338 PLANT PHYSIOLOGY 



found that the leaves that have been cut off accumulate con- 

 siderably more dry matter, sometimes twice as much, as those 

 remaining on the plant. This difference indicates the quantity 

 of substances that has 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 assimilates, gradually 

 diminishing wdth the decrease in synthetic activity, the movement 

 from the leaves of another type of compounds, such as organic 

 phosphorus compounds that are 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 have a considerably lower 

 content of 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 

 experiments with corn, for example, have shown that after 

 blooming, the plants may be transferred to distilled water and 

 still will give normal development of seeds. It is apparent that 

 all those compounds of nitrogen, phosphorus, sulphur, and other 

 elements, which are stored in the seeds, are taken from the vege- 

 tative parts. At the end of development, even the carbon com- 

 pounds are taken from the older parts in greater amounts than 

 they are synthesized. It has been observed that the average 

 dry weight of a wheat plant, taken from the field, does not 

 increase between the time of flowering and the ripening of the 

 grain notwithstanding the rapid increase in weight of the grain, 

 which in the end contains almost one-third of the total dry 

 matter. Hence, it is apparent that the weight of the remaining 

 organs of the plant must have decreased by the same amount. 

 In plants with a continuous type of development, such as the 

 tomato and cucumber, organic substances are removed con- 

 tinuously from the lower senescent leaves and branches, which 

 may continue through a long period of the life of the plant. 



The substances that are photosynthesized and then removed 

 from the leaves are distributed throughout the plant. Part of 

 them are utiUzed by the growing regions, the young developing 



