So much for the metals in soils; but, important as they are, plants can not live on them alone. Among the nonmetallic bodies phosphorus stands high among essentials, and for it we are indebted to the sea and the interior of the earth. Many living creatures extract phosphorus from the sea water--combine it chiefly with lime, and use the phosphate for making skeletons or shells, as the case may be. After the death of the possessors the bones or shells sink to the bottom, as do the _Globigerina_, and in time are either lifted up, as were the limestones, and form "phosphate beds" like those of Georgia and Florida, or are dredged up and ground into powder with bones of land animals. Much of the matter forced up from the interior of the earth contains phosphorus; indeed, it is the bane of Southern iron ores; but though iron masters dread it, farmers welcome it, as the rains and frosts crumble the phosphatic rocks and add them to the mass of _débris_ that forms our soil. Now let us take a test tube and put into it lime, potash, soda, iron, silicon, or sand, and phosphorus, add to it a grain of corn, and watch results. Under suitable conditions of warmth and moisture the grain will sprout, but when the store of food laid up in it is exhausted our little plant will die. It is obvious that something else is needed for a soil, and analysis shows that it is nitrogen, the gas that forms nearly four fifths of our atmosphere--a gas useless, as such, to animals, but essential to plants. Nitrogen is abundant in Nature. Besides being nearly four fifths of the air, it forms twenty-two per cent of nitric acid, forty-five per cent of saltpeter or niter, eighty-two per cent of ammonia, and about twenty-five per cent of sal ammoniac. Plants can not use nitrogen in its pure form, but one or another of these forms will be found in the soil, whence it may be extracted. Now we have the chief articles of plant food, and it is necessary to know how they are to be used. A plant usually consists of two parts, one that appears above ground, bearing branches, twigs, and leaves, and another that remains below ground. It is this latter that concerns us now, and it is worth study. This lower part consists of a number of twigs called rhizomes, from which proceed a vast number of fine, threadlike rootlets, and these are the mouths of the plant, through which it draws nourishment from the earth about it. Before any living thing can use nourishment from without, it must be dissolved, and this solution requires much preparation at times. Men, and other animals with a wide range of food stuffs, effect this by the secretions of the digestive organs; but most plants have no digestive apparatus, strictly speaking, and were they supplied with an abundance of the foods they most need, they would starve unless the food were in a suitable state for absorption.