160 PART III. PHYSIOLOGY. [ 39 



HEAT. Every function of the plant can only take place within 

 certain limits of temperature : that is, between a certain minimum 

 and a certain maximum degree. Between these limits there Js for 

 each function a degree of temperature, the optimum, at which that 

 function is carried on with the greatest activity ; any fall of 

 temperature from this optimum, or any rise above it, leads to a 

 diminished activity of the function. These general laws have been 

 arrived at by observation of such processes as movement, absorp- 

 tion by the roots, assimilation, etc. 



It may be stated generally that all the functions of plants 

 inhabiting temperate climates begin to be carried on at a tempera- 

 ture a few degrees above the freezing-point ; as the temperature 

 rises to 25-30 C. the activity of the functions is increased and the 

 optimum attained ; with a further rise the activity of the functions 

 is diminished, and at 45-50 C. they commonly cease altogether. 

 In the case of plants which naturally grow in warmer climates, the 

 minimum-temperature is somewhat higher than that stated above. 

 Thus a pumpkin-seed will not germinate at a temperature below 

 13 C. 



The power of withstanding the injurious effect of exposure to too 

 high a temperature depends mainly upon the proportion of water 

 which the plant, or any particular part of it, contains. Thus, dry 

 peas can withstand exposure for an hour to any temperature up to 

 70 C., whereas, when they have been soaked in water, exposure to 

 a temperature of 54 C. proves fatal. Most parts of plants are 

 killed by prolonged exposure to a temperature, in air, of about 

 50 C., and in water, of about 45 C. 



Injury or death by exposure to cold, is only induced when the 

 temperature falls in some cases many degrees below freezing- 

 point. Some plants just those, namely, such as Lichens, and some 

 Fungi and Mosses, which can undergo dessication without injury 

 are not killed by exposure to low temperature. Here, also, the 

 liability to injury depends upon the amount of water contained in 

 the tissue. Thus, dry seeds and the winter-buds of trees can 

 readily withstand low temperatures ; but when they contain a 

 considerable quantity of water, as when the seeds are germinating 

 or the buds unfolding, they are very susceptible to injury. When 

 a part of a plant, which contains a large proportion of water, is ex- 

 posed to a low temperature, a portion of the water contained in the 

 cells escapes from them and becomes frozen on their surface, the 

 whole tissue at the same time contracting ; the water does not 



