132 HEAT 



80. Maximum Temperatures.— No plants are active at tempera- 

 tures of 90° C. or higher. The maximum temperatures that can be 

 endured, however, like the minimum temperatures, vary greatly 

 with different species of plants and with different internal and exter- 

 nal conditions. Certain species of yeast have been shown to be 

 capable of enduring a temperature of 114° C. when in a dormant 

 condition. IMany very dry seeds can endure temperatures above 

 100° C. while the same species in an active state would be killed by 

 a much lower temperature. Certain algse found in hot springs live 

 and carry on their various functions at 77° C. and there are a few 

 fungi which can endure as much as 89° C. Some lichens, and per- 

 haps even seed plants, that live in tropical deserts, where the air 

 temperatures, and more especially the surface soil temperatures 

 become exceedingly hot, must endure temperatures of 70° C. or 

 higher frequently. 



Most of the plants we are familiar with, however, live where the 

 temperature never goes much beyond 40° C. and if subjected to 

 higher temperatures when in an active condition, since they cannot 

 quickly go into a dormant state, they suffer death. 



81. Optimum Temperatures.— The optimum temperature for 

 any plant is the temperature at which the plant gets along best in 

 the conditions under which it is living. There is an optimum tem- 

 perature for each physiological function of the plant and these vari- 

 ous optima, for the most part, do not coincide. The ecological 

 optimum is thus a temperature at which the plant as a whole is most 

 nearly in harmony with the environment and it may not coincide 

 with a single one of the physiological optima. The ecological 

 optimum varies greatly for different plants and for the same plant 

 at different ages and at different times during the year or even at 

 different times of day. For example, the optimum temperature 

 during the day, at least for plants of temperate climates, is always 

 higher than that for the night. For many plants the optimum tem- 

 perature becomes progressively higher from the time of germination 

 to the time of fruiting, though it must be admitted that we have 

 accurate knowledge of these variations for only a very few plants. 

 Probably no plant, in its natural environment, ever enjoys its 

 ecological optimum of temperature throughout an entire year. 

 However, a plant may be said to be successful if it can reproduce 

 each year, and plants vary so greatly in their adaptations to heat 

 as well as to other factors that they are found to flourish under 

 extreme conditions. Some Arctic and Alpine plants produce their 



