Chap, ii Photo synthesis 319 



Kreussler investigated the lower limit in 1888 and found 

 a feeble activity in bramble leaves at -2-4° C. Jumelle 

 confirmed this result in 1892. 



The matter was undetermined with greater accuracy at 

 the end of the century, but Kreussler 's work paved the 

 way for the observations of Blackman and Matthaei in 

 1902. 



Considerable inquiry was directed to certain collateral 

 questions springing from the information that had 

 been gained as to the absorption of radiant energy. The 

 first of these concerned the relation of intensity to effect. 

 Among the discoveries of Sachs during the course of the 

 researches we have discussed was the important fact that 

 activity does not increase pari passu with the intensity of 

 the light, but that there is an upper limit beyond which 

 increased brilliance of illumination becomes deleterious. 

 There is further a great deal of difference of behaviour 

 among plants as to the maximum intensity. The inquiry 

 into a possible optimum degree of illumination, however, 

 is closely connected with that of a possible optimum per- 

 centage of carbon dioxide in the air and the failure to 

 recognize this led to some conflicting results, for the 

 two conditions were studied separately as if they were 

 independent. Different plants again show remarkable 

 differences in the way in which they respond to all these 

 changes. We may almost impute to them personal idio- 

 syncrasies or at any rate peculiarities confined to species. 



A series of experiments made with increasing percentages 

 of carbon dioxide led Godlewski in 1873 to conclude that 

 the gas can be very largely increased in quantity, indeed 

 up to 10 per cent, of the air supplied, with the result that 

 the activity of the chloroplasts is thereby and up to that 

 point enhanced. 



In his Pflanzenphysiologie Pfeffer pointed out that while 

 in 1871 he proved that an excess of carbon dioxide exerts 



