466 



[June 19, 



There are many facts connected with the colouring of plants which 

 appear to be almost or quite unintelligible, if we suppose that the 

 colouriug-matters when once formed undergo no immediate further 

 change : for example, if we suppose that the chlorophyll is formed in 

 the young leaves in spring, and remains permanent until they fade in 

 autumn. If, on the contrary, we assume as an hypothesis that the 

 various colouring-matters (or at least some of them) are in a state of 

 constant change, we can readily explain many striking facts. Whatever 

 may be the ultimate conclusion, I shall therefore discuss my subject in 

 accordance with the hypothesis that the colouring-matters are being more 

 or less constantly formed by the internal constructive energy of the 

 plants, and decomposed by the destructive action of the oxygen of the 

 air, intensified by the presence of light ; and that the condition of each 

 particular part of a plant, at all seasons of the year and under different 

 circumstances, depends upon and expresses the temporary or more per- 

 manent equilibrium between these two opposing forces, both of which 

 may vary very considerably. By means of such an hypothesis it is pos- 

 sible to employ accurate physical methods in the study of vegetable 

 physiology ; and since the laws which regulate the destructive force may 

 be determined by experimenting with dead matter, we may learn what 

 does and what does not depend on life, and thus be in a better position 

 to understand its essential nature. 



According, then, to these principles we may expect to find in plants 

 variations depending on a change in the constructive force, as well as 

 others due to variations in the amount of light to which they are 



When a solution of chlorophyll is exposed to the sun and air it is 

 decomposed, the rate of change and the character of the result varying 

 much according to the solvent, and also depending on the presence of 

 minute quantities of sensitizing or protecting oils. When dissolved in 

 benzole or absolute alcohol, and protected by oil of citronelle, the chloro- 

 phyll slowly disappears, and very little of a red colouring-matter is 

 formed, or, if it be formed, it is destroyed nearly as quickly, not accumu- 

 lating so as to become conspicuous. On the contrary, when the chloro- 

 phyll is dissolved in bisulphide of carbon, the change is far more rapid, 

 and several different crimson and red substances are formed, the pro- 

 duction of each particular kind depending on a variety of conditions not 

 yet completely understood; and this makes it still uncertain whether 

 some are exclusively or only mainly due to the blue, and some to the 

 yellow chlorophyll. Since these red substances are destroyed by the 

 action of the blue rays, they are obtained to greater advantage by expo- 

 sing to red light, whieh acts upon the chlorophyll, but not on these pro- 



Action of Light on Chlorophyll. 



