TRANSLOCATION OF SCBSTANCES IN THE PLANT 339 



stems and leaves and the tips of roots. Another fraction is 

 directed into the ripening fruits and seeds, the underground 

 storage organs, or the Uving tissues of the trunk of the tree, 

 where it is stored as a food reserve. The storage processes are 

 exactly the reverse of those that take place at germination; from 

 the incoming sugars and amino acids, there are formed poly- 

 saccharides, mainly starch, fats, and reserve proteins. The 

 chemistry of the synthetic processes is not nearly so well known 

 as the chemistry of hydrolysis of these reserve substances. It 

 should be noted that in all of these processes water is given off. 

 Hence, in general, the maturation of seeds and the ripening of the 

 wood in the fall in preparation for dormancy proceed more 

 rapidly in dry weather, when water is being removed in larger 

 quantity. 



79. The Mechanism of Translocation of Organic Substances 

 through the Plant. The Theory of Miinch. — Organic substances 

 elaborated in the leaves do not move in any definite direction 

 but are always translocated to the points where they are either 

 consumed or deposited as reserves. They may either rise in the 

 stem toward the growing point and to the developing fruit or 

 descend to the roots and underground storage organs. Their 

 movement is thus connected with their consumption. The 

 mechanism of this translocation, however, remained obscure 

 until recently the German dendrologist Miinch (1929) succeeded 

 after many years of investigation in revealing this mechanism. 

 It is based on the flow of the liquid contents of the sieve tubes 

 and other conductive cells to the place of their utilization under 

 the influence of the osmotic pressure developed in the cells 

 elaborating organic substances. Accumulation of an excess of 

 water in these cells is precluded by the pressing out of water into 

 the elements of the wood. This creates. the condition of a contin- 

 uous flow of the solution in a definite direction. 



In order to obtain a clear concept of the motive forces deter- 

 mining this flow of organic substances, let us return to the 

 osmometers examined in Art. 4. Suppose the reservoir of the 

 osmometer immersed in water, the osmometer itself being filled 

 with a solution. As has been seen, such an osmometer will 

 absorb water so long as the weight of the water column in the 

 tube does not exceed the greatest osmotic pressure that the 

 solution is able to develop. The tube at the time of develop- 



