Chapter IX — 173 — Uptake and Movement 



presented to support the theory that the vital activity of the plant cells also 

 influences the diffusion pressure of water (cf. Chapter VIII) . Although the 

 amount of colloid in a highly vacuolated parenchyma cell may be small and 

 perhaps can be neglected in studying the water relations of the individual 

 cell, imbibitional forces associated with the colloids in less vacuolated cells 

 may be very important. 



Stocking (1943) was unable to find a correlation between the osmotic 

 pressure of the expressed saps of squash leaves and the sequence of wilting 

 except in the case of shaded leaves where the osmotic pressure falls to an 

 extremely low value. Wilting in this case occurs first in the shaded leaves. 

 In squash the tip leaves may continue to grow and expand even when the 

 plant is removed from the culture solution and allowed to wilt in a mod- 

 erately shaded condition. The flower buds may continue to open and bloom 

 while the basal leaves are dying. It seems probable that the greater im- 

 bibitional pressures of the slightly vacuolated, highly protoplasmic, expand- 

 ing cells of the young leaves could account for the lowered diffusion pres- 

 sure of the water in their sap. Thus imbibition enables them to maintain 

 their turgidity in the presence of the wilted lower leaves ; otherwise these 

 cells must actively secrete water into their vacuoles. Kerr and Anderson 

 (1944) have recently come to this conclusion in regard to the ability of 

 young cotton seeds to absorb water. 



The water reserves of plants in general are not equally distributed 

 among the different organs but the lower leaves and shade leaves are usually 

 richer in water and lower in osmotically active solutes than the upper or 

 sun leaves (Marsh, 1941 ; Maximov and K. Maximov, 1924). During the 

 wilting of sunflower this difference disappears as a result of withdrawal of 

 water from lower leaves by the upper. When such wilted plants were 

 watered, Maximov and K. Maximov found that the older leaves were un- 

 able to recover but continued to dry out. However, if these leaves were 

 cut and placed in water they were able to again recover their turgor. The 

 Maximovs explain this as being due to the greater absorbing power of 

 the young leaves and hence their ability to deprive the older leaves of water 

 even after the wilted plant was well watered. 



Direct observation of the xylem contents led Stocking (1943) to con- 

 clude that the lower leaves of severely wilted sunflower plants contain many 

 air filled xylem elements. Such a discontinuity of the water in the xylem 

 would lead to a high resistance to water movement. He believed that 

 whereas the young leaves were able to obtain water during the wilting of 

 the plant at the expense of the older leaves, the application of water to the 

 soil should cause the upper leaves to regain their turgor and failure of any 

 leaves to recover could in part be traced to a disruption of the xylem con- 

 tents so that the rate of water absorption could not reach that of water 

 loss. The recovery of the basal wilted leaves when placed with cut petioles 

 directly into water as found by Maximov tends to substantiate this con- 

 clusion. 



Chandler (1914) felt that the internal redistribution of water in the 

 plants which he investigated occurred at such slow rates that movement 

 would not have to occur through the non-living xylem conductors but could 

 occur through the living cells of the stem. 



The effects of drought upon the various functions of plants are im- 

 portant in agriculture and forestry. Work in this field is considered in 

 Chapter X. 



