=Sources of Food.=—The ordinary green plant has but _two sources from which to secure food,—the air and the soil_. When a plant is thoroughly dried in an oven, the water passes off; _this water came from the soil_. The remaining part is called the =dry substance= or =dry matter=. If the dry matter is burned in an ordinary fire, only the =ash= remains; _this ash came from the soil_. The part that passed off as gas in the burning _contained the elements that came from the air_; it also contained some of those that came from the soil—all those (as nitrogen, hydrogen, chlorine) that are transformed into gases by the heat of a common fire. The part that comes from the soil (the ash) is small in amount, being considerably less than 10 per cent and sometimes less than 1 per cent. _Water is the most abundant single constituent or substance of plants._ In a corn plant of the roasting-ear stage, about 80 per cent of the substance is water. A fresh turnip is over 90 per cent water. Fresh wood of the apple tree contains about 45 per cent of water. =Carbon=.—_Carbon enters abundantly into the composition of all plants._ Note what happens when a plant is burned without free access of air, or smothered, as in a charcoal pit. _A mass of charcoal remains, almost as large as the body of the plant._ Charcoal is almost pure _carbon_, the ash present being so small in proportion to the large amount of carbon that we look on the ash as an impurity. Nearly half of the dry substance of a tree is carbon. Carbon goes off as _a gas_ when the plant is _burned in air_. It does not go off alone, but in combination with oxygen in the form of _carbon dioxide_ gas, CO_{2}. =The green plant secures its carbon from the air.= In other words, much of the _solid matter_ of the plant comes from _one of the gases of the air_. By volume, _carbon dioxide forms only a small fraction of 1 per cent. of the air_. It would be very disastrous to animal life, however, if this percentage were much increased, for it excludes the life-giving oxygen. Carbon dioxide is often called “foul gas.” It may accumulate in old wells, and an experienced person will not descend into such wells until they have been tested with a torch. If the air in the well will not support combustion,—that is, if the torch is extinguished,—it usually means that carbon dioxide has drained into the place. The air of a closed schoolroom often contains far too much of this gas, along with little solid particles of waste matters. Carbon dioxide is often known as carbonic acid gas.