594 FACTORS AFFECTING GROWTH 



According to Harvey (1930) and others the threshold temperature for 

 inducing hardiness is about 6° C, at least in some species. He found that 

 exposure of cabbage plants to a temperature of 0° C. for one to four hours 

 per day kept the plants in a hardy condition even if they w^ere exposed to 

 temperatures of from 10° to 20° during the remainder of the day. This 

 indicates that as long as outdoor temperatures fall to about 0° C. for a few 

 hours each day, species with the capacity for cold resistance probably remain 

 resistant to injury at considerably lower temperatures. 



Seasonal variations in hardiness are of normal occurrence in the organs 

 of temperate zone species which are exposed to freezing temperatures during 

 winter months. The leaves of evergreen plants are not cold resistant during 

 the summer, but pass into the hardened condition in the autumn, probably 

 remaining continuously in this state during the winter. In the spring they 

 pass through a dehardening process and lose their cold resistance. The living 

 cells of the exposed stems of deciduous species undoubtedly pass through a simi- 

 lar seasonal cycle of hardening and dehardening. 



Among the physiological conditions which seem to be often associated 

 with the property of cold resistance in plant tissues are ( i ) relatively low 

 water content of the tissues, (2) accumulation of soluble carbohydrates in the 

 cells accompanied by an increase in their osmotic pressure, and (3) an in- 

 creased proportion of unfreezable ("bound") water in the tissues. 



Heat Injury and Heat Resistance. — I. Causes of Injury to Plmits at 

 Relatively High Temperatures. — Several types of injury result to plant cells 

 either directly or indirectly from relatively high temperatures: 



(1) Desiccation Injury. — High leaf temperatures resulting either from 

 intense insolation or high air temperatures, or both, may result in excessive 

 rates of transpiration. A relatively high rate of water loss, particularly at 

 times when the rate of absorption of water is sluggish, often leads to death 

 of some or all of the leaves or branches on a plant as a result of desiccation. 

 In extreme cases entire plants are killed in this way. 



(2) Injury Resulting from Metabolic Disturbances. — Relatively high 

 temperatures often induce various types of metabolic disturbances which are 

 detrimental or even fatal to plants. One important example of such an effect 

 has already been described (Chap. XXIX). With rise in temperature in- 

 crease in the rate of photosynthesis usually fails to keep pace with increase in 

 the rate of respiration. Relatively high temperatures therefore frequently 

 cause a stunting of plants because a disproportionate amount of the food? 

 manufactured is consumed in respiration. Maintenance of such a condition 

 for extended periods often results in the death of plants. 



(3) Direct Thermal Effects upon the Protoplasm. — The thermal death 



