Most of the oxygen and hydrogen of plants is probably obtained from the water absorbed by leaves and roots. Recent experiments indicate that plants may sometimes absorb oxygen directly from the air. This is especially true in the case of buds, as may be shown by the following experiment: Cut twigs of willow, oak or apple just before the buds are to unfold, and place the ends in a little holder containing a small amount of water, and set this in a saucer; partially fill the saucer with quicksilver; over the twigs invert a glass fruit jar filled with oxygen, so that its mouth will be sealed by the quicksilver. The buds will unfold, and some of the oxygen disappear, but if the jar be filled with hydrogen or nitrogen the buds will decay. De Saussure,[1] by a somewhat similar experiment, proved that oxygen is absorbed by the roots of plants. [Illustration: SECTION OF AN EXOGENOUS STEM.] Both gases and moisture are taken up and distributed through the cells by osmose.[2] This may easily be shown; cut off the end of a carrot and scoop out the central portion of the remainder, and place in the cavity dry sugar; this will soon be converted into a syrup, and the sides of the carrot will have perceptibly shrunk, from the passage of moisture out of the cells to the sugar. The mineral constituents of plants are all taken up by the roots in the form of solution, water being the great carrier by which plants are supplied. The following substances are invariably present in all agricultural plants, and in many others, viz.: Potash, soda, lime, magnesia, oxide of iron, chlorine, sulphuric acid, phosphoric acid, silicic acid, and carbonic acid. The chemical composition of different specimens of the same plant is found to be quite uniform. VEGETABLE NUTRITION. Young plants first feed upon the store of nourishment placed in the seed, either in cotyledons,[3] or around them. Soon the little roots acquire the power to take their nourishment from the earth in which they are imbedded. They absorb moisture and the materials in solution, which rise through the latest formed wood as ascending sap, and in the cells of the growing parts, especially the leaves, undergo the transformations which convert inorganic into organic substances. Hales[4] calculated that the force which impels the sap in a grapevine in summer time is five times as great as that which drives the blood through the arteries of a horse. [Illustration: SECTION OF AN ENDOGENOUS STEM.]