their work earlier 

 in the season than deciduous leaves do, and continue their 

 work later. 



270. New leaves and branches. When the growth buds open 

 in the spring, the branch extends, the new needle leaves begin 

 to elongate (fig. 220), and within a few weeks the leaves reach 

 their full size. This new growth was started in the preceding 

 summer and autumn, so that within the bud during the winter 

 there were the very small stem and leaves of the next spring's 

 growth. The scales from the growth buds fall away and leave 

 scars, by means of which the former position of the end of the 

 stem may be located. Also, by observing these bud scars from 

 the tip to older portions of the branch, the age of the branch 

 and the rate of elongation may be determined. 



271. The branch and stem. From transverse sections of 

 branches and stems of different ages their general structure 

 may be observed. 1 Innermost is a small pith region, which in 

 older stems is compressed until it is not usually noticeable. 

 Around this is the woody tissue (xylem) of the fibrovascular 

 bundles. The woody tissues are joined together in such a 

 way as to form a solid woody cylinder. It is possible to de- 

 termine the age of the twig approximately by counting the 

 layers or rings of wood, except when two or more distinct 

 growth periods occur in the same season. In such cases more 

 than one ring would be formed in the same year. At the outer 

 edge of the woody tissue is a thin layer of cells (cambium*) 

 which separates the woody tissue (xylem) from the outer tissue 

 (^phloem). The cambium is actively growing tissue which pro- 

 duces new wood within and new phloem without. The tissues 

 outside the phloem which we need to notice are the green bark 

 and the dead bark. Dead bark is constantly being formed from 

 the green bark within. This results in making the dead bark 



1 A hand lens will enable students to observe these regions of the stem. 



