FOOD 51 



so also for all the world. It manufactures most of its 

 own carbohydrates, using carbon- dioxide and water in 

 the process, and this process is known as photosynthesis. 

 This process of photosynthesis may be considered briefly 

 and conveniently under four headings : (1) the raw 

 materials, (2) the laboratories, (3) the energy, and 

 (4) the products and the process. 



The raw materials needed have already been men- 

 tioned carbon-dioxide and water. Carbon-dioxide 

 exists everywhere in the air in the ratio of about 

 three parts in ten thousand. In the neighbourhood of 

 towns and where there is a marked decomposition of 

 rocks or decay of vegetable matter the percentage may 

 be temporarily higher. It is also present in the water 

 of ponds, lakes, and slow-running streams in a higher 

 ratio ; it may be up to one hundred times as much as 

 in the air. 



In all plants with no cuticle on the surface the carbon- 

 dioxide enters all over the surface, but for the majority 

 of land plants the supply must pass through special 

 openings, the stomata, and these openings are sufficient 

 to admit as much as five or six times the amount 

 required. 



Water, the other raw material required, is never 

 lacking where plants are active. Its source in the 

 majority of land plants is the soil water that enters 

 through the roots ; only in the case of the mosses, liver- 

 worts, and a few epiphytes does it enter freely into the 

 aerial parts. 



The laboratories in which photosynthesis proceeds are 

 the chloroplasts. These are organs of various form and 



