38 Carbon Assimilation. 



Dry a quantity of leaves which have been put in boiling water 

 and examine their solubility as in Experiment 18. Note that the 

 chlorophyll in this powder is soluble in pure solvents. 



It should be noted that colloidal chlorophyll is an electronega- 

 tive suspensoid, and that it is a very excellent substance for 

 demonstrating the properties of such colloids. It might even be 

 worth while for this purpose to prepare a small quantity of pure 

 chlorophyll (a -1- b) (see section D of this chapter). 



In most suspensoids it is difficult to see when the precipitation 

 has taken place, but here where any precipitated chlorophyll can 

 immediately be extracted by ether, the method is open for quantita- 

 tive work of high accuracy. 



By the use of chlorophyll can be demonstrated such properties 

 of colloids as the salt concentration required for precipitation, the 

 effect on this of the valency of the precipitating ion, the stabilising 

 effect of alkali and the reverse effect of acids, the action of protective 

 colloids, etc. 



Crude colloidal chlorophyll contains, of course, a good many 

 accompanying substances which vary in composition and quantity, 

 so comparable data are not obtainable by the use of such 

 chlorophyll. 



F. Variations in the Quantity of the Leaf Pigments in 



Different Plants and under Different Conditions. 



The part of Willstatter's work with which we have dealt so far 

 concerns the characteristics of the pigments, their chemistry, and 

 the methods for extracting them. Willstatter has further devised 

 methods for the quantitative extraction and separation of the four 

 pigments. Here, as in the aspects of the leaf pigments we have 

 already considered, we are in the fortunate position of being able to 

 neglect all earlier work on the subject, for it is now obvious that 

 the methods employed by workers before Willstatter are imperfect 

 and must give erroneous results. Thus, for instance, it is essential 

 to separate the green and yellow pigments in order to obtain 

 quantitative data as to the amount of chlorophyll present ; and 

 again other substances liable to be extracted with the pigments, 

 particularly small quantities of plant acids, will cause considerable 

 alteration in the pigments. Tiiese considerations apply equally to 



