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 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.
6. Starch contains carbon.—Char a piece of laundry starch and observe the charcoal formed.
7. Green leaves do not contain starch after being left in the dark for 24 hours.—Keep a leafy plant in the dark for 24 hours and then test a leaf as in the previous experiment. No starch can be detected. Put the plant in the sunlight for an hour or two and test another leaf. It contains starch. Plainly, starch is only formed in leaves if they are exposed to light, and any starch previously present disappears when the plant is kept in the dark.
8. A green plant kept in air from which the carbon dioxide has been removed will not form starch in its leaves.—Obtain a large glass bottle, such as those used by confectioners, and fit it with a cork or india-rubber stopper through which passes a glass tube bent[5] as in [Fig. 21]. Care should be taken to make all the joints tight, and it may be necessary to soak the cork in melted paraffin to ensure this. Pack the bend of the tube loosely with pieces of soda lime, and in the bottle place a small jar containing lumps or a strong solution of caustic soda. When the apparatus is ready, place in the bottle a small plant or a leafy twig in water (fuchsia answers very well), which has been kept in the dark for 24 hours. The caustic soda in the jar very soon absorbs all the carbon dioxide which is present, and the soda lime in the bend of the tube prevents any carbon dioxide from getting into the bottle from the outside air. Place the jar in bright sunlight for a few hours and then test a leaf for starch. None can be detected. It is plain that one of the carbonaceous food stuffs—starch—is not formed in the leaves of plants unless the plant is grown (in the light) in air containing carbon dioxide.
9. Seedlings are at first independent of light.—Germinate pea or bean seeds in wet sand or sawdust in the dark. Notice that for some time the seedlings grow almost as well as when in the light. Soon, however, the stem becomes long, weak and straggling, and the leaves are pale in colour, even if the plant is supplied with mineral food.
10. The formation of sugar in a germinating pea-seed.—Take up a pea-seedling when the stem is one or two inches long, and chew the partly shrunken cotyledons. Notice the slightly sweet taste. Contrast this with the taste of an ungerminated seed.