MECHANISM OF THE ABSORPTION OF WATER 279 



or guttation, and "active absorption" are usually considered to be merely 

 different aspects of the same phenomenon. Since there are many species in 

 which root pressures are not known to occur, this process probably does not 

 occur in some species. "Active absorption" apparently occurs at appreciable 

 rates only when the transpiration rate is relatively low and the soil contains 

 water in abundance. 



A number of suggestions have been made regarding the possible mechanism 

 of "active absorption" and root pressure, only one of which will be considered 

 here. The essentials of this theory were first suggested by Atkins (1916). 

 Priestley (1920, 1922) has also advocated a very similar hypothesis. In some 

 species, at least, it can be shown that the osmotic pressure of the sap in the 

 xylem elements of the root is higher than that of the soil solution. Osmotic 

 movement from the soil solution to the xylem might therefore be regarded 

 as occurring through a "multicellular membrane" composed of the intervening 

 cortical cells. 



Experimental evidence has been obtained by Kramer (1932) which indi- 

 cates that the mechanism postulated by this theory to account for "active 

 absorption" and root pressure can be shown to operate in other tissues, and 

 can therefore be regarded as a possible explanation of these phenomena. He 

 showed that when the hollow petioles of the tropical papaw {Corica papaya) 

 were used as osmometers, by filling them with a sugar solution and immers- 

 ing them in water, that water would pass through the living cells of the 

 petiole into a sugar solution with an osmotic pressure of 2 atmos., in spite of 

 the fact that the cells of the petiole themselves had an osmotic pressure of 

 about 9 atmos. The mechanism of such a movement of water may be inter- 

 preted in terms of the establishment of a consistently increasing gradient of 

 diffusion pressure deficits from the circumambient water across the cells of the 

 petiole to the enclosed solution. 



While a plausible theory can thus be proposed which explains so-called 

 "active absorption' in terms of the simple osmotic movement of water, it is 

 doubtful if such theories are adequate. The rate at which sap is exuded from 

 the xylem in some species seems too great to be explained in terms of the 

 mechanism just described, which necessarily would operate relatively slowly 

 (Crafts, 1936). The findings of Grossenbacher (1938) and others that a 

 more or less regular diurnal fluctuation in root pressure is of common occur- 

 rence likewise suggest that a more complex mechanism, than the one described 

 is involved. At the best "active absorption" can be only partly explained in 

 terms of simple osmosis, and other mechanisms are almost certainly involved 

 which are not at present understood. 



The volume of water passing into plants under the influence of "active ab- 



