Because they secrete a carbonaceous matter, named _chlorophyll_, from which they derive their green colour. 1182. _Why are the hearts of cabbages, lettuces, &c., of a pale yellow colour?_ Because the action of _light_ is necessary to the formation of _chlorophyll_; and as the leaves are folded upon each other, they exclude the light, and the green matter is not formed. 1183. _Why do leaves turn brown in the autumn?_ Because, when their power of decomposing the air declines, the _oxygen_ absorbed in the carbonic acid gas, _lodges in the leaf_, imparting to it a red or brown colour. 1184. _Why do succulent fruits, such as gooseberries, plums, &c., taste acid?_ Because, in the formation of juices, a considerable amount of _oxygen_ is absorbed, and the oxygen imparts acidity to the taste. [Verse: "The earth is full of the goodness of the Lord."--PSALM XXXIII.] 1185. _Why do ripe fruits taste sweet, and unripe fruits taste sour?_ Because the juices of the ripe fruit contain a large proportion of _sugar_, which in the unripe fruit has not been formed. 1186. _Why do some leaves turn yellow?_ Because they retain an excess of _nitrogen_. Leaves undergoing decay turn either yellow, red, crimson, or violet. Yellow is due to the excess of _nitrogen_; red and crimson to various proportions of _oxygen_; violet to a mixture of _carbon_; and green to _chlorophyll_. 1187. _Why do leaves fall off in the autumn?_ Because they have supplied for a season the natural wants of the tree. Every part has received nutrition through the spring and summer months; and the wants of the tree being supplied, the chief use of the leaf ceases, and it falls to the ground to decay, and enrich the soil. 1188. _Why do plants suffer from the smoke of cities?_ Because the smoke _injures the porous structure of the leaves_, and interferes with their free respiration. CHAPTER LXI. 1189. _Why are vegetable productions so widely diffused?_ Because they everywhere form the _food of the animal creation_. Without them, neither man nor beast could exist. Even the flesh-eating animals are sustained by them, since they live by preying upon the bodies of vegetable-eaters. They also enrich and beautify the earth. They present the most charming diversities of proportions and features. From the cowslip, the primrose, and the blue-bell of our childish days, to the broad oak under which we recline, while children gambol round us, they are all beautiful or sublime, and eminently useful in countless ways to man. They spread a carpet over the surface of the earth; they cling to old ruins, and cover hard rocks, as though they would hide decay, and give warmth to the coldness of stone. In tropical climates they supply rich fruits full of cool and refreshing juices, and they spread out upon the crests of tall trees those broad leaves which shelter the native from the scorching heat of the sun.