MINIMUM TEMPERATURES 131 



decreases with a lowering of the temperature. Therefore, when the 

 plant most needs extra heat it cannot supply it through respiration. 

 The internal temperature of the plant often is somewhat higher than 

 the external temperature during the day. The internal temperature 

 of pine leaves, for example, has been found to be from 2° to 10° 

 higher than that of the surrounding air during the day. This, how- 

 ever, has been attributed to the absorption of radiant energy rather 

 than to respiration. 



The death of plants by winter-killing, or from cold at any time, 

 is very frequently the result of desiccation rather than directly 

 from low temperature. Thus, plants that are protected from drying 

 winds can endure much lower temperatures than those of the same 

 species that are fully exposed. There is no place in the world that 

 is too cold for plant life. There are, however, places that are too 

 continuously cold. Many plants, when in a dormant condition, 

 can live for a considerable time at temperatures far below zero, but 

 they cannot grow and reproduce in such temperatures. It is not 

 known for many plants what the minimum temperature beyond 

 which life ceases is. It varies greatly at different times of the year 

 and with different conditions of the plant, especially as to the amount 

 of moisture present in the plant. 



For some plants temperatures below zero are necessary peri- 

 odically. Some seeds, for example, will not germinate until they 

 have been subjected to freezing temperatures and this is true also 

 of the spores of some fungi. 



Plants can often be made more resistant to low temperatures 

 artificially by a process of hardening. This is usually done by sub- 

 jecting the plants to a temperature a few degrees above freezing 

 for several days and watering sparingly. Seedling vegetables are 

 often hardened in this way by placing them in a cold frame for 

 several days before transplanting to the field. When plants freeze 

 water is withdrawn from the cells and ice forms in the spaces be- 

 tween cells. Death of the cells results from this desiccation of the 

 protoplasm or possibly in some cases from the rupturing of the cell 

 through the mechanical pressure of the ice. The hardening process 

 seems to increase the water-retaining power of the cells so that a 

 larger proportion of their water is prevented from being withdrawn 

 and remains in an unfrozen state. Perennial plants can often be 

 hardened by subjecting them to drought, the results being much the 

 same as those brought about by subjection to low temperatures. 

 Tree seedlings may be hardened by this method in nurseries before 

 transplanting. 



