Chapter IX — 151 — Uptake and Movement 



tion as a result of the presence of "extra water." This is brought about by 

 disappearance, through metabolic processes, of osmotically active sub- 

 stances in the stele. 



Passive Absorption : — As emphasized in Chapter V, the diffusion pres- 

 sure of water may be lowered in several ways i) by adding solute, 2) by 

 lowering temperature, 5) by reducing hydrostatic pressure. When evapora- 

 tional loss from leaves exceeds uptake by the roots, the diffusion pressure is 

 lowered throughout the plant and particularly through the elongated tubes 

 of the xylem is this pressure reduction equaUzed between leaves and roots. 

 As tensions of a few atmospheres are developed in the xylem, the forces 

 responsible for active absorption are exceeded and passive flow to satisfy the 

 hydrostatic gradient becomes the dominant factor in water uptake. 



Not only is the active mechanism no longer effective ; experiments where 

 living and dead root systems have been compared have led Kramer (1933) 

 to believe that the osmotic mechanism responsible for active water absorp- 

 tion actually constitutes a resistance for the more rapid passive uptake ; dead 

 roots allow more rapid flow than live ones. 



These and other experiments of Kramer (1932 to 1939, see 1945) and 

 work by many others including the very practical process known as the acid- 

 arsenical method for kiUing weeds (Kennedy and Crafts, 1927, 1931; 

 Crafts and Kennedy, 1930; and Crafts, 1933a, b, 1937) all emphasize 

 the fact that under most conditions in the field the movement of water into 

 and through the plant as a result of transpiration is a reversible process, and 

 that the water in the plant is in a state of tension stretched between the 

 menisci in soil colloids and in hydrated cell walls of the mesophyll. The 

 mechanics of water absorption and movement represent an adaptation on 

 the part of the plant to the demands of transpiration and the limitations of 

 the soil reservoir. The extent to which various plants have been able to 

 meet these conditions determines to a considerable degree their distribution 

 with respect to characteristics of the environment and their productivity 

 within any circumscribed range. 



Methods of Estimating the Availability of Water: — Moisture is 

 retained in the soil by adsorptive and capillary forces. Its rapid absorption 

 by plants is also hindered by the low permeability of living plant roots 

 (Kramer, 1933). Richards (1941) has described a pressure-membrane 

 apparatus for studying soil-moisture availability at tensions of one atmos- 

 phere or above. The method consists of enclosing a thin layer of air-dry 

 soil in a cylinder on a permeable cellulose membrane that is supported by a 

 brass screen. The soil is saturated with distilled water and then the excess 

 moisture is forced out by gas pressure. The moisture retaining force at any 

 given soil moisture content ("moisture tension" of Richards and Weaver, 

 1944) is measured by the gas pressure required to attain that content. 



This method is weak in that it is useful only on laboratory prepared soil 

 samples in which the natural structure has been destroyed in preparation; 

 the pressure applied tends further to destroy structure ; the effect of solutes 

 on the diffusion pressure of water is not included in the measurement. The 

 latter effect may be measured cryoscopically and the freezing point depres- 

 sion plus the moisture tension measure the DPD or total moisture stress 

 of the soil. 



Physiologically, soil moisture has been classified as -7 ) that available for 

 vegetative growth, 2) that which may be used by certain plants to maintain 



