Quantitative Estimation of Carbon Asslinilation. 133 



course Brown iiiid Escombe iissume tliat the ratio between the 

 various carbohydrates remains constant and further that carbon 

 dioxide is used only in the production of carbohydrates. It is true 

 that variations in the ratio of the various carbohydrates will make 

 little difference in the carbohydrate factor, and similarly, the error 

 introduced owing to the probable incorrectness of Brown and 

 Morris's analysis (cf. Chapter V) is likely to be small. On the other 

 hand the error introduced by the assumption that the whole of the 

 carbon dioxide is used in carbohydrate formation is likely to be 

 larger, but on this subject our information is very incomplete. It 

 may be interesting to compare this carbohydrate factor with 

 values obtained by experiment for the ratio between increase in 

 dry weight and carbon dioxide absorbed. The following table is 

 due to Krasheninnikoff (1901) and although the values are probably 

 not of a very high order of accuracy, they may give some idea of the 

 variations likely to occur. 



Table XXXVI. 

 Increase in Dry Weight of Leaves per gram of Carbon dioxide 



absorbed. 



Bamboo 0-60 



Cherry Laurel 0-60 



Sugar Cane 067 



Lime 074 



Tobacco 0-68 



Thoday (1909) compared the increase in dry weight with the 

 increase in carbon content of the leaf in the cases of Helianthus 

 tuberosus and Cherry Laurel. His results indicate a considerable 

 variation in the ratio of carbon increase to increase of dry weight 

 in leaves of the same species, but in Thoday's experiments a good 

 many factors are not controlled, and it is impossible to say what 

 causes the variation. We should like to emphasize that in all such 

 cases in plant physiological researches where it is sought to determine 

 the absolute value of a quantity, it is absolutely imperative to 

 determine the pi-obable error of the experiment. This, as far as we 

 know, has not been done in any single instance in work on carbon 

 assimilation. 



In regard to the direct determination of the products of 

 assimilation, the principle of this method was first exposed by Sachs 

 in his paper " Ein Beitrag zur Kenntniss der Ernahrungsthatigkeit 

 der Blatter" published in 1884. Sachs' method is well-known. 



