There are several things that we may say about them. For example, they grow. But a crystal grows and so does a heap of sand. There are, then, different ways of growing. The sand heap grows when the wind scatters particles of sand over it or the sea drives them; or the river, or a small boy with a barrow, deposits them. It grows by mere addition. The crystal grows without any such aid and in a different way. If you want to see an alum-crystal grow you must offer it more alum, dissolved; you must offer it its own material and it will then grow without other help. It will grow by what we may call formative accretion. On the other hand the grass and the corn and the trees grow neither by mere addition nor by formative accretion, but by converting, assimilating and incorporating nitrogen, carbon, and so on, compounded either by their own effort or by the micro-organisms of the soil. They are magicians, these live creatures. And of all their magic the magic of their green leaves is chief in our eyes. For it takes the carbonic acid of the air, a stable compound of one atom of carbon with two of oxygen, and tears it to pieces, using for that enormous work the radiant energy of the sun. This carbon, now potent with the potency of the sun, is then united with other elements to make sugars that nourish the tissues of the plant. In these sugars power is stored as it is stored in an explosive like dynamite or trinitrotoluol. But in the plant, power is under strict control; it is liberated by degrees as it is wanted and is replaced as it is used; it is controlled, used, replaced in a discriminating, selective and determining way. The ability to make use of physical power, within a body, in this discriminating, selective and determining way and thus to grow is (among other things) what is meant by being alive.