Grafts et al. — 206 — Water in Plants 



aid in the transport of these solutes throughout the vascular system and in 

 their final distribution by various mechanisms of translocation, both pri- 

 mary and secondary. 



From results presented, it is evident that water deficits may have pro- 

 found effects upon plants. Studies on energy absorption by leaves indicate 

 there is no lack of available energy for photosynthesis and the transport of 

 water, particularly during full sunlight. Work on the state of water in the 

 conducting tracts indicates furthermore that the mechanism of transloca- 

 tion breaks down only after the occurrence of severe wilting. The problem 

 of the plant's water supply then seems to resolve itself into a question of the 

 relation of the rate of absorption by roots to the rate of loss from the 

 leaves. Though certain leaf structures may prevent the loss of the vapor 

 cloud from the leaf surface and hence conserve moisture, most leaves dur- 

 ing still weather and all leaves during wind lose large amounts of water 

 if the stomates are open so that photosynthesis may go on. Consequently, 

 most plants require large amounts of water from the soil reservoir. In 

 many soils around one half of the water held against the force of gravity 

 is available for plants and in light to medium textured soils the movement 

 of such moisture through the soil to the roots and the growth of roots 

 through the soil are rapid enough so that plants do not suffer from lack 

 of moisture until the permanent wilting percentage is approached. In a 

 heavy soil, on the other hand, Aldrich, et al. (1940) found that pear trees 

 began to suffer when only one half of the so-called available moisture had 

 been used. Apparently movement of water through such a soil was so 

 slow that even though the average moisture content was well above the 

 PWP the soil immediately around the roots became dry during the day 

 and was not replenished during the night and root growth was not suffi- 

 cient to counter-balance this effect. 



Plants exhibit many means for meeting conditions of drought. Leaf 

 and stem modifications have been mentioned as have biochemical trans- 

 formations within the plant. Diverse root forms, tap roots for deep soils, 

 fibrous roots for shallow or claypan soils, branching roots, and root hairs 

 have all been resorted to to enable the plant to obtain adequate water 

 from the soil. However, there is a limit to root extension. Carbohydrate 

 supply, the concentrations and proportions of mineral nutrients, soil oxy- 

 gen, and even genetic factors control the size and form of roots of a given 

 plant. And in plant populations, particularly crops, competition may 

 severely limit the extent to which any one root system may profitably grow 

 into its water supply. For these various reasons there is a definite limit 

 determined by immutable physical laws that regulates the production of 

 plants in a given environment. This limit is related to the ability of the 

 plant to adjust itself to a low water supply and at the same time absorb 

 CO2 for photosynthesis. 



Certain drought-resistant plants may endure long periods of permanent 

 wilting but though they survive they grow very little while wilted. Suc- 

 culent plants may close their stomata and carry on an internal economy 

 that enables them to survive drought but they, too, grow very little in the 

 process. Other plants, such as the bunch grasses, space themselves so that 

 they have enough moisture to carry them through the dry periods and still 

 others dry up and die, leaving only seeds to carry on when moisture is 

 again available. 



Many studies have pointed out the relations of plant distribution to 



