100 GENERAL BOTANY 



pith, phloem, and cortex. The main water stream at the same 

 time moves upward to supply the leaves with water and prevents 

 them from drying up by the constant loss of water vapor. 



In early spring and summer the food stream moves in reverse 

 directions to supply the growth needs of the tissues, while the 

 water stream again moves upward and . supplies water for the 

 inflation of the growing cells in buds and in the cambium layer. 



Supporting and protective function. We have already learned 

 that the early formation of cork and bark in the outer layers of 

 twigs and shoots soon replaces the epidermis as a protective 

 external layer. The continued formation of cork and of the 

 supporting layers of the phloem finally produce in old trees a 

 highly protective bark which shields the delicate tissues within 

 the tree from mechanical injury and guards against excessive 

 loss of water and sudden changes in temperature. 



As we have already seen, the supporting, or mechanical, func- 

 tion is provided for by thick-walled cells or fibers, both in the 

 phloem and in the wood. In older trees and shrubs the thick- 

 walled cells of the wood cylinder are the real supporting tissues, 

 while the mechanical layers of the phloem serve to strengthen 

 the outer bark jacket. 



GENERAL STRUCTURE AND PHYSIOLOGY OF TREES 



SUMMARY 



Structural features of the tree in longitudinal section. The main 

 facts noted in the previous pages can now be summarized and cor- 

 related by using diagrammatic drawings (Fig. 55, A and 5), as 

 illustrations of the main structural features involved in the growth 

 and development of a tree. In Fig. 55, A, the tissues of the main trunk 

 and branches of a tree are so drawn as to display in their proper 

 relations the annual rings of growth, the wood rays in radial view, 

 and the differentiation of the woody cylinder into a central area of 

 heartwood and an outer lighter zone of sapwood. In addition to 

 these structures the figure shows the clos.e connection of the wood 

 and pith of the lateral branches with similar structures of the main 

 axis. The student will also observe that each so-called annual ring 

 of wood is really a cone-shaped cylinder laid down by the cambium 



