242 AGRICULTURAL CHEMISTRY 



of small cells, the walls of which serve in part, as mem- 

 branes, and offer but little resistance to diffusion. The 

 cells are filled with sap, which is acid in nature and con- 

 tains numerous solid substances in solution. Between 

 the cells are intercellular spaces filled with sap of differ- 

 ent density from that within the cells and charged 

 with numerous alkaline matters taken from the soil. 

 Here, then, are nearly the same conditions as when the 

 salt and sugar were separated, and the result, osmosis, is 

 the same in each case. Within the cell walls active chem- 

 ical changes are taking place which aid in this inter- 

 change. It cannot be said that there is a constant flow of 

 sap in any one direction, as blood flows in the animal body. 

 It was formerly believed that there were two courses of sap 

 in the plant, upward and downward. The movement of 

 the plant juices is now considered as due to (i) capillary 

 action, aided by evaporation which disturbs the equilib- 

 rium of the plant juices, together with (2) osmosis aided 

 by chemical action within the cells. These factors are, to 

 a certain extent, mutually dependent upon each other. 

 By their joint action, aided by the chemical changes 

 within the plant, the water from the soil is taken into 

 the plant through the roots with the mineral matter in 

 solution, which serves as food, and finds its way all 

 through the plant, finally returning to the roots charged 

 with the material that can be made only in the leaf and by 

 the aid of light and sunshine. 



Chlorophyl and Protoplasm 



342. Chemical Action in Leaves of Plants. All of 

 the organic compounds of plants are produced within the 



