CHLOROPHYLL AND ENERGY 203 



5 per cent and 10 per cent, which is two hundred times greater 

 than the normal. When it rises above 10 per cent, assimilation 

 decreases ; the carbon dioxide becomes toxic and the stomata 

 close. If by the use of ammonia they are forced to remain 

 open, the assimilation decreases less. 



The Influence of Temperature 



If the plant is not affected by frost, photosynthesis can be 

 observed at rather low temperatures; the common juniper, 

 for example, is capable of assimilating at - 40° C, the spruce 

 at - 35° C. 



As the temperature rises, the rate of assimilation progres- 

 sively increases according to van't Hoff's law and is nearly 

 double for an increase of 10° C. This increase continues up to 

 35° C. or 37° C, then the rate rapidly diminishes towards 

 43° C. to 45° C. — a mortal temperature for the plant. 



But here the time must be taken into account. The figures 

 just quoted are based on a brief study of the phenomenon at 

 each of the temperatures given. At 35° C, for example, there 

 is no doubt that the rate of assimilation is about twice as 

 high as it is at 25° C, but this temperature is unhealthy for the 

 plant and if the experiment is continued the rate decreases 

 and falls below that at 25° C, while the latter is maintained 

 indefinitely. The best continuous temperature for photo- 

 synthesis is therefore in the neighbourhood of 25° C. 



Another difficulty of measurement arises from respiration. 

 Absent at low temperatures, it becomes established and 

 increases at higher temperatures, but follows a rather different 

 curve from that of photosynthesis. It is therefore difficult to 

 estimate the quantity of oxygen that must be added or of 

 carbon dioxide that must be subtracted from the results 

 obtained. 



The temperature of the leaves is also rather variable. 

 It is maintained by transpiration, but it may be up to 10° C. 

 above the ambient temperature. If, oi; a fine summer day, a 

 cloud passes in front of the sun, there may be sudden variations 

 of more than 10° C. in the temperature of the leaf. The 



