244 TEXTBOOK OF PLANT PHYSIOLOGY 



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 deci- 

 meter, or about 0.2 g. per leaf, if it is assumed that the average 

 sunflower leaf has an area of 2 sq. dm. Whence, it is seen that 

 early in the morning leaves of plants are considerably poorer 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 the course of synthesis. This may be shown if the 

 increase in weight of the halves of the leaves remaining on the 

 plant be compared with the increase in weight of leaves cut from 

 the plant but placed under such conditions (sufficient water supply, 

 moist atmosphere, etc.), that synthesis will proceed uninterruptedly. 

 It will be found that the cut off leaves accumulate considerably 

 more dry matter, sometimes twice as much as those remaining on 

 the plant. This difference indicates the quantity of substances 

 that have passed from the leaf into the stem during the day. 



In the autumn, when the time of leaf abscission approaches, 

 besides this uninterrupted outflow of assimilants, gradually 

 diminishing with the decrease in synthetic activity, the movement 

 from the leaves of another type of compounds more firmly bound 

 within them, is observed. Evidently, this is connected with the 

 gradual aging of the cells and the breaking down of a part of their 

 integral substances. Falling leaves as a rule are considerably 

 poorer in nitrogen, phosphorus, and potassium than those still 

 active. Calcium salts, on the contrary, remain in fallen leaves. 



A translocation of substances of similar nature takes place in 

 annual plants, in which at the time of maturation of the seeds all 

 other parts of the plant, leaves, stems, and roots, are gradually 

 emptied of food substances and finally die. Water-culture experi- 

 ments with corn, for example, have shown that after flowering the 

 plants may be transferred to distilled water and still will give 

 normal development of seeds. It is apparent that all those com- 

 pounds of nitrogen, phosphorus, sulphur, and other elements, 

 which are stored in the seeds, are taken from the vegetative parts. 

 At the end of development, even the carbon compounds are taken 

 in greater amounts from the older parts than they are synthesized. 

 It has been observed that the average dry weight of a wheat speci- 

 men, taken from the field, does not increase between the time of 



