FORMATION OF STARCH AND SUGAR 33 



carbon dioxide and the water into sugar. Thus a carl^ohydrate 

 is formed, and thus is the plant's food built up. The sugar in 

 solution slowly passes down the veins of the leaf and leaf- 

 stalk into the main part of the plant. During the day time 

 more sugar is usually formed than can be transported by this 

 slow process and the result is that the sap of the leaf cells 

 becomes more concentrated — sweeter. An indefinite increase 

 in the concentration would clog the mechanism, and it com- 

 monly occurs that where the sap contains more than a certain 

 percentage (usually about 0-5 per cent.) of sugar, all in excess 

 is transformed into insoluble starch. The starch exists in 

 small granules in the chloroplasts, and if the plant be removed 

 to darkness it will gradually disappear, because the transport 

 of sugar is going on the whole time and as soon as the con- 

 centration of sugar in the sap drops below the critical figure, 

 starch is turned back to sugar once more. 



The sugar may be immediately used up in a growing plant, 

 or it may be stored away as a food reserve for the future. 

 For instance, carrots and turnips store up grape sugar, and 

 sugar-cane and beet-root, cane sugar. 



All the time this building-up process is going on — and it 

 takes place only during daylight — the living matter of the 

 plant is absorbing some oxygen and giving out some carbon 

 dioxide; and this it does during its whole life, whether in 

 sunlight or in shade. This latter process is called respiration^ 

 and it is common both to plants and animals. But the plant 

 is breathing out what it wants back as food, namely carbon 

 dioxide, and breathing in what it excretes as the waste 

 products of the food, namely oxygen. 



As Sir E. Ray Lankester has written, this action of chloro- 

 phyll is "the critical step in the interaction of chemical 

 elements on the earth's surface, by which life is at present 

 determined. Were there no assimilation of carbon from 

 carbonic acid to form sugar or starch — by green plants, the 

 whole fabric of the living world would tumble to the ground — 

 in truth become mineralized. All living matter breaks down, 

 within a short space of hours or days, to the resting or mineral 

 condition of carbonic acid and ammonia (or nitrates). Were 

 the building-up process, the raising to higher potentiality, 



S L 3 



