iv] CARBON ASSIMILATION 35 



soluble, will not extract the pigment from thoroughly dried leaves, but 

 if a little water is added, it readily goes into solution. From fresh leaves 

 also these solvents can extract the pigments. 



As an explanation of the above phenomena, it has been suggested 

 that chlorophyll in the chloroplastid is in the colloidal state, and that, 

 when water is added to the dried leaf, a solution of mineral salts in the 

 leaf is formed which alters the colloidal condition of the chlorophyll and 

 makes it soluble. This view is supported by the fact that if a colloidal 

 solution of chlorophyll in water, made from the pure extracted pigment, 

 is shaken with ether, the ether remains colourless. If, however, a little 

 salt solution is added and the mixture shaken, the ethereal layer 

 becomes green. In preparing the colloidal solution the solvent, acetone, 

 is replaced by the medium, water, in which chlorophyll is insoluble. 



The condition of chlorophyll* is altered by plunging the leaves into 

 boiling water. The pigment is then much more readily soluble in ether, 

 etc., even when the leaves are subsequently dried. It is supposed that 

 the chlorophyll has diffused out from the plastids, and is in true solution 

 in accompanying waxy substances which have become liquid owing to 

 change of temperature. 



Expt. 31. Preparation of a colloidal solution of chlorophyll. Take 10 c.c. of an 

 acetone extract of chlorophyll (Expt. 18) and pour this acetone solution into a large 

 volume of distilled water (100 c.c.), the liquid being continually stirred. This opera- 

 tion can be most conveniently done by taking the acetone solution in a pipette and 

 allowing it to run out of the pipette while the latter is used as a stirring rod in the 

 water. Note the change in colour to a purer green, and the disappearance of 

 fluorescence. 



Expt. 32. To demonstrate the difference between a true and a colloidal solution of 

 chlorophyll. Evaporate 10 c.c. of an acetone extract (Expt. 18) to complete dryuess 

 and test its solubility in ether, petrol ether and benzene. It is soluble in all three 

 solvents. Now add these solvents to some of the colloidal solution prepared in the 

 last experiment, and note that the chlorophyll does not dissolve in any of these 

 solvents. If, however, some salt solution, e.g. a little magnesium sulphate, be added, 

 the chlorophyll is precipitated from its colloidal state and is now soluble in ether 

 and other solvents. 



Expt. 33. To show that chlorophyll in the plant is probably in the colloidal 

 condition. Some nettle powder is carefully dried, e.g. by keeping it at 30^40 C. in an 

 oven, and then further drying in a vacuum desiccator over sulphuric acid. Small 

 quantities of this dry powder are put in test-tubes, and different pure water-free 

 substances such as acetone, ether, benzene and absolute alcohol are added. Note 

 that these solvents are not coloured by the chlorophyll. It can be demonstrated that 

 the extracted pigment is easily soluble in any of these substances. Repeat the experi- 

 ment with nettle powder moistened with a few drops of water, and note that the 

 solvents are immediately coloured. 



32 



