CH. I] ASSIMILATION. 9 



potassium, magnesium, iron, in fact the necessary mineral 

 constituents of the food. Nitrogen will be supplied as a 

 nitrate, sulphur as a sulphate, phosphorus as phosphate. 

 The water will not however be found to contain sugar or 

 any substance from which fungi can obtain carbon. It is 

 therefore clear that Spirogyra has some special method of 

 assimilating carbon. It is in the way that it gets its 

 carbon that Spirogyra (and all other green plants) differ 

 in nutrition, not only from fungi, but also from animals. 

 To yeast as to animals C0 2 is an absolutely waste product, 

 cast out in the process of respiration as of no more use. 

 But to the green plant it serves as an indispensable food- 

 supply, and it is because the ditch-water contains C0 2 in 

 solution, that the Spirogyra is able to live in it. The 

 process by which the carbon is taken out of the C0 2 and 

 built into living substance is known as the assimilation of 

 carbon. 



The fact that CO 2 serves as food may be proved by 

 observing the results of depriving green plants of this gas. 

 If a Spirogyra or other chlorophyll-containing aquatic 

 plant is cultivated in water, which except for the absence 

 of CO 2 is precisely like that in which it naturally lives, 

 the plant dies. This experiment alone is not conclu- 

 sive as to the cause of death, but the conclusion is 

 strengthened by the result of another experiment. If 

 sugar is added to the water the plants do not die : from 

 this it would be rational to suspect that the absence of 

 C0 2 in the first experiment was injurious because it 

 meant the absence of carbon-containing food-stuff. Death 

 is not the only test of an organism being starved : if an 



