The Pigments of the Leaf. 7 



is mixed with accompanying oils, waxes, etc., but petrol-ether does 

 not extract at all from dried leaves, so it is feasible to suppose that 

 the chlorophyll in the chloroplast is in the colloidal condition, that 

 water added to the pure organic solvents dissolves the mineral 

 substances in the leaf, and the salt solution so formed alters the 

 colloidal condition of chlorophyll in the chloroplast and makes it 

 easily soluble. 



In this connection it should be pointed out that the pureorganic 

 solvents can extract the chlorophyll from fresh leaves, as there is of 

 course, abundant water present in them. 



Support for this assumption is given by the fact that colloidal 

 solutions of chlorophyll in water made up from the pure extracted 

 pigment behave in a similar way to the dried leaf powder. Thus if 

 a colloidal solution of chlorophyll is mixed with ether, the ether 

 remains colourless, but if a little salt solution, for instance, a solution 

 of calcium chloride or calcium nitrate, is added, on shaking the 

 ethereal layer becomes coloured green. The salt solution has 

 precipitated some of the chlorophyll from its colloidal condition and 

 it is now easily soluble in ether. 



In regard to the actual state of chlorophyll in the leaf there has 

 been some difference of opinion. Arnaud (1885) supposed 

 that capillary forces kept the chlorophyll back in the leaf, and 

 Willstatter himself at one time assumed that chlorophyll in the 

 leaf was present in the form of adsorption compounds with colloids. 

 Similarly Tswett (1901) held that the pigment was bound to the 

 skeleton of the chloroplast by molecular adsorption. Recently 

 Palladin (1910a, 1910b) suggested that the chlorophyll is present in 

 the leaf in a state of chemical combination, particularly with the 

 so-called lipoid substances, and he shows that the use of solvents fov 

 extraction could be explained by their dissociating power in regard 

 to the adsorption compound. 



Willstatter's present opinion is that the chlorophyll in the 

 chloroplast is present in a colloidal mixture, and there appears to 

 beagood deal of experimental evidence in support of this view, even 

 if it may have to be slightly modified when we have more experi- 

 mental knowledge of the kinetics of the physiological processes 

 involved. 



We may mention briefly some other reasons for the assumption 

 that chlorophyll is present in the colloidal condition. There is first 

 evidence derived from spectroscopic examination. According to 

 Tswett (1910) and other writers the absorption bands in the spectrum 

 of the living leaf are displaced towards the red end of the spectrum 

 as compared with the bands in the spectrum of extracted cliloro- 



