34 



FORAGE PLANTS AND THEIR CULTURE 



26. Loss of substance from growing plants. Studies 

 of the chemistry of plants at different stages have in many 

 cases shown that the total amount of such substances as 

 nitrogen, phosphoric acid, potash and soda was smaller 

 at maturity than some time previously. The same fact 

 has also been shown in field investigations where the total 

 weight of hay produced per unit of area was less at ma- 

 turity than at an earlier stage. 



Three general explanations of the phenomenon have 

 been advanced, namely : 



1. The backward flow of the salts of the plant through 

 the stem and roots into the soil. 



2. The mechanical loss of material from the leaves by 

 decay, drying, etc. 



3. The leaching effects of rain and dew. 



The subject has recently been studied by LeClerc 

 and Brezeale. From their investigations it is demon- 

 strated that all growing plants exude salts upon the sur- 

 face of the leaves which are washed off by rains. No 

 evidence was found that salts migrate downward, as the 

 lower part of the stem is always poorer in phosphorus, 

 potash and nitrogen than the upper part and leaves. 

 Wheat plants were grown in the greenhouse and watered 



