THE METABOLISM OF PLANTS. 267 



Batalin, G. Haberlandt). In other cases leaves assume a leathery bro\vn 

 colour : this is well-marked in some Conifers, such as various species of 

 Thuja, Yew, Sequoia gigantea. This colouration is only produced after 

 the plants have been exposed to frost (Kraus, Haberlandt), but it is also 

 in some way connected with the influence of light. It appears to be 

 due to a chemical alteration of the chlorophyll by the action of some 

 substance (possibly an acid) contained in the cell-sap. 



Se'ne'bier observed that if strips of tin-foil be fastened on pears and 

 apples, the natural red colour which these fruits assume on ripening is 

 not produced in those parts which have been covered. 



It will be remembered that, when we were considering the 

 effect of light upon respiration, it was mentioned that it tends 

 to promote the absorption of oxygen. The oxidation of 

 chlorophyll is one instance of this, but it is probably only one 

 of many. De Saussure observed that light is favourable to 

 the absorption of oxygen by oil of Lavender, and Jodin has 

 found this to be also the case with ethereal oils and solutions 

 of tannin. It has been suggested (p. 236) that resin is formed 

 in the living plant by the oxidation of an ethereal oil (ter- 

 pene), but nothing is known as to the influence of light upon 

 the process. 



The formation of the other colouring-matters of plants, 

 those of flowers for example, appears to be less dependent 

 upon the action of light than is the formation of chloro- 

 phyll. Sachs observed, in his experiments upon etiolation, 

 that in all the plants which came under his observation, the 

 flowers which were produced in darkness were coloured in 

 much the same manner as those produced in light. Askenasy 

 has however found that this is by no means always the case, 

 but that the colouration of flowers is in many cases much 

 modified or even absent when the plants bearing them are 

 kept in darkness. There are not at present sufficient data upon 

 which to base an explanation of the diversity of behaviour 

 of flowers in this respect, but it appears to depend upon their 

 particular hue. Sorby has observed that the red colouring- 

 matter of flowers (which is probably identical with erythro- 

 phyll) is formed in smaller quantity in relatively weak than 

 in relatively strong light. 



Comparatively little is known as to the influence of light 



