30 tngvar Jorgensen and Walter Stiles. 
decimetre per hour the error in the increase in dry weight obtained 
by the half leaf method in an experiment lasting 5 hours would be 
as much as 100%, whereas the error in the results obtained by 
measuring the carbon dioxide absorption would amount to no more 
than 2%. They therefore reject Sachs’ method as quite untrust¬ 
worthy. 
As the dry weight method, if it could be made sufficiently 
accurate, would have its uses we agree with Thoday “ that it should 
not lightly be abandoned.” Thoday makes some useful suggestions 
in regard to decreasing the inaccuracy of the method, but he does 
not furnish data which enable one to determine the degree of 
accuracy obtainable when all suggested precautions are taken. It 
appears to us that the only way of finding this is to make a number 
of such estimations and determine the probable error of the mean 
result. 
C. The Quantitative Determination of the 
Heat of Combustion of the Products of Assimilation. 
Although the measurement of heats of combustion offers no 
particular difficulties, very few such measurements have been made in 
plant physiology. Brown and Escombe assume that the heat of 
combustion of the products of assimilation is the same as that of 
glucose, but this assumption is not justified by the values obtained by 
experiment for the heat of combustion of one gram of material pro¬ 
duced in assimilation. It will be seen however from the heats of 
combustion of varioussubstances recorded in the accompanying table, 
that measurements of the actual heats of combustion of the products 
of assimilation might afford helpful information as to the relative 
proportion of the different products. 
Table XXXIX. 
Heats of Combustion in Gram-Calories of Various Substances. 
Substance Heat of Combustion per gram. 
Ethyl Alcohol ... 
. . . 
. . . 
7-18X 10 3 
Glucose 
. . . 
3-76 x 10 3 
Sucrose 
3-99 x 10 3 
Dextrin 
. . . 
4-1 xlO 3 
Starch .. 
4-1 xlO 3 
Cellulose 
• • • 
4-2 xlO 3 
Leucin ... 
• * * 
6-5 xlO 3 
Vi tel 1 i n ... 
... 
5-7 xlO 3 
Linseed Oil 
• • . 
9-47 x 10 3 
Olive Oil 
• • • 
9-51 x 10 s 
