THE METABOLISM OF PLANTS. 239 



Again, indigo is formed by the decomposition of a nitrogenous glu- 

 coside termed indican, C 26 H33NO 18 which is present in various plants, 

 Nerium tinctorium, Polygonum tinctorium, Isatis tinctoria, and especially 

 in Indigofera tinctoria a Leguminous plant. The decomposition may be 

 represented thus ; 



Indigo-blue 



C 26 H 33 N0 18 + H 2 = C 8 H 5 NO + 3C 6 H 12 O 6 . 



The colouring-matter obtained from the indican of Isatis tinctoria 

 has the formula C 8 H 5 NO 2 , and is termed I satin or Woad. 



Other colouring-matters occur in the cavities of cells ; for example 

 alcannin (CigHjoOj) in the root of Anchusa tinctoria, and curcumin in the 

 root of Curcuma longa. 



The green colouring-matter of plants, chlorophyll, always 

 occurs in intimate relation with the protoplasmic cell-con- 

 tents, and, as we have seen, it is confined in the higher plants 

 to certain specialised portions of the protoplasm which are 

 termed chlorophyll-corpuscles. We have already discussed 

 the chemical composition and the physical properties of this 

 substance ; it only remains now to consider the conditions 

 of its formation, and the probable source from which it is 

 derived. 



The general conditions upon which the formation of 

 chlorophyll depends are three: I, exposure to light; 2, a 

 sufficiently high temperature ; 3, a supply of iron. Of these, 

 the two first will be further discussed in a subsequent lecture, 

 in which we will consider the influence of heat and light on 

 the metabolism of the plant. The significance of the third 

 condition is not understood at present. It was thought that 

 iron entered into the composition of chlorophyll much in the 

 same way that it enters into the composition of the colour- 

 ing-matter (haematin) of the red-blood corpuscles : but it 

 appears from the analysis of chlorophyll (see p. 1 54) that 

 no iron can be detected in it. It may be urged that the 

 chlorophyll when extracted is not identical with the green 

 colouring-matter in the living plant ; but the extracted chlo- 

 rophyll has the green colour and the characteristic spectrum, 

 so we may conclude that neither of these important pro- 

 perties depends upon the actual presence of iron in the 

 chlorophyll-molecule. Arthur Gris came to the conclusion 



