II THE PIGMENTS OF ORGANISMS 33 



when exposed to sunshine can decompose carbonic 

 acid and set free oxygen, and that it is by virtue of 

 their chlorophyll that they can do this, but beyond 

 this we have nothing but a host of rival theories. 

 The most interesting, of these is perhaps that of 

 Schunck, who considers that chlorophyll carries 

 carbonic acid from the air to the assimilating proto- 

 plasm, just as haemoglobin carries oxygen. Macallum 

 holds that chlorosis in plants and animals is due 

 to the same cause, the absence or deficiency of 

 iron in the nuclear chromatin ; and this in spite of 

 the fact that it is improbable that chlorophyll con- 

 tains iron. Thus the absence of chlorophyll in a 

 plant grown without iron is only one of the con- 

 sequences of the general unhealthiness. In reference 

 to this view of Macallum's, it is interesting to note 

 that according to Nencki there is a close relation 

 between hsematoporphyrin, a derivative of haemo- 

 globin, and one of the derivatives of chlorophyll 

 green. 



As is well known, chlorophyll is, at any rate in 

 solutions, an extremely unstable pigment, fading in 

 light with great rapidity. In natural conditions 

 it is associated with one or more pigments belonging 

 to a widely spread group of pigments — the lipo- 

 chromes, characterised by their colour, which varies 

 from red to yellow, as well as by other properties. 

 It is uncertain whether these subsidiary pigments 

 exercise any influence in the process of assimilation, 

 or whether it is not the true chlorophyll or chloro- 

 phyll green which is alone active. Lipochromes are 

 not the only pigments which occur in association 

 with chlorophyll, but the subject will be treated 



D 



