THE LEAF 147 



We have spoken in general terms till now of car- 

 bonic acid being decomposed in the leaf, of the rays 

 of the sun being absorbed by the leaf, and so on ; but 

 we can express ourselves much more definitely. In 

 speaking of the assimilation of carbon in contradistinc- 

 tion to the assimilation of other nutrient substances, 

 we can determine quite clearly the microscopic seat of 

 the process. It is the green chloroplast. We can show 

 that certain of the sun's rays are really absorbed by 

 chlorophyll, and that it is just those rays which are 

 absorbed which bring about the decomposition of car- 

 bonic acid, the initial stage in the assimilation of 

 carbon, and also the formation of starch, its final stage. 

 The green colour, which depends upon the peculiar 

 absorption of light by the chlorophyll in the chloroplast, 

 is thus not an accidental property of the plant, but is 

 closely bound up with the most essential process of its 

 nutrition. It is not the leaf as a whole, but the 

 chloroplast that colours it green, which serves as a 

 connecting link between the sun and all things living 

 upon the earth. 1 



We have now studied the function of the green leaf. 

 Plants without green organs are unable to manufacture 

 organic matter for themselves out of carbonic acid, but 

 are obliged to live at the expense of organic matter made 

 by other plants. Fungi, for instance, the plants we 

 generally call mushrooms and the microscopic moulds, 



1 The necessity for maintaining in due proportion the different parts of 

 this course prevents me from working out as fully as it deserves this 

 most interesting chapter in the physiology of the plant. Those who wish 

 to study more closely this side of the life of the plant will find a more 

 detailed exposition of the subject in a chapter appended to the course 

 entitled The Plant as a Source of Energy, which in its turn presents a 

 popular exposition of the principal results of my special work On the 

 Assimilation of Light by the Plant, and of my further researches in the 

 same direction. These facts are set forth in still greater detail in a lecture 

 entitled The Plant and the Energy of the Sun (in my Lectures and 

 Addresses, Moscow, 1888), and in my Croonian lecture The Cosmical 

 Function of the Green Plant. Proceedings of the Royal Society, 1903. 



