128 ENVIRONMENTAL CONDITIONS. 



The temperature limits for vigorous activity, as these are usually 

 given, are very different for different plants. The lowest minima are 

 somewhat below 0° C, while the highest maxima are above 60° C. 

 For the retention of life in dormant phases the range is, of course, much 

 greater than for vigorous activity. Dry seeds can endure, for long 

 periods, temperatures far below the freezing-point of water and far 

 above the boiling-point. It needs to be emphasized that the water- 

 content of a tissue is highly important in determining what may be its 

 minimum and maximum temperature and the duration of such 

 temperatures that it may survive. 



2. RELATION OF TEMPERATURE WITHIN THE PLANT TO CONDITIONS OF 



THE ENVIRONMENT. 



Material changes, whether physical or chemical, generally result 

 in the warming or cooling of the medium; some heat is generally 

 produced or else disappears with each material alteration. Wher- 

 ever material processes are going forward in the plant, heat must 

 be continuously supplied, or else it must be as continuously removed ; 

 otherwise the temperature of the body must fall or rise and a cor- 

 responding alteration in the processes themselves must ensue. 



In the case of a living plant the many physical and chemical processes 

 of its life must, of course, influence one another, in velocity and direc- 

 tion in various ways, among which the mutual effect of heat absorp- 

 tion and liberation must be important. Thus the heat set free by 

 respiration may be of primary importance in maintaining a possible 

 temperature for cell growth, and the absorption of heat by transpira- 

 tion is probably often the prime condition that prevents a too great 

 rise in tissue temperature. 



If there were no outward or inward passage of heat through the 

 periphery of the plant, it is obvious that life could be possible only 

 so long as this complicated interplay of the heat effects of the various 

 physiological processes were automatically so limited that no essential 

 process might be too greatlj^ altered by temperature change. But, 

 just as the moisture conditions of the plant-body are usually much 

 more influenced by water changes between it and its surroundings than 

 by the generally insignificant destruction and formation of water 

 within the tissues, so also the internal temperature conditions usually 

 depend mainly upon heat exchanges with the exterior, and the internal 

 absorption and liberation of heat which has just been considered are 

 of prime importance only in relatively few cases, if they ever are at 

 all in nature. 



The temperature of the plant tends closely to follow that of its 

 environment; roots can seldom possess a temperature markedly dif- 

 ferent from that of the surrounding soil, and stems and leaves are 

 never very much warmer or cooler than the air that bathes them. If 

 the vital processes result, at any time, in the liberation of heat, a 



