148 TRANSLOCATION IN PLANTS 



The secretion of water from the cambial surface, when 

 separated from the xylem, as reported by Miinch, is 

 additional evidence strongly supporting the hypothesis. 

 Weevers and Westenberg (1931), however, were unable 

 to confirm a secretion from the cambium. Even if the 

 secretion of water is proven, such secretion has not been 

 demonstrated to bear a relation to food movement. Such 

 secretion from the cambium may be comparable to the 

 bleeding from cut stumps or to that occurring in normal 

 guttation. There is no clear indication that these secre- 

 tions are related to rapid food translocation, as would be 

 the case in food transport to storage tissues or meristematic 

 cells. Such bleeding, it seems, may occur under conditions 

 when one would expect no storage and little or no meri- 

 stematic activity. It may be true that this secretion is 

 related to meristematic activity or food deposition, but it 

 would not necessarily follow that the water results from a 

 mass flow of solution through the phloem, for unequal dis- 

 tribution of solutes in secreting cells alone may cause water 

 secretion and this may bear no relation whatever to the 

 mechanism of transport. 



The postulated secretion of water into the xylem might 

 well account, as Miinch suggests, for the refilling of the 

 xylem with water, after high transpiration has reduced the 

 content or even ruptured the cohesive columns and filled 

 certain of the tubes with air. He suggests that such a 

 mechanism may explain the necessity of living cells for 

 maintaining a continuous column of water for transport, 

 according to the cohesion theory of water rise. No other 

 satisfactory explanation has been given for the seeming 

 fluctuations in the water content of the trunk. I say 

 seeming fluctuations because it is possible that the low 

 water content of sapwood, observed by Jones et al. (1903), 

 Craib (1918), Busgen (1911), MacDougal, Overton, and 

 Smith (1929), and others during periods of high transpira- 

 tion or low water supply in the soil, as contrasted with the 

 higher water content during rainy periods, may be in large 

 part due to the fact that during dry periods the water is 



