THE PLANT AND ITS ENVIRONMENT 157 



marked responses in the activity of protoplasm. In general, 

 active life is possible for the higher plants between and 50° 

 Centigrade, although these limits vary much from one species to 

 another. The lowest temperature at which a given plant can 

 continue to live is known as its minimum temperature and the 

 highest as its maximum. At some point between these two the 

 plant displays its greatest activity and this point is known as its 

 optimum temperature (Fig. 76).* The various physiological proc- 

 esses which go on within the plant, such as photosynthesis, 

 respiration, and growth, each have their minimum, optimum, and 

 maximum temperatures and these are not necessarily the same 

 for the different processes. 



Members of the vegetable kingdom lack the high and deli- 

 cately maintained body temperature which is characteristic of 

 the higher animals, and the temperature of most plants follows 

 rather closely that of their environment, absorbing heat or losing 

 heat as the environment becomes warmer or colder. About 

 25 per cent of the radiant energy from direct sunlight, and a 

 much larger percentage in diffuse hght, is absorbed by the plant. 

 Most of this is converted into heat, only a small fraction of the 

 energy being used in photosynthesis. This heat would often 

 raise the temperature of the plant above the maximum if it were 

 not largely expended in evaporating water from the plant tissues, 

 and the importance of transpiration as a cooling process is thus 

 again to be emphasized. The body temperature of the plant 

 may sometimes fall a little below that of its immediate environ- 

 ment, owing to excessive radiation or evaporation; or it may 

 sometimes rise noticeably above it owing to the release of heat 

 during respiration, especially in regions where growth is active. 

 The latter phenomenon is particularly conspicuous in the case of 

 intense bacterial action, for the amount of heat liberated by the 

 vigorous respiration of these lowly plants is often sufficient to 

 raise the surrounding temperature markedly. 



The problem of the temperature relations of plants is further 

 complicated by the fact that a plant may often become accommo- 

 dated or "acclimatized" to temperatures higher or lower than the 

 usual limits for the species, if the plant is brought into the new en- 

 vironment very gradually. Thus plants which would normally 

 suffer from cold at a given temperature may often be made to 



* These terms maximum, minimum and optimum are also used for other 

 environmental factors, notably light and moisture. 



