ACTION OF LIGHT ON VEGETATION. 767 



Erythrophyll group. — The colouring- matters belonging to this group are very 

 numerous, and their production often depends upon obscure and accidental causes, 

 easily modified by slight variations in the internal or external conditions. They may be 

 divided into three well-marked sub-groups, according as they are changed by the action 

 of sodium sulphite. They are soluble in water, and are usually, if not always, dissolved 

 in the juices of the plant, and disseminated in cells of various kinds. A greater number 

 of different species occur in the petals than in the leaves. They are usually indicative 

 of low constructive energy, but yet are not merely products of chemical decomposition. 



Chrysotann'in group. — Much remains to be learned with respect to these more or less 

 pale yellow or even colourless substances, and the part they play in plant-life. The most 

 striking fact connected with them is that when oxidised they give rise to the various 

 brown substances which are the cause of many of the characteristic tints of autumnal 

 foliage. These changes are mainly, if not entirely, due to chemical action, and can 

 easily be imitated artificially. 



Exposure to a greater or less degree of light may produce a great quantitative or 

 even qualitative difference in the colouring matters. Rudimentary petals and rudi- 

 mentary leaves correspond closely, but subsequently development takes place in two 

 different directions; and very often when the petals of the more highly developed 

 varieties are only partially grown, the constituent colouring-matters are both qualitatively 

 and quantitatively the same as those in some other variety, as though this were due 

 simply to a natural arrest of development. By growing almost in the dark flowers 

 coloured by more or less of the orange species of the xanthophyll group, the petals are 

 obtained of the full size, but only yellow and corresponding exactly to the normally 

 yellow variety; and there is this remarkable peculiarity, that the relative proportion 

 between the different colouring-matters approximates more or less closely to what is 

 obtained by exposing to light a solution of those found in the normal petals ; that is 

 to say, absence of light tends to prevent the formation in the petals of those more 

 orange-coloured substances which are the most readily decomposed by exposure to 

 light when they are dissolved out from the petals.]^ 



^ [The occasional occurrence of ' chlorophylloid green colouring-matters ' in the tissues of 

 animals is a matter of considerable significance. Mr. E. R. Lankester has obligingly drawn up the 

 following list of such cases. Those marked with an asterisk have been observed by him with the 

 spectroscope for the first time : — Infusoria ; Stentor Mulleri and others. Foraminifera. Radiolaria ; 

 Rhaphidiophrys viridis, Heterophrys myriapoda (Quart. Journ. Micr. Sc. 1 869). Ccelenterata ; * Spon- 

 gilla fluviatilis (Journ. Anat. and Phys. 1869), * Hydra viridis, Anthea cereus var. smaragdina (chloro- 

 fucine). Vermes; Mesostomum viride (Planarise), ^ Bone Ilia viridis (in the skin), *^ Chcetopierus 

 Valenciennesii (in the walls of the alimentary canal). Crustacea; ^Idoiea viridis (Isopoda). The 

 chlorophylloid substance is not present in the same physical or chemical condition in all these cases. 

 In Rhaphidiophrys, Heterophrys, Spongilla, and Hydra, it is localised in granules imbedded in the 

 protoplasm ; this is also the case in Bonellia, but the granules are finer. In Idotea it is not in 

 granules but diffused in the chitino -calcareous integument. In all cases the chlorophylloid substance 

 agrees in having a strong absorption-band in the red— a little to the right or left ; and, except in 

 Idotea, in being soluble in alcohol ; and in having strong red fluorescence and in finally losing its 

 colour when dissolved. In Bonellia, Chcetopterjis, and Spongilla, the absorption-spectrum presents 

 differences in other respects in each case, and the green tint is itself different— being black olive- 

 green in Chcetopterns, bluer but equally dark in Bonellia, and apple-green in Spongilla and Idotea. 

 In Spongilla the green colour is not developed if the animal grows in the dark. But like etiolated 

 vegetable tissues, Spongilla, when immersed in strong sulphuric acid, gradually developes a strong 

 leaf-green colour, fully as intense as that of the naturally green specimens (Quart. Journ. Micr. Sc. 

 1874, p. 400). Bonellia, on the other hand, always lives in a dark hole excavated by it in calcareous 

 rock, and Chcetopterns lives in a thick opaque tube. Geddes has found that a green Planarian {Con- 

 voluta Schultzii) decomposes carbonic acid under the influence of light, oxygen being evolved and 

 starch formed in the cells.] 



