48 



happens to those rays which are absorbed ? We are 

 still very much in the dark on this subject, but in all 

 probability the rays absorbed are transformed into 

 some other form of energy and used by the proto- 

 plasm in the construction of organic substance. 

 What we do know for certain is that, given the condi- 

 tions above described, oxygen gas is evolved from 

 the green leaf and almost immediately thereafter 

 carbohydrates appear in the cell. 



We have now to ask what becomes of the energy 



of the solar rays which are absorbed ? Undoubtedly, 



a large amount of it is used up in the decomposition 



of the highly oxidised mineral compounds, water and 



Photosyn- carbon dioxide, and in getting rid of the excess water 



thesis. absorbed by the roots, but part becomes stored as 



potential energy in the carbohydrates which have been 



manufactured. This constructive process is spoken 



of as photosynthesis. 



While the detailed stages of the photosynthetic 

 process are as yet very imperfectly known to us, we 

 are even more in the dark as to the nature of the 

 further constructive efforts of the protoplasm by 

 which higher compounds, such as proteids, are manu- 

 Summary. factured. We know, however, that for these higher 

 constructive efforts no sunlight is necessary, and in 

 all probability the energy required is obtained by 

 the oxidation of primary organic compounds, and 

 possibly of protoplasm itself (chemosyn thesis), but 

 into these recondite physiological problems it would 

 be out of place to enter in such a preliminary sketch 

 as the present. Thus far we have learned that the 

 process of nutrition is fundamentally the same in 

 plant and animal, but that the green plant adds a 

 new department of work, entirely absent from the 

 normal animal economy, namely, the manufacture 



