112 BOTANY: PRINCIPLES AND PROBLEMS 



wood, this cohesive power is perhaps so strong that a pull at the 

 top — in this case the osmotic pull at the leaf — will lift the column 

 bodily, as a rope might be lifted. There are certain objections 

 to this explanation, too, but they are not as serious as in the 

 other hypotheses. Possibly several of the factors mentioned 

 may be concerned together in the ascent of sap. We must 

 admit that this problem, like so many others in biology, is as 

 yet far from a satisfactory solution. 



The Translocation of Foods. — The plant must possess means 

 not only for insuring the passage of a plentiful supply of water to 

 the leaves through the wood of the stem, petioles and veins, but 

 also for transporting the product of the leaf's activity — the manu- 

 factured food in the form of carbohydrates, fats and proteins — 

 to any region of the plant where food is used or stored. This 

 function of translocation is performed chiefly by the sieve-tubes 

 of the bast. The movement of organic substances by diffusion 

 from cell to cell is a comparatively slow process, but is the only 

 means available in regions remote from the vascular system. 

 Movement of food for long distances, as from the leaf to the 

 storage regions of stem and root, seems to take place almost 

 entirely in the bast. Here the protoplasmic connections from 

 sieve-tube to sieve-tube through the sieve-plates do away with 

 the necessity for diffusion through a long series of membranes 

 and thus facilitate the rapid transfer of substances from place 

 to place. This importance of the bast has repeatedly been 

 demonstrated by experiments involving "ringing" or "girdling," 

 in which there is removed from around the stem a continuous 

 encircling strip of tissue, including all bark and bast. It is a 

 matter of common observation that a tree in which the trunk has 

 been girdled in this way will ultimately die. Although small in 

 amount, therefore, and rather inconspicuous when compared 

 with the wood, the bast is a vitally necessary tissue in the 

 economy of the plant. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



334. Most stems tend to be stout below and more slender above. 

 Why is this, and of what advantage is it to the plant? 



335. Why are young trees often somewhat spire-shaped but old trees 

 of the same species fiat or convex at the top? 



