Hoagland —84— Plant Nutrition 



aeration, guttation becomes negligible. If, on the 

 other hand, the roots are immersed in a dilute salt 

 solution containing mobile ions, are well-aerated, and 

 kept at a favorable temperature, rapid guttation con- 

 tinues for a long time in a humid atmosphere. Very 

 little guttation was observed when roots were exposed 

 to a low temperature (5° C). In brief, the develop- 

 ment of root pressure seems to follow or to accom- 

 pany an active transport of salt into the xylem con- 

 ducting system, where the osmotic value rises above 

 that of the external solution. According to one theory, 

 water withdrawn from the external solution will de- 

 pend on the difference between the osmotic pressure 

 of the latter and that of the xylem vessels after inter- 

 vening cells have reached full turgor. 



An earlier view of Priestley that the osmotic 

 gradient is brought about through release of organic 

 solutes from differentiating cells does not seem to be 

 applicable here. The response to conditions affecting 

 salt movement is too rapid and further, the com- 

 position of the exuded fluid is chiefly inorganic. 



The observation on guttation and on bleeding may 

 argue for a simple osmotic movement of water, with 

 the cell activities confined to the antecedent or con- 

 comitant movement of solutes. Yet this is not cer- 

 tainly the whole explanation. Some investigators be- 

 lieve that an active transport of water occurs under 

 the influence of metabolic activities of the cell, pos- 

 sibly associated with the movement of salt, as for 

 example in some type of electro-osmosis. It does not 

 appear that there has yet been presented any really 

 conclusive proof of the active transport of water in 

 such systems, but the question is not closed. 



In any event, a simple explanation of the phe- 

 nomena in their entirety in terms of osmotic relations 

 does not seem to be adequate. The work of Grossen- 

 BACHER (1938) in our laboratory several years ago on 

 root pressure and rates of bleeding is indicative of 



