ii6 METABOLISM 



assimilation which was carbohydrate in its nature, and found (1890) it to be, 

 in round numbers (on an average of various experiments), 68 per cent., d)^ per 

 cent., and 64 per cent. He imagined that the remainder in each case, i. e. 32 per 

 cent., 13 per cent., and 36 per cent., was proteid, and later on (1895) endeavoured 

 to prove it. He also investigated the increase in carbohydrates and proteids 

 during light in isolated leaves. An active construction of proteid could be deter- 

 mined especially when the leaves were immersed in nutritive solutions con- 

 taining nitrogen. Further, a diminution in the light retarded the rate of 

 formation of carbohydrates but relatively accelerated that of proteid. Saposch- 

 NiKOFF, notwithstanding his anxiety to reach such a conclusion, was, how- 

 ever, unable, from these and other experiments, to establish the view that 

 proteid was the ' first formed reserve of carbon-assimilation'; on the contrary, 

 everything was in harmony with the conclusion that proteid was a secondary 

 product constructed from carbohydrates. 



The results which have been quoted as to the quantity of carbon assimi- 

 lated are in accordance with those arrived at by the use of Sachs's half-leaf 

 method or approximate closely to them. Although the errors in determining the 

 area of the leaf surface be reduced to a minimum, still the inequalities in thick- 

 ness of otherwise similar portions of the leaf would account for the inaccuracies 

 in the amounts ; for this reason one is limited to the use of a single leaf, because 

 a comparison of surface areas taken from various leaves is bound to lead to errors. 

 It is of interest therefore to draw attention to another method of determining 

 the amount of the products of assimilation which we owe to Kreussler 

 (1885-90), and which is certainly much the most accurate. He estimated 

 how much carbon-dioxide was withdrawn by the leaf from the air, an amount 

 quite independent of contingencies in the leaf structure. Kreussler investi- 

 gated isolated twigs in appropriate bell- jars through which was made to pass 

 a known quantity of air containing a known percentage of carbon-dioxide. He 

 next determined how much of this carbon-dioxide passed out of the apparatus, 

 and at the same time worked out how much carbon-dioxide was produced in 

 the same period as a result of respiration. In this way the amount of carbon- 

 dioxide actually absorbed by the plant could be accurately determined. Since 

 Kreussler worked with air containing relatively high percentages of carbon- 

 dioxide, and since the material experimented on was illuminated with electric 

 light, his results furnish us with no information with reference to the amount 

 of carbon-dioxide decomposed under normal conditions of assimilation. We 

 need not enlarge on these results at the present moment, since they will have 

 to be considered in another relation presently. Brown (1899) has employed 

 this method in a modified form. He worked with ordinary atmospheric air, 

 whose percentage of carbon-dioxide was accurately determined, and further, 

 employed ordinary daylight. Brown also used in his bell-jars only single leaves, 

 which, however, remained attached to the plant, so more easily guarding 

 against withering than was possible with severed branches. Certain valuable 

 data may be extracted from the preliminary account he has given of his re- 

 markable experiments. A sunflower leaf, with a surface area of 617-5 sq. cm. 

 absorbed in diffuse light 139-95 ccm. of CO^ in five and a half hours, or 412 ccm. 

 of carbon-dioxide per sq. m. per hour. Since one may reckon that 784 ccm. of 

 carbon-dioxide on an average are required to construct one gram of carbohy- 

 drate it will be seen that, in the case cited, 0-55 gr. of carbohydrate was manu- 

 factured per sq. m., and this number corresponds pretty nearly with what 

 Brown found by weighing; the fact that it is considerably less than the 

 amount found by Sachs is explained by the feebler intensity of light employed. 



[In the preliminary account of their experiments. Brown and Escombe 

 (1905, Proc. Roy. Soc. B. 76, 29) give only 0-4-0-5 gr. of carbohydrate per sq. m. 

 per hour for Helianthus annuus. They draw attention to the difference between 



