EFFECTS OF TEMPERATURE UPON GROWTH 589 



100 



so- 



so 



are affecting growth, but that they are present in different relative intensities as 

 compared with the maximum effectiveness. 



According to Liebig's law of the minimum only an increase in factor A 

 will cause an increase in the yield of a crop. According to IVIitscherlich, 

 increase in any one of these factors will cause an increase in yield. A 

 unit increase in A will have the greatest effect, a unit increase in B the 

 next greatest effect, etc. Factor £ is so 

 close to its maximum that a unit increase 

 in it will have an almost negligible influ- 

 ence on yield. IMitscherlich's interpreta- 

 tion of the law of the minimum seems to t&o 



in 



be more nearly in accord with the results g 

 obtained in experiments on plants than - 

 Liebig's simpler formulation of this same 

 principle. 



Effects of Temperature upon Growth. 

 —The rate of every physiological process ^^^ ^^^ Diagram to illustrate 

 occurring in plants is markedly influenced ^^^,^ interpretations of the "law of 

 by the all-pervading factor of temperature, the minimum." 

 Similarly the rate of growth, as measured 



in terms of any of the usual quantitative indices, is profoundly influenced by 

 this factor. Temperature, however, exerts qualitative as well as quantitative 

 effects upon the development of plants. In other words the structural 

 development and physiological reactions of a plant may vary greatly, depend- 

 ing upon the temperature pattern of that plant's environment. Fmally, 

 whether or not a plant can survive in a given habitat often depends upon 

 the temperature extremes which occur in that habitat. 



I. Effect of Temperature on the Rate of Growth. — It has been customary 

 to consider that there are three "cardinal" temperatures for growth, a mini- 

 inu?n, an optimum, and a maximum, although these so-called cardinal points 

 on the temperature scale may vary greatly with different species. While in a 

 loose sense this is true, and such temperatures can be approximately deter- 

 mined by experimentation, they are by no means immutable. All three of 

 these "critical" temperatures have been found to vary considerably with the 

 stage in the development and the physiological condition of the plant, the time 

 and rate of exposure, and other environmental conditions. 



In general, the range of temperatures within which growth will take 

 place varies considerably with the species. Arctic and alpine species may 

 grow at the freezing point or even at temperatures slightly below, and their 

 optimum growth temperature is often no higher than 10° C. Most species 



