Grafts et al. — x — Water in Plants 



tion from the cytoplasm into the vacuole of the plant cell actually occurs. In consider- 

 ing Levitt's (1947) claim that secretion values reported are thermodynamically un- 

 sound, Bennet-Clark casts doubt on the validity of the value used by Levitt to repre- 

 sent water permeability in calculation of water velocity. 



LuNDEGARDH further identifies an active water mechanism in roots with the ab- 

 sorption of salts from the culture medium. As in previous work, he assumes that 

 "anion respiration" provides the energy. 



Recent findings have added to our understanding of auxin-induced water uptake 

 by cells. In an important paper Levitt (1948) offers convincing evidence that auxin- 

 enhanced water absorption by potato discs is not active water uptake, as suggested by 

 several investigators, but rather a result of altered plasticity of the cell wall. The 

 manner in which auxin may increase wall plasticity is not understood but Kelly 

 (1947) has shown that it is an aerobic process in Avena coleoptile segments. Audus 

 has critically reviewed the problem of cell elongation (Biol. Rev., 1949). 



Studies on water secretion and transport by Nitella cells have been extended by 

 Osterhout (1949). That the pumping mechanism demonstrated in Nitella under 

 somewhat arbitrary conditions could operate in certain kinds of cells seems validated 

 by a theoretical consideration by Franck and Mayer (1947) of an osmotic diffusion 

 pump. 



The absorption of water by root systems is being given detailed consideration by 

 Kramer, whose book on the subject is in press. Frey-Wyssling (1941) has demon- 

 strated guttation under conditions of high water absorption and low transpiration in 

 several woody species, and Daniel (1949) observed xylem exudation from excised 

 roots of several coniferous species. Studies on the effect of certain salts on increasing 

 root pressure and tissue resistance to infiltration in sunflower, tomato, and dahlia were 

 made by Volk (1944). 



Hagan (1949), by measuring water intake by excised root systems of sunflowers 

 growing in moisture-deficient soil, has verified the autonomic diurnal cycle of root 

 activity studied by Grossenbacher (1938). The diurnal cycles continued for about 

 6 days but were terminated if the roots were aerated with N2 or CO2. Similar cycles 

 observed in roots washed free of soil indicate that the activity is a function of the root 

 and not of its environment. Such roots in water exuded water during the day and 

 withdrew it during the night. Hagan attributes this activity to changing hydration of 

 the cytoplasm. 



Mitscherlich (1947) has pointed out that water requirements of plants follow 

 the law of plant growth. Water requirements are high for plants having short vegeta- 

 tive periods and they decrease with increasing nutrient supply. More specific data 

 on the effects of potassium and calcium on the water relations of plants were given 

 by Woestmann (1942). Potassium induced water storage, water uptake, and trans- 

 piration, probably through its effect of promoting hydration of the protoplasm. Calcium 

 diminished protoplasmic hydration and consequently lowered protoplasmic permeability 

 and hence water absorption. Water storage and transpiration were reduced. Plants 

 deficient in nitrogen showed decreased transpiration, a failure of guard cells to func- 

 tion normally, and an increase in cold resistance and protoplasmic viscosity (Gessner 

 and Schumann, 1948). 



Wadleigh (1945) has made a new approach to the problem of plant-soil water 

 relations. He devised a method for integrating the variables that account for the total 

 soil-moisture stress against which the plant absorbs water. When leaf elongation of 

 cotton plants was expressed empirically as a second degree function of soil-moisture 

 stress, values close to IS atmospheres for the stress inhibiting leaf growth were calcu- 

 lated (Wadleigh and Gauch, 1948). Additional data indicating that under good 

 conditions of soil aeration and fertility the rate of vegetative growth increases with 

 increasing soil moisture within the range from near the permanent wilting percentage 

 to near soil saturation have been presented by Haynes (1948). He concludes that 

 corn plants subjected to daily periods of temporary wilting produce small vegetative 

 growth when under humid conditions. On the other hand, Wilson (1948&) has 

 shown that tomato stem tips can maintain their highly hydrated state at the expense 

 of older mature tissues. Thus they can increase steadily in length throughout the 

 day and night even when the plant as a whole passes through a diurnal cycle with the 

 water balance on the negative side during the day. 



Morton and Watson (1948) conclude that the rate of leaf production by apical 

 meristem is unaffected by water supply. The number and size of cells per leaf in- 

 creased with increasing water supply. Net assimilation was greatly reduced by water 

 deficit. 



