There are other motions of the sap in plants, which are called special, in distinction from the ascending and descending of the sap, which is called general, or common to all plants. The special movements of the sap are peculiar to certain plants, in some of which a fluid, full of little green cells, is found to have a rotatory motion; in other plants, a milky fluid is found to move through particular tissues of the vegetable structure.
1181. Why are the leaves of plants green?
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.