Roots sometimes perform other duties in addition to those of fixing the [Pg 31] plant in the soil and providing it with water and mineral food. It is usual, for example, for biennial plants—which produce flowers and seeds in their second year, and then die—to take in much more food during their first season than they require at the time, and to store up the surplus in readiness for the great effort of the second year. These reserve materials are often stored in the roots, which then become swollen and fleshy, like those of the turnip, radish, and carrot. 6. THE FOOD WHICH A GREEN PLANT OBTAINS FROM THE AIR. 1. Plants contain much carbon.—Char a stick, and notice the black charcoal which is formed. Charcoal is an impure form of carbon. 2. Carbon dioxide gas is formed when wood burns.—Fasten a shaving or a splinter of wood on a piece of wire, light it, and lower it into a clean glass jar. When the wood has burned for a few seconds take it out, and pour a little clear lime-water into the jar. The lime-water turns milky. Similarly, pour a little lime-water into a jar in which nothing has been burning, and notice that it remains clear. There is evidently a difference in the nature of the air of the two jars. The difference is caused by the burning of the wood, during which some of the carbon unites with the oxygen of the air in the jar, forming an invisible gas, called carbon dioxide. Carbon dioxide can always be detected by the milkiness it causes in clear lime-water. 3. Carbon dioxide present in ordinary air.—Pour some clear lime-water into a blue saucer and let it stand exposed to the air for half an hour, then examine it. A white scum has formed on the surface of the lime-water. Stir with a glass rod; the solution becomes milky. The scum and the milkiness are produced by the union of the lime with carbon dioxide from the air. Carbon (in the form of carbon dioxide) is therefore present in the air. 4. No carbon in the food solution.—Examine again the list of [Pg 32] elements (p. 27), which compose the mineral salts which have been found to replace satisfactorily the food which a plant obtains from the soil. There is no carbon in it. A plant evidently does not depend on the soil for the carbonaceous part of its food. From what other source can a plant obtain its carbon? Carbon has just been proved to be present in the air. Does the plant obtain its carbon from the air? 5. Green leaves contain starch after exposure to sunlight.—Take a green leaf from a plant which has been exposed to the sunlight, boil the leaf in water for a minute or two to kill it. Then put it in methylated spirit until the leaf-green is dissolved out. When the leaf is bleached rinse it in water, then put it into a dilute solution of iodine (p. 2) and notice that it becomes blue or purplish brown. The formation of this colour proves the presence of starch in the leaf.