2 6 Carbon Assimilation. 



E. Simple Laboratory Experiments on the Leaf Pigments. 



As Willstatter truly points out, the experiments described in 

 text-books of plant physiology for the demonstration in class 

 work of the properties of chlorophyll are quite inadequate. 



In preceding sections of this chapter, we have described 

 Willstatter's methods for the extraction in quantity of the leaf 

 pigments. It is quite obvious, however, that the length of time and 

 large amount of material and reagents required for these extrac- 

 tions, render their use scarcely possible for ordinary class work. 

 We have therefore brought together in this place a number of 

 easily performed experiments which amplify the collection of 

 examples given in Willstatter and Stoll's book. 



The performance of these experiments will not only lead the 

 student to clearer ideas about the chemistry of the leaf pigments, 

 but will also give an opportunity for that chemical and physical 

 manipulation which is becoming increasingly necessary to the plant 

 physiologist, but for the practice of which there is no great 

 opportunity in most other parts of a plant physiology course. 



Preliminary. It will be found of great convenience for class 

 work to collect in the summer every year a quantity of nettle 

 leaves. These are dried at air temperature ; they are spread out 

 on sheets of paper and a sheet of paper placed on top of them to 

 prevent dust from falling on them, and to prevent undue exposure. 

 They are then ground up finely and dried completely for several 

 days at a temperature of 30° to 40"C., in an incubator. 



The powder so obtained is kept in a stoppered bottle. So 

 prepared, the powder retains for a long time the leaf pigments 

 unaltered in quantity and quality. 



Any further drying required must be carried out by placing the 

 leaf powder in a vacuum desiccator over sulphuric acid. This 

 procedure is necessary for instance when it is required to show that 

 pure solvents do not extract the pigments from dry leaf powder. 

 In this case it is also necessary that the solvents should be as water- 

 free as possible. Ordinary solvents may have to be redistilled over 

 quick-lime, calcium filings, etc. 



Experiment 1. Extraction of the pigments. Required : small 

 Buchner funnel with flask and a water pump ; 20 c.c. 85% acetone 

 or 90% alcohol. 



