"ROOT PRESSURE" 231 



leaves, cause death of the leaf cells. There is still some doubt, however, 

 whether all of the retardation in rate of translocation as a result of such ex- 

 periments can be explained in these two ways. The possibility remains open 

 that the living cells of the xylem may in some way be necessary to the main- 

 tenance of the water-conducting capacity of the xylem, or even that they may 

 contribute in some way to the lifting of water through stems (Ursprung, 

 1912). 



In general, the evidence appears to be quite conclusive that the living cells 

 of the stem do not furnish the principal mechanism which causes the upward 

 transport of water in stems. On the contrary, as shov/n in the subsequent 

 discussion, the living leaves or stem tips at the top of the plant appear to be 

 absolutely necessary for the ascent of water through stems. 



"Root Pressure." — That a slow exudation of sap often occurs from the 

 cut surface of a stem is well known. Such sap exudations are often ascribed 

 to the development of a pressure in the dilute sap of the xylem vessels result- 

 ing from the operation of some as yet not fully understood mechanism in the 

 root cells. Hence the name "root pressure." It is generally considered that 

 such pressures may occur in intact plants as well as in those which have been 

 cut into. • 



It is probable that not all of the phenomena which have been classed under 

 the name of "root pressure" are due to the operation of the same physiological 

 mechanism. Some sap exudations seem to come from the cambium or phloem 

 rather than from the xylem. It seems quite certain that some of the sap 

 exudations from the xylem occur from the living wood parenchyma or wood 

 ray cells rather than from the vessels, or at least that the exudation is greatly 

 influenced by the activity of such cells. Hence the living cells of the stem 

 as well as those of the root are probably involved in many "root pressure" 

 phenomena. James and Baker (1933) go so far as to regard the subjection 

 of the sap in vessels to a pressure as an exceptional occurrence in plants, but 

 this seems to be an extreme point of view, and will not be followed in this 

 discussion. Because of the unsettled state of our knowledge regarding this 

 phenomenon it will be necessary to use the term "root pressure" in a very 

 uncritical sense. 



The magnitude of "root pressures" is commonly determined by attaching 

 a manometer to the cut surface of the plant from which the bleeding is oc- 

 curring. With only a few rare exceptions the exudation pressures which 

 have been recorded for plants do not exceed 2 atmos., and most of them are 

 less than this. According to White (1938) individual excised roots of tomato 

 plants exude sap from their basal ends with a pressure of more than 6 atmos. 

 It has not yet been shown, however, that such pressures operate for any con- 



