Carbon Assimilation. 
213 
being used to wash out the flask if much chlorophyll or fat is present. 
It is now necessary to remove tannins, amino-acids, basic 
substances, etc. This is effected by addition of the exact quantity 
of basic lead acetate required to precipitate the whole of these 
substances. Any excess of lead is removed by hydrogen sulphide 
(Brown and Morris) or by solid sodium carbonate (Davis, Daish and 
Sawyer). A solution so prepared can be kept for several weeks if a 
little toluene is added, provided no excess of basic lead acetate is 
present and that the solution is just alkaline. Any excess of basic 
lead acetate or much alkali brings about a rapid destruction of 
fructose. 
6. Estimation of Sugars. 
The methods employed by various workers for the estimation 
of sugars vary in details, but the general principles underlying all 
of them are the same. We will consider first the method used by 
Parkin, for as this worker concluded that sucrose, glucose and 
fructose are the only sugars present in the snowdrop leaf, his 
analysis is simpler than that of Brown and Morris, and Davis, Daish 
and Sawyer who analysed their extracts for other sugars as well. 
(n) Parkin's Method. The cupric-reducing power and optical 
rotation of a definite volume of the purified extract is first measured. 
The cupric-reducing power is due to the hexoses alone, the optical 
rotation to the hexoses and sucrose together. A further volume of 
the extract is inverted with invertase and the cupric-reducing power 
and optical activity again measured. The increase in reducing power 
and change of optical activity must be due to the inversion of the 
sucrose, and from these numbers the quantity of sucrose is obtained. 
From the reducing power and optical activity due to the hexoses, 
the quantities of fructose and glucose can be calculated. 
Parkin also found that after fermentation with brewers’ or 
bakers’ yeast, that the cupric-reducing power and optical activity 
became negligible. This indicates the absence of pentoses and the 
l forms of the hexoses. 
(6) Method of Brown and Morris. 
(i.) The cupric-reducing power and optical rotation are first 
measured. 
(ii.) For the estimation of cane sugar the solution is inverted 
with invertase at 50° to 55°C. The increase in cupric-reducing 
power and change in optical rotation will both give the quantity of 
sucrose. 
(iii.) 50 c.c. of the 1% solution is heated with 3 c.c. of concen- 
