"A plant breathes, drinks, eats, selects, refuses, seeks, works, lives, acts according to its instincts. One does 'like a charm,' another pines, a third is nervous and agitated. The sensitive-plant shivers and droops its leaves at the slightest touch. In certain hours of well-being the calla lily is warm, the pink is phosphorescent, the valisneria goes down to the bottom of the lake to ripen the fruit of her loves. In these manifestations of an unknown life the philosopher cannot help recognizing a song from the universal choir in the plant world. "I go no further for the human soul just now, although it is incomparably superior to the soul of a plant, and although it has created an intellectual world as much above the rest of the terrestrial world as the stars are higher than the Earth. I am not looking at it now from the point of view of its spiritual faculties, but only as force animating the human being. "Ah! I wonder that that force can group the atoms that we breathe, or that we assimilate by nutrition and form this charming being! Think of that young girl the day she was born, and follow in thought the gradual development of that little body through the years of her awkward age to the first graces of youth and the charms of womanhood. How is human organism nourished, developed, and composed? You know,--by respiration and nutrition. "The air supplies three quarters of our nourishment by respiration. The oxygen in the air maintains the fire of life, and the body is comparable to a flame, constantly renewed by the principles of combustion. The lack of oxygen extinguishes life as it extinguishes a lamp. By respiration the black venous blood is transformed into red arterial blood and regenerated. The lungs are a fine tissue pierced with from forty to fifty millions of little holes, which are just too small for the blood to filter through, and just large enough for the air to penetrate them. A perpetual interchange of gas takes place between the air and the blood, the first furnishing the second with oxygen, the second eliminating carbonic acid. On the one hand the atmospheric oxygen burns carbon in the lung; on the other the lung exhales carbonic acid, nitrogen, and water in the form of vapor. In the daytime, plants breathe by an opposite process,--they absorb carbonic acid and exhale oxygen; by this difference maintaining one part of the general equilibrium of terrestrial life.