120 ENVIRONMENTAL CONDITIONS. 



Higher evaporation-rate or increased solar intensity may raise the 

 rate of transpiration in any plant until it surpasses the possible rate 

 of supply to the transpiring parts. On the other hand, the drying- 

 out of the soil or a pathological condition of the absorbing or conduct- 

 ing system of the plant may reduce the rate of entrance or transmis- 

 sion of moisture until the transpiring tissues suffer from dryness. 

 The effect of this physiological drought, however caused, is a gradual 

 loss of water and hence of turgor, which results finally in plasmolysis 

 and wilting. Under such conditions tissue enlargement must cease 

 before plasmolysis is accomplished, and can not begin again until a 

 certain amount of turgidity has been regained. 



As long as the ratio between the rate of possible water-supply and 

 the rate of water-demand in any tissue or organ is greater than unity, 

 growth may occur. When this water ratio falls to unity growth must 

 soon cease, though the organ may retain its form and vitality. When 

 the water ratio becomes less than unity, incipient drying occurs and 

 plasmolysis must soon follow if the ratio continues less than unity. 

 Whether plasmolysis and wilting result in the death of the tissue in- 

 volved depends upon the extent to which the ratio falls below unity 

 and upon the length of the period during which this condition obtains.^ 

 Of course, it must be remembered that the matter here brought forward 

 is very much complicated by the free interchange of water by various 

 parts of the plant itself; the wilting of a certain tissue may not denote 

 anything out of the ordinary in the plant as a whole, for the normal 

 process of development often includes many reversals in growth. 

 Thus, a tuber grows for a long time and then loses its water and other 

 contents, while the entire plant of which such tuber is a part may be 

 said to be continually advancing through its development phases. 



With the continuation of a drought period most plants die only by 

 degrees; the lower and older leaves are apt to succumb first, and it is 

 only after a somewhat protracted dry period that total death of an 

 individual occurs. Even in such cases the existence of seeds usually 

 carries the vital substance forward to the next favorable season. 

 The withering and falling away of a few of the older leaves often acts 

 as an automatic removal of the drought conditions, for such a decrease 

 in the transpiring surface may so diminish the transpiration-rate as 

 to prevent further wilting. The same result is frequently brought 

 about by a temporary lowering of the evaporating power of the air or 

 of the hght intensity. The tendency to wilt, which is manifest in 

 most plants on dry, sunny afternoons, though no actual wilting may 

 occur, is regularly checked by the coming on of night with its conse- 

 quent lowering of the evaporation-rate, and also, sometimes at least, 

 through closure of the stomata. The water ratio of transpiring organs 



^Livingston, B. E., Incipient drying in plants, Science, n.s., 35: 394-395, 1912. — Caldwell 

 1913. 



