1873.] 



Comparative Vegetable Chromatology . 



467 



ducts. It thus appears that chlorophyll is in the first instance decom- 

 posed by light into various coloured compounds, which become colourless 

 on further exposure, and that these may be developed so as to be very 

 conspicuous or scarcely visible, according to particular conditions. 



Production of Red Leaves. 



The red substances thus formed artificially differ from those met with 

 in plants in being soluble in bisulphide of carbon, and more rapidly 

 destroyed by the action of light ; but yet it appears to me extremely pro- 

 bable that those often found in leaves are products formed by the action 

 of light on chlorophyll, under conditions which have not been reproduced 

 artificially. At all events, by adopting this hypothesis, the changes that 

 occur in many leaves at different seasons of the year admit of very 

 simple explanation. I have constantly noticed that the leaves of some 

 plants are more especially subject to this disappearance of chlorophyll 

 and formation of erythrophyll when the twig on which they grow is 

 partially broken, or when the leaf is actually severed from the plant, but 

 remains in a damp place, so as not to actually wither, especially when 

 the underside is exposed to the light. The change is also often depen- 

 dent on the presence of parasitic fungi or insects, and often takes place 

 along the dying edge of a leaf, where it is not actually dead, but, so to 

 speak, only half alive ; and hence there seems to be no doubt that one 

 essential condition is that the leaf should be in a state of low vitality. It 

 also depends upon the presence of light ; for if a leaf that seems likely 

 to change be partially covered with some opaque substance, that part 

 will remain green when the rest has turned red. Such rough natural 

 photographs are often found where one leaf has partially shaded another. 

 This explanation of the facts agrees very well with what occurs at 

 different seasons of the year. The leaves of some plants are almost per- 

 fectly green in summer, but as winter approaches they turn more or 

 less red, and on the return of spring these self-same leaves lose their 

 red colour and become green. From this I should draw the very simple 

 and probable conclusion, that as the temperature falls the equilibrium 

 between the constructive and destructive agencies is so much modified by 

 the reduction of vital activity, that the amount of chlorophyll formed is 

 relatively less than that of the red substance, whereas at a more favour- 

 able season of the year more of the chlorophyll and less of the red are 

 produced, and the leaves again become green. On this principle it is 

 also easy to understand the reason of the distribution of red colour 

 in different parts of the same plant — as, for instance, only in the tips of 

 the leaves or in the leaf-stalks, and in those parts well exposed to the 

 sun. According to these views of the subject, permanent red varieties 

 of plants are due to a similar difference in the equilibrium, sometimes 

 greatly modified by the conditions in which they grow, the development 

 of the erythrophyll being also generally dependent on exposure to light. 



