26 THE COTTON PLANT IN EGYPT chap. 



however, that two such dissimilar organisms as a cotton 

 plant and a sore-shin fungus possess identical growth- 

 curves with no true optimum. On the other hand, the 

 cotton plant certainly grows best at temperatures near 

 32° C. ; we are therefore led to a final conclusion, namely, 

 that the general " growth-optimum " of any higher plant 

 is that temperature at which, after prolonged exposure, 

 the equilibrium of growth-acceleration with " a;-produc- 

 tion " allows the maximum sustained value to the former. 

 The " a;-production " factor is itself compound, being the 

 balance of " a?-production " against " ^-decomposition," for 

 the latter of which there appear to be special facilities in 

 the higher plants. 



Summarising this section, we have found that, in the 

 absence of other limiting factors, growth is accelerated by 

 rise in temperature along a logarithmic curve, while a 

 time factor (" thermotoxy ") acts against this acceleration, 

 unless the toxic katabolites are removed as fast as they 

 are formed. The " optimum " is essentially a variable, 

 but the " maximum " is normally constant, though it can 

 be depressed by suitable treatment. Tissue temperatures 

 which exceed 38° C. reduce the subsequent growth- rate of 

 the cotton plant to a marked extent, while prolonged 

 exposures above 35° C. are proportionately harmful. 

 Exposure to temperatures below 30° C. for a long 

 time appears to produce no injurious effect on the cotton 

 plant. 



The effect of sunshine. — The conclusions of the 

 preceding discussion would lead us to expect that strong 

 sunlight might conceivably have a prejudicial effect on the 

 growth of the plant-stem, by over-heating the growing- 

 point, but thermo-electric measurements of tissue-tem- 

 peratures indicate that the young leaves and buds rarely 

 exceed the shade temperature during early summer, owing 

 to the regulation of their temperature by transpiration. 



Actual measurements of growth, on the other hand, 



