TRANSACTIONS OF SECTION K. 905 



2. On Increase in Dry Weight as a Pleasure of Assimilation. 

 By J). Thoday, B.A. 



The increase in dry weight of unit-area of leaf-surface was first suggested as a 

 measure of the accumulated products of assimilation by Sachs in 188^1 He took 

 halves of leaves in the early morning and the other halves after'a certain number 

 of hours of insolation, and found the difference in dry weight per square metre. 



Comparison with the results obtained by other more exact methods has led to 

 the conclusion that this method tends to give an over-estimate of assimilation. 

 Brown and Escombe (1905) compared increase in dry weight with intake of 

 carbon dioxide simultaneously directly determined, and found the former far 

 in excess of the latter in three experiments out of four. They attributed the 

 discrepancy chiefly to the asymmetry of leaves and to changes in area taking 

 place under experimental conditions, and published a few determinations of 

 the errors introduced in these ways. They also suggested the possibility of 

 a change of capacity for water retention in the colloid constituents of leaves 

 during insolation, presumably as an explanation of the high] values usually 

 obtained ; and expressed the opinion that Sachs' method is not capable of giving 

 accurate results. 



At Dr. Blackman's suggestion I undertook a more detailed investigation of the 

 method, for it is the only method which is available for determining the rate of 

 assimilation in the open air, and if it could be trusted for sufficiently accurate 

 results in its original or a modified form, might prove of service in ecological 

 investigations in the near future. The following are among the most important 

 results :— 



Analysis by combustion of portions of material of which the dry weight had 

 been determined showed a fair correspondence between the increase in dry weighfr- 

 and the increase in carbon content, and gave no support to Brown and Escombe 's 

 suggestion of a change in capacity for water retention. 



Leaf material is often extremely hygroscopic. It should be dried in a current 

 of dry gas and weighed with care to exclude moisture. 



The methods or measuring area are or can be made sufficiently accurate. 

 Errors from lack of symmetry in dry weight per unit area cannot be elimi- 

 nated. They can only be reduced by avoiding veins and by using a large number 

 of leaves. It is this source of error which will set the limit to the degree of 

 accuracy obtainable by the method. 



Change of area by shrinkage during experiment introduces an error the 

 seriousness of which varies according to conditions, and is most liable to vitiate 

 the results of experiments aimed, like those of Sachs, at determining a maximum 

 rate of increase in full sunlight in the open air. 



These errors from shrinkage or expansion will require special care in the future 

 use of the method, but present no insuperable difficulty. Floating on water can- 

 not be trusted to eliminate them. 



The calculation of the equivalent intake of carbon dioxide from the increase 

 of dry weight can only at present be rough, as too little is known of the composi- 

 tion of the products of assimilation. For more accurate work it may be possible 

 to avoid this difficulty by estimating the carbon content itself. 



Sachs' result for the rate of assimilation of detached leaves oi Helianthus 

 annuus, 16 milligrammes per square decimetre per hour, has been confirmed as 

 substantially correct in this investigation. 



Flaccidity, even if but slight, brings about a considerable reduction in the 

 rate of assimilation in leaves of Helianthus annuus, and probably in other leaves. 

 Brown and Escombe's results for Helianthus annuus are very low, and cannot be 

 taken as a satisfactory] indication of what goes on under open-air conditions. 

 Nor can their suggestion be considered universally applicable that the greater 

 results which detached leaves have given when compared with attached leaves 

 are to be explained by differences of stomatal apertures. One of Brown and 

 Morris's results cannot be explained satisfactorily in this way; similarly with 



