404 Plant Physiology 



justment, and also in environmental factors will affect 

 the cardinal temperatures in any particular case. 



Reference has been made already to the fact that pho- 

 tosynthesis, metabolism, and other processes or responses 

 of the plant are to a certain point rapidly accentuated 

 with increase of temperature. Blackman has shown very 

 clearly that maximum activity, especially for respiration 

 and photosynthesis, has commonly been placed too high, 

 since proper consideration of the time factor has not al- 

 ways been given. 



241. Inhibition at high temperatures. From recent 

 work reported by Balls it would seem that the inhibition 

 of growth at high temperatures during a considerable pe- 

 riod of time is in all probability the result of an accumula- 

 tion in the cells of injurious metabolic products. The time 

 factor is most important. According to his views, some 

 of these deleterious products are produced at low tempera- 

 tures, but under such circumstances they are constantly 

 decomposed, whereas at high temperatures production is 

 more rapid, and consequently accumulation and injury 

 result. Upon this hypothesis the effect of high tem- 

 perature upon the protoplasm would be that of favoring 

 auto-intoxication. 



242. Heat units. Considerable attention has been be- 

 stowed upon computations of the heat units (thermal con- 

 stants) required to mature certain crops. Such data are 

 not without interest, yet examination of the evidence thus 

 far accumulated indicates that there is practically no such 

 thing as a relatively invariable thermal constant for any 

 plant when factors other than temperature are inconstant 

 or uncontrolled. Assuming that every other factor of the 



