406 SMITH'S INTERMEDIATE CHEMISTRY 



How the Plant Feeds. We have mentioned (p. 395) that the 

 plant juice or sap contains soluble sugars, which travel to those 

 parts of the plant which require new material for their growth. 

 The moist walls of the root-hairs of the plant are freely permeable 

 to water, the entrance of which into the plant from the soil is 

 necessary to offset evaporation from the leaves and stems. Sol- 

 uble salts present in the soil water are also able to pass, although 

 less freely, through these walls, and are incorporated in the sap. 

 If they are of nutritive value to the plant, they react, as they 

 circulate through the plant from cell to cell, with the organic 

 constituents there present, forming more complex compounds 

 which are unable to permeate the boundary walls of the cells, 

 and thus become permanently fixed in the growing parts. If 

 they are not of nutritive value, they complete the circuit un- 

 changed. 



Now the passage of salts from the soil water into the plant 

 continues only until the sap inside contains the same concen- 

 tration of each salt as the solution outside. Hence while nutritive 

 constituents, which are removed from the sap during its circula- 

 tion through the plant, can continuously enter to keep up the 

 supply necessary for growth, non-nutritive constituents soon reach 

 their equilibrium concentration and are thereafter rejected. 



Osmosis. The membranes lining the cell walls and the root- 

 hairs of plants exercise, as we saw above, a selective action with 

 regard to the passage of different substances through them. Water 

 is able to permeate the membranes quite freely, dissolved salts 

 pass through less readily, while the movement of complex organic 

 substances, such as proteins, is completely blocked. Upon this 

 selective flow of materials through the plant membranes, it must 

 be noted, the life of a plant is absolutely dependent. If the 

 membranes were freely permeable to the organic materials con- 

 tained in the plant sap, the plant would soon lose these materials 

 to the outside soil and die of exhaustion. 



