To our mind, this grain of mustard-seed is an image of the sun, which, falling upon the earth, sows animated germs in its bosom, which produce plants that afterwards give birth to animals, and later still to man, and thus to the entire series of creatures, invisible to us, which succeed him in the domain of the heavens. The little cold, colourless, scentless seed is nothing to look at, nothing distinguishes it apparently from the neighbouring dust. Nevertheless, it contains that mysterious leaven, that sacred being, so to speak, which we call a germ. And what wonderful things are to be born from that sacred being! In the obscurity of the cold, damp earth the germ transforms itself, it becomes a new body without any resemblance to the seed which contained it. It produces a plantlet, a subterranean, but perfectly organized creature, possessing a root which fastens itself into the soil, and a stalk which takes the opposite direction. Between the two portions lies the seed, split, gutted, having allowed the germ to escape, its part in the matter ended. The subterranean plantlet is a new being; it has no longer anything in common with the seed from whence it came. The plantlet is dull and colourless, but it breathes, it has channels, in which liquids and gases are circulating. In a little while the plantlet comes up above the earth, it greets the daylight, it appears to our eyes, and then it is a very different being from the subterranean creature. The new-born vegetable is no longer as it was when in the bosom of the earth, dull and grey; it is green, it breathes like other vegetables, producing oxygen under the influence of life, whereas in the bosom of the earth it gave out carbonic acid. Instead of the dull and sombre subterranean plantlet, you have a green and tender shoot, provided with special organs. Where is the grain of mustard seed? Presently our shoot grows, and becomes a young plant. It is still weak and hidden under the grass, but nevertheless the young plant has its complete individuality. It resembles neither the shoot nor the plantlet, its subterranean ancestor. The shoot grows, and becomes a twig, that is to say, the adolescent of the vegetable kingdom, with the ardour and the energy of the young among herbaceous creatures. In this state the plant has already renewed its substance several times, and nothing remains of the organic and mineral elements which existed in the different beings that have preceded it on the same little corner of the earth which have witnessed the changing phases of its curious metamorphoses. Wait a while, and you will see the twig growing long and large. Its respiration becomes active, its leaves spread out, and vigorously exhale the carbonic acid gas of the air. The exhalation of watery vapour over all the surface takes place, and a young and vigorous tree is there, which day by day grows more robust and more beautiful.