io8 



Carbon Assimilation. 



1 . Preparation of Material. 



In order to obtain correct results in the estimation of substances 

 so liable to change by enzyme action as carbohydrates, special care 

 has to be taken to avoid such change in the preparation of the 

 material for analysis. 



Brown and Morris (1893) therefore dried the leaves rapidly at 

 from 75"C to 80"C before estimating starch, and for estimating 

 sugars the leaves were dried on wire-bottomed trays in a steam 

 oven. Parkin (1911) used a similar method. The leaves were air 

 dried at a temperature sufficiently low to prevent discoloration. In 

 both cases the dried leaves wei'e then powdered. That the sugars 

 are extracted unchanged by this method was shown by Parkin by 

 estimating them in material so prepared, and in leaves killed by 

 immersion in liquid air which were subsequently ground up while 

 frozen and then thrown into boiling water (containing a few drops 

 of ammonia to neutralise any acid from the leaf) in order to kill the 

 enzymes. 



The following table shows that the two methods give almost 

 identical results. 



The numbers for two separate examples (I and II) are given. 



Table XXIX. 



Comparison of Sugars in Air-dried Leaf and in Leaf 



treated with Liquid Air. 



From the leaf powder of Tropoeolnm Brown and Morris 

 extracted fat and chlorophyll with ether. The residue was then 

 twice extracted for 24 hours with 80% alcohol at 40'>C. The 

 alcoholic extract was used for the estimation of sugars, the residue 

 contained the starch. 



In the case of the Snowdrop where the leaf contains no starch. 

 Parkin extracted the sugars by four extractions with cold water 



