COLD INJURY AND COLD RESISTANCE 591 



can endure vary greatly according to species and depend upon their capacity 

 for heat resistance and cold resistance, respectively, as shown in the later 

 discussion. 



3. Morphogenic Effects of Temperature. — No two of the many and varied 

 metabolic processes occurring in a plant are equally influenced by a change 

 in temperature. Hence the morphogenic development of a plant is often 

 markedly different under one set of temperature conditions than under another. 

 Such effects upon the structural development of plants are the most complex 

 and most striking of the many influences of temperature upon plants. In- 

 numerable illustrations of the morphogenic effect of temperature could be 

 mentioned, only one of which will be cited. Thompson and Knott (1933) 

 have shown that if lettuce (white Boston variety) is grown at a temperature 

 of 70-80° F. no heads form and the plants soon begin to develop flowerstalks. 

 At the lower temperature range of 60-70° F., however, the plants form heads, 

 and the development of flowerstalks is considerably delayed as compared with 

 plants at the higher range of temperatures. A somewhat similar effect of 

 temperature upon the development of celery is described in the last section of 

 this chapter. 



Cold Injury and Cold Resistance. — I. Causes of Injury to Plants upon 

 Exposure to Low Temperature. — Several types of injury may occur in plants 

 as a result of exposure to conditions which often prevail during cold seasons: 

 ( I ) Suffocation. — When covered for long periods during the winter with 

 densely packed or encrusted snow some low-growing species of plants may 

 suffer from a deficient oxygen supply. This is reported to sometimes result in 

 serious injury to wheat in parts of Russia, but in general is not a phenomenon 

 of very frequent occurrence. 



(2) Desiccation. — Relatively high winter transpiration rates in ever- 

 greens during a period when absorption of water can proceed only at a rela- 

 tively slow rate often lead to a tj'pe of injury frequently called winter-killing 

 (Chap. XIV). Injury under such circumstances is due to desiccation of the 

 tissues. A similar type of injury may result to some plants, especially herba- 

 ceous species, as a result of frost heaving of the soil. Such heaving often tears 

 the roots loose from the soil or, in extreme cases, may even result in breaking 

 them. If environmental conditions favoring high transpiration rates intervene 

 before the root system can be securely re-established in the soil the plant may 

 be so severely desiccated that death or marked injury results. This is often 

 a serious source of injury to winter wheat during "open" winters. One of 

 the advantages of mulching plants with straw, leaves, etc. during the winter 

 months is that it greatly reduces frost heaving of the soil. 



(3) Chilling Injury. — Many species of plants, particularly those which 



