Grafts et al. — 172 — Water in Plants 



ment. Consequently the rate of sap movement is in this case high (up to 

 43.6 meters per hour), being about ten times that found in diffuse porous 

 trees such as beech, maple, linden, etc., where a greater conducting area is 



available. 



In the case of conifers, the velocity of sap movement is usually less than 



half a meter per hour. 



Diurnal variations in rate of flow, as would be expected, follow fairly 

 closely the transpirational fluctuations. From a relatively quiet transpira- 

 tion stream at night, movement is initiated in the morning in the upper 

 branches and gradually reaches the lower parts of the stem and lastly the 

 roots. At night the reverse takes place as water moves into the roots and 

 stem after the tip has ceased to transpire. This movement continues as 

 long as water deficits exist within the plant. Figure 48 shows the diurnal 

 variation in the rate of sap movement in Larix, Picea, and Fagus. The 

 maximum velocity of the transpiration stream is reached several hours after 

 the peak of transpiration. 



Annual variations in transpiration rates are reflected in annual cycles 

 in the movement of water through the plant. Baumgartner (1934), using 

 birch, found such a trend. Any factor causing a reduction in absorption or 

 transpiration will correspondingly affect the rate of movement of water 

 through the plant. 



Water Balance and Redistribution: — During conditions of high 

 transpiration a shrinkage of stem diameter is a characteristic phenomenon. 

 Bode observed an expansion of vessels upon cutting and explained this as 

 being due to the release of high tensions in the stem. MacDougal, Over- 

 ton, and Smith (1929) report that their dendrographic measurements 

 "confirm the contention of several investigators that the tensions set up by 

 the transpiring leaves may amount to as much as two hundred atmospheres." 

 Undoubtedly the greatest strain on the cohesive water columns in an indi- 

 vidual plant must occur during the most advanced stages of wilting as long 

 as liquid continuity is maintained. 



Meschazeff (1882) is mentioned by Delf (1912) as being the first to 

 call attention to the ecological importance of internal redistribution of water 

 in plants during periods of water deficits. Pringsheim (1906), Chand- 

 ler (1914), Bartholomew (1926), Savastano (1934), and Furr and 

 Taylor (1935) have emphasized that certain plant organs may act as res- 

 ervoirs from which others may draw water. Young leaves of some plants 

 are able to draw water from older leaves and remain turgid longer during 

 progressive wilting. Fruits may act as reservoirs from which leaves can 

 draw water during wilting. 



Pringsheim (1906) and Chandler (1914) found a correlation be- 

 tween the osmotic pressure of the sap expressed from an organ and its ability 

 to absorb water. Beck (1928), however, repeated some of Pringsheim's 

 work and reported that water did not move along an osmotic gradient but 

 along a "suction tension" gradient which was maintained by reason of dif- 

 ferences in the elasticity of the cell walls of young and old leaves. 



While it is obvious that if direct water contact exists between two 

 organs the water will flow along a gradient in diffusion pressure of water, 

 it is not necessarily true that such a gradient is maintained only by differ- 

 ences in osmotic pressures and wall pressures of the various cells. Imbibi- 

 tional forces as well as osmotic forces influence the diffusion pressure of 

 water, and as has been discussed earHer, certain recent evidence has been 



