36 CARBON ASSIMILATION [CH. 



Expt. 34. Pure solvents are able to extract chlorophyll from fresh leaves. Crush 

 10 gms. of fresh leaves of nettle, horse-chestnut or elder in a mortar with some clean 

 sand, and put the crushed material on a filter-paper in a porcelain funnel. Add 

 20 c.c. of pure acetone and suck it through by means of a water-pump. Repeat this 

 several times. The pure solvent is here able to extract the pigment. 



Expt. 35. Treatment of fresh leaves with boiling water changes the condition of 

 the chlorophyll. Dry a quantity of leaves which have been put in boiling water and 

 examine their solubility as in Expt. 33. Note that the chlorophyll in this powder is 

 soluble in pure solvents. 



There is finally another change which chlorophyll can undergo, 

 namely that of allomerization, which takes place in alcoholic solution. 

 The characteristic of allomerized chlorophyll is that it does not give the 

 brown phase when treated with alkali (see Expt. 22). Allomerization is 

 accelerated in alkaline solution but inhibited by small quantities of acid. 



Expt. 36. To demonstrate that allomerized chlorophyll does not give the brown 

 phase test. Dissolvte a little crude chlorophyll, obtained by evaporating an ether 

 solution, in absolute alcohol. To a sample of this add a little alkali, and perform the 

 phase test, from time to time, till at last the brown phase no longer appears. 



CONNEXION OF CHLOROPHYLL WITH FORMALDEHYDE. 



In addition to the above, another chemical property of chlorophyll 

 of great interest, is that connected with the production of formaldehyde. 

 Those investigators, who have sought to confirm the formaldehyde 

 hypothesis of carbon assimilation, have based their evidence on tests for 

 formaldehyde both in the plant and in chlorophyll-containing systems 

 outside the plant. By exposing films, or solutions, of chlorophyll to light 

 in presence of carbon dioxide, they have detected formaldehyde as a 

 result (Usher and Priestley, 5). 



The most recent investigations (Jorgensen and Kidd, 2) have shown 

 that the experimental evidence is at present inadequate to support the 

 hypothesis, since formaldehyde arises from chlorophyll itself in the 

 absence of carbon dioxide. 



In this later work (Jorgensen and Kidd, 2) on the behaviour of ex- 

 tracted chlorophyll in light, use has been made of a colloidal solution (see 

 p. 35) of pure chlorophyll (chlorophylls a and 6) for experimental work. 

 The solution for this purpose must be prepared from pure chlorophyll, 

 which has been tested and shown to be free from yellow pigments, since 

 the latter absorb oxygen and may confuse the issue of the experiment. 

 The pure chlorophyll is prepared by extracting dried nettle leaves with 

 80-85 / acetone in the usual way and transferring to petrol ether (p. 28). 



