THE MECHANISM AND CAUSES OF TRANSLOCATION 587 



SECTION 108. The Mechanism and Causes of Translocation. 



Any diosmosing substance will continue to penetrate a cell until 

 a condition of equilibrium is reached, and hence the consumption of the 

 substance in question will by disturbing this equilibrium cause further 

 supplies to flow inwards. It is in this way that consumption regulates 

 absorption and translocation, while by the conversion of the diosmosing 

 compounds into other substances, large quantities of reserve-materials 

 may be accumulated in the cells of storage receptacles. The selective 

 power of a plant as well as of each individual cell is determined by its 

 inherent properties and by the special character of its metabolism, and hence 

 it arises that one cell may absorb much, another little or none of a given 

 substance, and that in time very large quantities of a dissolved salt may 

 be absorbed from an exceedingly dilute solution. On the other hand, 

 any reconversion into diosmosing substances results in an outward flow 

 away from the regions of higher concentration (Sect. 22). Translocation 

 is induced and regulated by these means, and when the terminal cell of 

 a hair, or the tip of a fungal hypha, consumes nutriment, a current is 

 induced which supplies the deficiency by means of materials drawn from 

 the productive organs or storage receptacles. The translocatory substances 

 probably pass mainly through the cavities of the cells, but may diffuse to 

 a certain extent through the imbibed water of the cell-walls when no 

 penetration through the protoplasts is possible. Substances may be trans- 

 located in the form of excessively dilute solutions, so that frequently none 

 can be detected in the translocatory channels, although large amounts may 

 be conveyed to the consuming tissues, or may gradually accumulate in 

 terminal storage tissues. Starch, sugar, asparagin, may be passively secreted 

 by the cells forming the translocatory channels, and the cell-walls may even 

 be permeated by a very dilute solution of sugar or other solub'e food- 

 materials. All the substances contained within the protoplasts are retained 

 when a translocatory tissue is placed in water, and hence for the transit 

 from cell to cell a reconversion of the transitorily stored compounds into 

 diosmosing ones is necessary. Consequently mere traces of the actual 

 translocatory substances are present at any given moment, and the storage 

 and mobilization must be repeated as many times as there are cells to 

 be passed. This cannot be directly seen when the translocatory channels 

 are filled with asparagin or sugar, but can readily be followed in the 

 case of starch, starch grains being continually deposited and redissolved. 

 It is obvious that these repeated reconversions must involve a considerable 

 expenditure of energy, but this is more than counterbalanced by the 

 decreased danger of loss and by the avoidance of concentrations injurious 

 to the protoplast. 



