THE STEM 



247 



H.W 



27 — Longi- 

 inal section of 

 oody stem. 

 . Heart-wood. 

 Alburnum, 

 arrows show 

 way of the 

 ascending sap. 



the mighty weight of the branches above the trunk. Some trees, 



indeed, do not contain such heart-wood, as will be seen in the 



White Da«Qmar ( Valeria indica; 



Kan. Dhiipa), and, therefore, be- 

 come hollow when old, the dead 



wood being destroyed by fungi. 

 The water-current does not rise 



in the bark also, as can be shown 



by removing a ring of bark from 



a branch: the leaves above the 



wound remain for a time as fresh 



and green as those below the wound. 



If, however, the fresh wood below 



the bark (alburnum) were removed and the bark allowed to stand, 



the leaves and all the parts 

 above the cut would perish. 



This shows that the sap 

 ascends only in the alhurmim 

 consisting of vessels through 

 which the sap can easily pass. 



(b) The Descending; Sap. 

 The organic compounds formed 

 in the leaves, of Avater and 

 carbon, have to be distributed 

 to the places where they are 

 required, viz-, the growing 

 points — the tips of stems and 

 roots, flower-buds, fruits, seeds, 

 the cambium cylinder, and all 

 other places of growth. What 

 the leaves elaborate is mostly 

 starch. But starch cannot pass 

 from cell to cell through the 

 cell-walls. It is, therefore, 

 turned into a liquid, i. e., into 

 grape-sugar, which easily goes 

 Besides starch, nitro- 



A. -8. 



Pig. 228. — A ring of bark removed 

 from a stem, il. as it appears in the first 



year, B. as it appears after 2 years. 

 a b the bark stripped off, above a the 

 stem is swollen by the descending sap, 

 h c dry bark, below cd the stem has 

 grown in size due to the sap, des- 

 cending from branch e. 



through thin membranes like cell-walls. 



