CHAPTER XI 

 HEAT 



Heat is a factor that is of very great importance to plants but one 

 that is difficult to study in such a way as to yield tangible results. 

 There seems to be little, if any, relation between structure and 

 temperature. For this reason we cannot readily see the effect of 

 heat or lack of heat upon the plant, and it is very difficult to separate 

 heat as a factor from other factors that are acting at the same time. 

 There are many structural features of plants that were formerly 

 attributed to heat that are really due, for the most part at least, 

 to other factors. 



Plants may be compared, in a sense, to cold-blooded animals. 

 Their internal temperatures vary with the external temperature and 

 their greatest activity takes place at an optimum temperature, while 

 at very low or very high temperatures they are sluggish or dormant. 

 Each species of plant has a maximum temperature and a minimum 

 temperature beyond which activity ceases, as well as an optimum 

 temperature at which it is most active. 



79. Minimum Temperatures.— In general plants are adapted to 

 temperatures ranging from 0° to 100° C. The reason for setting 

 these rather arbitrary limits is simply that plants, in order to carry 

 on their functions, must have liquid water, and, of course, water 

 ordinarily solidifies at 0° and vaporizes at 100° C. There are a few 

 of the lower plants, however, that are active at temperatures 

 below zero. Some of the algse that occur in Arctic waters appear to 

 be active at temperatures below zero. The water in which they live 

 is still liquid, however, since there is salt enough in the water to 

 lower the freezing-point several degrees below zero. The "black 

 spot" fungus which is found on meat in cold storage is able to grow 

 and even produce spores at temperatures as low as 6° below zero. 

 This growth is not luxuriant, of course, and very little water is re- 

 quired. 



Plants apparently have no protection against low temperatures. 

 Their internal temperature rises and falls with the external tempera- 

 ture. To be sure, heat is liberated by respiration within the plant as 

 in an animal, but the i)lant camiot keep up its internal temperature 

 by respiration, as a warm-blooded animal can, because the optimum 

 temperature for respiration is rather high and the rate of respiration 

 (130) 



