130 PLANT PHYSIOLOGY 



from excessive heat, may allow the development of disease and 

 may even cause the death of an organism. 



Within the inhibitory ranges, there is no growth; but the other 

 vital processes, such as respiration, assimilation, and metabolism, 

 may still continue. The cause of this check in growth must be 

 looked for in the disturbed harmony between the separate func- 

 tions of the plant, since growth is a very complex process requiring 

 a precise coordination of all functions. The rate of the various 

 metabolic processes, however, changes in different degrees with 

 variations of temperature. For instance, respiration increases 

 until a lethal temperature is reached, its maximum and optimum 

 coinciding; while assimilation decreases considerably earher. 

 Therefore, at temperatures exceeding the required optimum for 

 assimilation, consumption will overbalance assimilation, and the 

 plant will soon be exhausted. 



The optimum temperature for growth is by no means the most 

 favorable one for the general development of a plant. According 

 to the usual definition, the optimal point is one at which growth 

 proceeds most rapidly. But the highest rate of growth is not 

 always the most desirable for the development of a strong and 

 healthy plant. Because of the rapid consumption of food mate- 

 rials, plants that show fast development or, as gardeners would 

 say, have been forced at a high temperature are usually weaker 

 than those that have developed at a lower temperature. A 

 purely physiological optimum, therefore, indicating the greatest 

 rapidity of development, must be distinguished from the har- 

 monious optimum, producing the most vigorous plants. 



It is evident that the exact determination of such an optimum 

 presents greater difficulties than the estimation of the physio- 

 logical optimum. It requires, in the first place, much technical 

 equipment, such as several greenhouses where temperatures can 

 be regulated with precision. This, however, is available only in 

 a very few scientific institutions. Observation of plants in their 

 natural state usually shows that the temperature optimum for 

 growth is not constant for the whole period of development of a 

 plant. It varies continually from germination to flowering and 

 to maturity. For most annual plants, at least, a general rule 

 can be established that, in the earher stages of development, this 

 optimum fies at a lower point than during the later ones. This 

 coincides with the natural march of temperature from spring to 



