298 THE INTERNAL WATER RELATIONS OF PLANTS 



Evidently during this period, which corresponds approximately to the period 

 of relatively high transpiration rates, w^ater was moving out of the fruit into 

 the other organs of the tree. Transpirational water loss from the fruit itself 

 was negligible. Between the hours of about 4:00 p.m. and about 6:00 a.m. 

 the next morning the volume of the lemon gradually increased, showing that a 

 gradual restoration of the water content of the fruit was occurring. Marked 

 daily fluctuations in the diameter of the lemon fruits were apparent even 

 under environmental conditions which resulted in no observable wilting of 

 the leaves. Greater diurnal fluctuations in the diameter of the fruits was 

 found to occur after the trees had gone without irrigation for several weeks 

 than when irrigation water had been recently supplied. 



Drought Resistance. — Some species of plants are better able than others 

 to survive and develop in habitats in which a dearth of water is frequent 

 or usual. This capacity of surviving periods of drought with little or no 

 injury is usually termed drought resistance. All perennial species of plants 

 native to semi-arid regions are more or less drought resistant. This same 

 statement is true for those species indigenous to local habitats which, for 

 one reason or another, are unduly dry, even in humid climates. Drought 

 resistant species or varieties of plants are important in the agricultural econ- 

 omy of certain regions, such as the "dry-farming" areas of western United 

 States. Certain varieties of crop plants are much more productive in dry 

 regions than other varieties of the same species. Examples are the durum 

 and evuner varieties of wheats. 



The term "drought" is not in itself subject to any rigid definition. In 

 general, however, this term refers to periods during which the soil contams 

 little or no water which is available to plants. In the more humid climates 

 of the world such periods are relatively infrequent and seldom last very long 

 except in certain local habitats. The more arid a climate, in general, the more 

 frequent the occurrence of periods of drought, and the longer their duration. 

 Most species of plants can survive short dry periods without serious injury, but 

 only those possessing a well developed capacity for drought resistance can 

 avoid death or serious injury during prolonged periods of soil water deficiency. 



Usually soil drought conditions are accompanied by atmospheric condi- 

 tions — high temperature, low humidity, and often relatively high wind veloci- 

 ties — which favor high transpiration rates. Such "atmospheric drought" is 

 not only a usual accompaniment of soil drought, but sometimes occurs in the 

 absence of soil water deficiency. Atmospheric drought frequently induces 

 transient wilting of plants during the daylight hours. This alone often 

 results in a serious checking of photosynthesis and growth. In more extreme 

 cases atmospheric drought alone may have devastating effects upon plants. 



