TRANSLOCATION OF SIBSTANCES IN THE PLANT 337 



the influence of heat or of chemical substances. Others are 

 inclined to think that the living cells of the parenchyma, without 

 directly participating in the process of raising water, exert an 

 indirect influence by absorbing the air that appears in the vessels. 

 This question requires further study. 



78. Translocation of Organic Substances in Plants and Their 

 Storage. — The organic substances produced in the leaf by photo- 

 synthesis and those stored as food reserves in seeds and other 

 storage organs must be translocated to the place of their con- 

 sumption. One of the first prerequisites for their translocation. 

 viz., their hydrolysis into more mobile compounds, has already 

 been discussed. The more complex transformations that fats 

 and proteins undergo also have been noted. The substances 

 most used by a plant and most easily translocated in its structure 

 are the sugars and amino acids, especially asparagine. 



The initial synthesis of all these compounds takes place in the 

 leaves, and it is not difficult to show by simple experiments that 

 this is followed by an uninterrupted movement of these sub- 

 stances from the leaves. It is possible to demonstrate this by 

 Sach's dry-weight method, by which one may also study the 

 accumulation of substance during the process of photosynthesis 

 (Art. 38). If one-half of a leaf is cut off and dried in the evening, 

 and the other half in the morning, it will be found that in the 

 latter case the weight per unit leaf area will be considerably less. 

 Thus, Sachs noted that a square decimeter of leaf area of the 

 sunflower weighed 0.8 g. in the evening, but in the morning 

 only 0.7 g. In this case, the leaf lost during the night 0.1 g. of 

 organic substance per square decimeter, or about 0.2 g. per leaf, 

 if it is assumed that the average sunflower leaf has an area of 

 2 sq. dm. From this, it is seen that early in the morning, the 

 leaves of plants are considerably lower in organic substances than 

 in the evening. This translocation from the leaves, of course, 

 goes on not only during the night but also during the day, 

 apparently with an almost equal rapidity. But during the day 

 the outgo is overbalanced by the production of substance in 

 photosynthesis. This may be show^n if the increase in weight 

 of the halves of the leaves remaining on the plant is compared 

 with the increase in weight of leaves cut from the plant but placed 

 under conditions of sufficient water supply, moist atmosphere, 

 etc., so that synthesis wiff proceed uninterrupted. It will be 



