RESISTANCE OF PLANTS 363 



of plants open widely under the influence of high temperature. 

 This pathological phenomenon takes place even with deficiency 

 of water in the leaves and leads to a rapid desiccation and death 

 of the plants. 



Although not surpassing the limits endurable by plants, 

 temperatures from 35 to 40°C. may be very harmful if their 

 action is prolonged. The disadvantageous change in the relation 

 between respiration and photosynthesis is of major importance 

 in this case; photosynthesis possesses a relatively low optimum 

 as compared with respiration. At high temperatures, the 

 expenditure of organic substance in the process of respiration 

 exceeds its synthesis, and the plant is gradually depleted of its 

 carbohydrates and then begins to starve. This is especially the 

 case with plants in moderate climates, such as wheat, flax, and 

 many truck crops, which at very high temperatures begin to 

 elongate, tiller little, and decrease their yield. Besides, this 

 general weakening makes them more susceptible to fungous 

 attacks. The harmful effect of excessively high temperatures 

 is one of the chief causes of decreased yield of early spring cereals 

 when they are sown too late. 



In different growth stages, plants show various degrees of 

 susceptibility to the effect of high temperatures. Usually, 

 excessive heat is most injurious in the early stages of growth. 

 Conversely, during ripening of the fruit, high temperatures may 

 be beneficial. For some plants, high temperatures are especially 

 injurious during flowering and cause sterility and abscission of 

 the blossoms or young fruit. 



84. Effect of Water Deficiency on the Plant. Soil and Atmos- 

 pheric Drought. — One of the adversities of the plant most often 

 met with is the deficiency of water, which in its extreme mani- 

 festation is designated as ''drought." Drought, first of all, 

 disturbs the water balance of plants. ^This sooner or later dis- 

 arranges all the other physiological functions, such as photo- 

 synthesis, respiration, carbohydrate and protein metabolism, 

 translocation of substances, and growth. 



A distinction should be made between atmospheric and soil 

 drought. Atmospheric drought is characterized by a high 

 temperature and low relative humidity of the air (10 to 20 per 

 cent) . It affects the plant by excessively increasing its transpira- 

 tion. This may disturb the coordination between the rate of 



