Having glanced at the functions performed by the mineral constituents, we will pass on to those of the leaves, and here as before no attempt will be made to answer the question, How do the leaves act? but rather our intention is to show the result of their action. The leaves are the means whereby the plant communicates with the air, absorbing from it that portion which is injurious to the life of animals, namely, carbonic acid gas, which consists of carbon and oxygen; under the influence of sunlight these two components are separated in the leaf, the one from the other, the carbon or solid part remaining in the plant to form all the various compounds, such as starch, oil, and acids, while the oxygen is exhaled into the air for the use of animals; this retention of carbon and conversion into starch, etc., has been termed assimilation, to which we have already referred; now we can appreciate the immense importance of plants of all kinds, for without their aid the atmosphere would become so overburdened with the harmful carbonic acid that it would no longer support life or combustion. A small experiment will readily demonstrate the action of leaves on carbonic acid: if a green laurel-leaf, immersed in a glassful of spring-water, be exposed to sunlight, a number of small bubbles will soon be noticed on the surface of the leaf. In a short time they will increase in size, and finally float to the surface, when by proper means they can be collected and shown to consist of oxygen, which possesses the property of causing a glowing splinter of wood to burst into flame when introduced into it. This oxygen has been produced by the decomposition of the carbonic acid dissolved in the water. It would be incorrect to suppose that the leaves absorb no oxygen, but always give it out, for at all times a proportion of oxygen is inspired, and in the dark, carbonic acid is exhaled, yet the quantity is always less than that of the oxygen exhaled during the day, and at low temperatures the amount of oxygen absorbed exceeds that of the carbonic acid. How to account for the production of starch from the materials at the disposal of the plant is somewhat difficult; but, theoretically, six volumes of carbonic acid combining with five volumes of water produce starch, six volumes of oxygen being liberated; but when once the starch is produced, we know, from laboratory experiments, that sugar can easily be produced from it as well as oxalic acid, etc. The purpose of the leaves is not only to collect air food, but also to get rid of superfluous water, for the roots are continually pumping in water laden with mineral food, so that to allow of the circulation and deposition of this food the water must be got rid of. This water is exhaled from the leaves in the form of invisible vapor, but the quantity depends on the state of the atmosphere, which when moist almost wholly prevents exhalation; on the other hand, in very dry weather, exhalation takes place too