320 The Physiology of Plants BOOK in 



a poisonous influence on the plant there is a certain optimal 

 percentage at which photosynthesis is most active, and 

 that this varies not only in different plants, but according 

 to external conditions. 



In 1885 Kreussler made a number of researches on the 

 subject ; in each experiment he kept the air supplied to 

 the plants charged with a measured quantity of the gas, 

 and employed the constant light of an electric lamp. The 

 plants he used were varied, and included Rubus, Carpinus, 

 and Tropceolum. His results showed that the amount of 

 the gas which was used increased with the amount present, 

 but not proportionally. It required the presence of seven- 

 teen times the normal amount, or -56 per cent., to make 

 the quantity entering into combination double the normal 

 quantity. He estimated the optimum amount rather 

 higher than Godlewski. 



The latest investigations were those of Blackman at the 

 end of the century. These were remarkable in that they 

 correlated closely the two factors of quantity of gas and 

 intensity of illumination and showed that for each degree 

 of the latter there exists an optimum point for the former, 

 which may range as high as Godlewski's figures, but which 

 under ordinary conditions is continually varying. It is, 

 however, much higher than the normal percentage in the 

 atmosphere, so that we find the machine working well 

 within its capacity. 



The optimal percentage for photosynthesis is not the 

 same as for the subsequent processes of metabolism. Monte- 

 martini found the best percentage for growth did not 

 exceed 4 ; Mangin and other observers proved that though 

 photosynthesis may be very active in air containing 4-15 

 per cent, of carbon dioxide, the plants exposed to it are 

 ultimately injured. 



For the continuous activity of the chlorophyll apparatus 

 it is necessary that accumulation of the products of photo- 



