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 Qf 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 hirn with the 
spectroscope for the first time : — Infusoria ; Stentor Mulleri and others. Foraminifera, Radiolaria ; 
Rhaphidiophrys viridis, Heterophrys myriapoda (Quart. Journ, Micr, Sc, 1 869), Coelenterata ; * Spon- 
gilla ßuviatilis (Journ. Anat. and Phys. 1869), * Hydra viridis, Anthed cereus var. sinaragdina (chloro- 
fucine). Vermes; Mesostomtmi viride (Planarise), Bone Ilia viridis (in the skin), ChcEtopterus 
Valenciennesii (in the walls of the alimentary canal). Crustacea; ^Idotea 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, Chcetopterus, 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 Chcetopterus, 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 Chcetopterus lives in a thick opaque tube, Geddes has found that a green Planarian {Qon- 
voluta Schultzii) decomposes carbonic acid under the influence of light, oxygen being evolved and 
starch formed in the cells.] 
