TEMPERATURE RELATIONS OF FRUIT PLANTS 235 



these, and we can follow the transpiration current along its course, whereas the 

 action of heat is carried on in the molecular region of the protoplasm beyond 

 our ken, and is visible to us only in its final consequences, such as the acceleration, 

 retardation or complete cessation of physiological processes. The cecological 

 phenomena display similar processes. Protective adaptations against a want 

 or superfluity of water are within our power of observation, those against cold 

 and heat are entirely beyond them. We can directly see whether any plant 

 naturally inhabits a dry or a moist station, but not whether it belongs to the 

 flora of a cold or warm climate. Indeed plants from hot deserts frequently have 

 a strong resemblance in habit to those of polar zones." 



The metabolism of a plant may be regarded as a complicated set of 

 chemical reactions, subject to several influences. Among the factors 

 governing chemical reactions and vital processes the chemist and the 

 physiologist recognize temperature. There are certain limits, apparently, 

 for all vital reactions, limits wide in some instances, narrow in others. 

 Some plants require a relatively high temperature for setting in motion 

 the processes known as growth; others will carry on similar processes 

 at a lower point. One may go on at a certain temperature in a given 

 plant, while another in the same plant may require more heat. At a 

 low temperature a plant is said to rest; certain processes are in truth 

 suspended, but others are inaugurated. Finally, there is a point so 

 low that the plant cannot exist; it dies apparently from cold. On the 

 other hand, all plants show their maximum growth activity within the 

 limits of a comparatively small range of temperature; above these 

 limits some reactions are retarded or some are so accelerated as to 

 become harmful, or new injurious reactions begin and the net results that 

 are recognized as growth or fruitfulness are diminished; here again the 

 point is finally reached where the equilibrium of reactions is broken and 

 death ensues. 



Withal, it must be considered that temperature is only one of the 

 factors affecting plant growth. Even a single plant may be limited at 

 various times by quite different features of its environment. 



Investigation has shown that in soy beans in Maryland growth was controlled 

 during one fortnight by temperature, but in the next by the rainfall-evaporation 

 ratio. 1-^ In Ceylon it has been found that with ^^awe and FwrcrtEa temperature 

 is always the limiting factor; with Dendrocalamus sometimes it is temperature, 

 sometimes water supply. In January Vitis is Umited in growth by temperature 

 and in July by the water supply, while with Capparis and Stiff tea the Umiting 

 factors are water supply during the day and temperature during the night. ^^^ 



MacDougaP^^ shows the operation of limiting factors in his study of the 

 growth of tomato fruits. As the temperature of the fruits increased, growth 

 progressed until the rise caused a loss of water exceeding the gain. The higher 

 temperatures did not accelerate growth unless the relative humidity of the atmos- 

 phere was high; a rise in temperature with decreased humidity retarded or stopped 

 growth or even caused an actual diminution of volume. 



