CHAPTER VII. 



NUTRITION, STORAGE, AND RESPIRATION. 



THE Green Plant, supplied as we have seen with soluble salts from 

 the soil, is able under certain conditions to construct new organic 

 materials by a process carried on in its green parts. Such a process 

 may be included under the general term Constructive Metabolism. 

 But it is better to style it more specifically Photo- Synthesis, because 

 it is a constructive process for which exposure to light is a necessary 

 condition. It was formerly called Assimilation ; but this term is 

 unsatisfactory, for it has been used in Animal Physiology to connote 

 the absorption of organic material already formed, whereas in Photo- 

 Synthesis new organic material is elaborated by the plant itself. 



The conditions necessary for Photo-Synthesis to proceed are 

 these : 



d) Chlorophyll must be present in the cells which carry on the 

 process. 



(2) Carbon dioxide must be accessible to them. 



(3) They must be exposed to tight of sufficient intensity. 



(4) The temperature must be within certain limits. 



(5) There must be a supply of certain mineral salts. 



The green colouring matter Chlorophyll is located in the chloroplasts. 

 These may be found in the tissues of the stem, or even occasionally 

 in roots, but only in parts exposed to the light. The leaf is their chief 

 centre, and in the mesophyll particularly (Figs. 71 A, 72). Wherever 

 chloroplasts exist they may act in Photo-Synthesis, for which their 

 presence is necessary. The green colouring matter which they contain 

 is soluble in alcohol, and may thus be extracted, leaving the discoid 

 chloroplasts colourless. The green leaf itself, or the solution of 

 chlorophyll in alcohol, if examined spectroscopically will show certain 



