Chapter IX — 149 — Uptake and Movement 



tation. The term "active" as used here is not identical with that used in 

 Chapter VIII because, in this case, a complex cellular mechanism is in- 

 volved. The extent to which water secretion, etc., enters into the mechan- 

 ism of active water absorption by roots has not been determined. 



Passive absorption goes on when transpiration exceeds absorption and 

 results from a DPD gradient developed in the leaves and transmitted to the 

 roots through continuous columns of water under reduced pressure. Be- 

 cause conditions under which active absorption predominates occur only 

 rarely in the life span of most plants, a large portion of the water used is 

 taken up by passive forces. Nevertheless, active absorption has been stressed 

 in physiological studies (White, 1942) and it may play an important role 

 in the growth of seedlings, lawn grasses, plants of the tropical rain forest, 

 and by most crop plants during rainy spring weather. 



Active Absorption: — Priestley (1920) was one of the first to give 

 a comprehensive picture of a mechanism of active water absorption by the 

 root in which structure and function were adequately related. According 

 to this picture, water entering the root hairs passes inward across the cortex 

 and endodermis and enters cells of the stele surrounding the non-living 

 xylem vessels. This passage Priestley attributes to an osmotic gradient. 

 Because the concentration of solutes in the xylem vessels may be less than 

 in the surrounding parenchyma, and because, for continuous absorption, 

 solutes must be supplied to the xylem, the cells surrounding the vessels 

 are postulated to be more permeable to solutes on their inner surfaces and 

 hence they would allow a "leakage" of solution into the xylem. The dif- 

 ferential permeability might result from the fact that the cytoplasm of the 

 xylem parenchyma cells is in contact through the outer walls via protoplas- 

 mic connections with living protoplasm ; on the inner walls it faces a dead 

 lignified layer. Furthermore, metabolism of the root cells might lead to 

 oxidation of sugars to organic acids ; osmotic activity would thus be en- 

 hanced because several acid molecules would result from splitting of a 

 single sugar molecule ; protoplasm is relatively permeable to organic acids 

 and might release them in solution into the xylem elements. 



If the above mechanism could provide solutes to the xylem in sufficient 

 quantity to maintain a higher osmotic concentration than occurs in the soil 

 solution, water should move in to satisfy the osmotic gradient, and higher 

 osmotic concentrations within intervening tissues would not interfere be- 

 cause water movement is determined by gradients of diffusion pressure and 

 not osmotic pressure. 



Crafts and Broyer (1938) elaborated somewhat the above picture bas- 

 ing their theory on two essential assumptions (1) that the interconnected 

 protoplasts of the living cells of the root form a continuous protoplasmic 

 system (the symplast) extending from the root hairs to the xylem paren- 

 chyma; and (2) that the close relation of the cortex with its system of air- 

 filled intercellular spaces with the soil atmosphere makes possible a higher 

 metabolic status than occurs within the endodermis where intercellular 

 spaces are lacking and where water movement would tend to sweep in CO2 

 formed by respiration in the cortex. Since anatomical and physiological 

 considerations seem to substantiate these conditions, functioning of the root 

 mechanism is pictured as follows. When solutes are absorbed and accumu- 

 lated in the root hairs a diffusion gradient is established across the symplast 

 and movement inward occurs via the interconnected protoplasm, the tend- 



