NUTRITION, STORAGE, AND RESPIRATION 105 



starch formed by a square metre of Sunflower-leaf on a summer's day 

 would be about 25 grammes. This quantity of starch should be 

 weighed out in scales, it will then be seen to be a very considerable 

 bulk. In forming these 25 grammes the plant would clear the carbon 

 dioxide from about 50 cubic metres of atmospheric air, while for each 

 gramme of starch formed about 250 to 400 cc. of water of the Trans- 

 piration-Stream would be evaporated. These approximate statements 

 give some idea of the activity of the changes attending Photo-Synthesis 

 on a summer's day. But it is not only this degree of activity that 

 makes the constructive process in green plants notable. It must in 

 a sense be recognised as the starting-point of all physiology. For to 

 it the origin of all other organic substances may be referred. 



It may not De strictly correct to say that all the combined carbon in the 

 organic world was derived ultimately by Photo- Synthesis from the carbon 

 dioxide in the atmosphere, though for practical purposes it is so. A small 

 amount is obtained by nitrifying Bacteria, by action that is not dependent 

 on light, or on the presence of chlorophyll. If these organisms, derived from 

 garden soil, be grown in a culture solution containing potassium phosphate, 

 magnesium phosphate, sodium or magnesium carbonate, and common salt, 

 they will nourish. But the carbon they contain is obtained from the carbon 

 dioxide of the air, not from the carbonate supplied. The nitrifying Bacteria 

 thus have the power of forming a small amount of organic substances from the 

 atmosphere, by a process altogether different from that of green plants. 



USE OF CARBOHYDRATE TO FORM PROTEID. 



The carbohydrate acquired by the plant in Photo-Synthesis is 

 removed by diffusion from the cells where it is formed. This may be 

 readily proved if after active Photo-Synthesis and production of 

 starch, the plant be kept in the dark for a few hours. The chloro- 

 plasts will then be found to have been completely cleared of the 

 starch which they previously contained. A considerable proportion 

 of this carbohydrate is probably used up directly in the construction 

 of proteid, which is essential for the supply of the growing and dividing 

 protoplasts. In addition to Carbon, Hydrogen, and Oxygen, all the 

 proteids contain nitrogen (15-19 p-c.), and sulphur (f-ijp.c.), and in 

 some cases phosphorus also. Accordingly the construction of proteids 

 requires a supply of these elements, and the source of that supply 

 must be considered. 



The question of the supply of nitrogen lies at the root of all plant- 

 nutrition, for no protoplast can be formed without it. If in a culture- 

 solution nitrogen be available in the form of nitrate, and all the other 



