154 



All Food Is Made by Green Flants unit hi 



Fig. 178 (left) The smooth paperlike bark of 

 the birch with its conspicuous leiiticels. Can yon 

 name another tree ivhose young tivigs have con- 

 spicuous lenticels? (Schneider and schwartz) 



Fin. 179 (below) Diagram of a young ivoody 

 stem. How does this steTU differ from most 

 root si' Wl:iat are the parts of the vascular 



c y Under? 



is never lost; the tree keeps a smooth bark 

 throughout its Hfe. 



The vascular cylinder is much like the 

 one in roots. It consists of phloem, cam- Epidermis 

 bium, and xylem. But in the center there Cork 

 is usualK' a fourth cylinder of thin- Cortex 

 walled cells, the pith. Pith is very rarel\' 

 found in roots but is found in most 

 stems. Compare Figures 176 and 179. 

 This vascular cylinder connects at its 

 lower end with the vascular cylinder of 

 the roots. At its upper end it branches Xylem 

 many times and in each leaf branches 

 again, forming the veins of the leaves. Cambium 

 You will find it interesting to study a 

 young stem as described in Exercise 8. 



How stems grow longer and branch. A Cortex 

 swing fastened to one of the lower limbs 

 of a tree remains the same distance from 

 the ground \ear after year; yet the tree 

 jrrows taller all the time. FA'idently the 

 lower portion of the trunk docs not 

 lengthen. In fact, the tree lengthens only 

 near its tip. In parts of the world where 

 there are distinct seasons growth ceases 

 during the winter. At this time the cells 

 at the tip of the stem arc covered by 

 heavy scales, forming the tennincil bud. 



Lentice 



