44 THE FACTORS [Part I 



exposed to the sun's rays at the same time, were heated only up to 



35° C 



Whilst awaiting further investigation, it appears to result from the facts 

 recorded above that the power of enduring high temperatures, like that of 

 withstanding cold, varies more considerably in the different species than is 

 generally supposed. The maximum temperatures determined by Sachs 

 cannot hold for plants living in extreme climates. 



Protective means for the prevention of overheating have up to the 

 present time been no better demonstrated than have means for the preven- 

 tion of overcooling. The subaerial parts of plants in very hot regions, 

 being exposed to the danger of desiccation, in most cases are protected 

 against transpiration, and are thereby deprived of the most important means 

 for keeping the temperature down, as is proved by the high temperatures 

 attained by succulent plants exposed to the sun. Many plants, however, 

 escape from the injurious effects of such high temperatures by leading an 

 exclusively subterranean life during the period of their prevalence. This, 

 however, is by no means true of all species of plants. 



3. THE CARDINAL POINTS OF THE FUNCTIONS 

 OF PLANTS. 



The life of a plant is made up of thousands of separate actions, each of 

 which is performed within its own ranges of temperature, and exhibits 

 its own optimum temperature. In most habitats — except those which are 

 generally very unfavourable to plant-life — such plants alone can succeed in the 

 struggle for existence as have their functions in a state of equilibrium that 

 corresponds to the external conditions : this condition of equilibrium is 

 termed the oecological optimum. This total optimum is not deducible from 

 the separate optima of all the functions taken collectively ; indeed many 

 functions, such as respiration or transpiration, when intensely active, are 

 injurious to the plant. In regard to every function we must discriminate 

 between the absolute optimum, which corresponds to the highest intensity 

 of a function, and the harmonic optimum, which corresponds to its most 

 favourable intensity. The oecological optimum is composed of the Jiarmonic 

 optima. 



A knowledge of the extreme temperatures of a function is more im- 

 portant, in geographical botany, than is that of the absolute optimum, 

 which it is often difficult to ascertain, and which is often somewhat devoid 

 of significance as regards the natural conditions of life. The absolute 

 optimum is important in geographical botany only when it nearly coincides 

 with the harmonic optimum, as it does, for example, for assimilation and 

 other processes of nutrition. 



