Some plants have acquired the faculty of storing water in their bodies, on which, camel-like, they can subsist for long periods of time. A certain large tree-cactus of the American desert sometimes stores up as much as seventeen hundred pounds or five barrels of water in the wet season. When drought comes, its roots dry up and it lives entirely on its internal resources. It is said that an eighteen-foot specimen can exist for a year on its stored-up liquid. A branch on such a plant may live and bloom after the trunk is dead. Many ordinary plants, such as Turnips, Carrots, and Beets, store water along with starch and dextrose in their underground tubers. Such subterranean reservoirs are preferable to those above ground. Plants have paid particular attention to the manipulation of gases. They maintain an internal atmosphere of their own composed of oxygen, nitrogen and carbon dioxide in proportions varying greatly from those of the outside air. If the stem of a Water Lily be broken below the surface of a pond, gas bubbles will often be observed to issue from the wound, indicating that the internal gas pressure of this particular plant is greater than that of the external air. In other cases, the reverse is true and we find partial vacuums within the bodies of plants. Man long ago found it impossible to “live on air” but the plants have solved the difficulty of aerial existence and have become creatures of the air rather than the earth, so far as their food is concerned. The great bulk of the largest tree is preponderantly composed of carbon, which has been slowly and labouriously extracted from the air. The mineral salts and water which have been filtered out of the ground by the roots are essential but are present in a much lesser quantity. It is well known that plants breathe in carbon dioxide and breathe out oxygen. This can be graphically demonstrated by placing a plant in a glass jar of carbon dioxide inverted in water. If its life processes are quickened by exposure to sunlight, the plant will replace the CO₂ with oxygen in a day. A more striking example is furnished by any aquatic plant accustomed to growing submerged in ponds and rivers. Placed in a water-filled bottle inverted in a pan of water, it will generate oxygen so rapidly that the bubbles can be seen forming on the leaves when the sun is allowed to strike them fully. The bottle will become filled with oxygen in a few hours, and its presence can be demonstrated with the usual ember test.