300 TEXTBOOK OF PLANT PHYSIOLOGY 



may allow the development of disease and may even cause the 

 death of an organism. 



During retardation there is no growth, but the other vital proc- 

 esses, such as respiration, assimilation, metabolism, etc., still 

 continue. The cause of this check in growth must be looked for in 

 the disturbed harmony between the separate functions 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 till a lethal 

 temperature is reached, its maximum and optimum coinciding, 

 while assimilation decreases considerably earlier. Therefore, at 

 temperatures exceeding the required optimum for assimilation, 

 consumption will overbalance assimilation and the plant will soon 

 be exhausted. Hence, 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 materials, plants which show fast development or, as garden- 

 ers would say, have been forced at a high temperature, are usually 

 weaker than those which 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 quite evident that the exact determination of such an 

 optimum presents greater difficulties than the estimation of the 

 physiological optimum. It requires, in the first place, much tech- 

 nical equipment such as several greenhouses where tempera- 

 tures can be regulated with precision. This has not been avail- 

 able at any of the scientific institutions. Recently, however, the 

 construction of such greenhouses has begun at some of the wealthier 

 American institutions. For the present one must be contented to 

 obtain approximate information from the observation of plants 

 in their natural state. Such observations usually show that the 

 harmonious optimum is not constant for the whole period of devel- 

 opment of a plant. It varies continually from germination to 

 flowering and maturity. For most annual plants, at least, a 

 general rule can be established, that in the earlier stages of devel- 



