As the earth revolves upon its axis, sunlight falls successively on every acre of land. Almost everywhere it is intercepted by green foliage. Each leaf of every plant receives and absorbs as much as it can, and, for so long as the light lasts, its living particles are hard at work: water or sap is hurrying up the stem and streaming out of the leaves as water vapour. Carbonic acid gas also is hurrying into the leaves; inside these latter first sugar and then starch is being manufactured, so that the green cells become filled with starch or sugar. So soon as the light fails, the work begins to slacken. When darkness sets in, the starch changes to sugar and passes down the leaf-stalk into the stem, where it is used up in growth, in the formation of new wood or in supplying the developing flowers or young buds. Next morning when the sunlight touches the plant all its little living cells set to work again, and another day's task is begun. It is very difficult to understand what is going on inside the leaf. If you were to imagine a square yard of leaves all taking in sunshine and making starch as they do in fine weather; then if you weighed all these leaves, and then weighed them again one hour after they had been in the sunshine, of course that square yard of leaf surface should be heavier, because a certain amount of starch has been formed in it. The amount actually made in one hour has been estimated by Dr. Horace Brown as 1/500 lb. So that 100 square yards of leaves working in sunshine for five hours might make one pound of starch. But one can estimate the activity of plants in another way. Look at the amount of work done by the Grass, etc., on an acre of pasture land in one year. This might entirely support a cow and calf during the summer; all the work done by these animals, as well as all the work which can be done on the beef which they put on, is due to the activity of the grasses on that acre. Moreover it is not only these large animals that are supported, but every mouse, every bird, every insect, and every worm which lives on that piece of ground, derives all its energy from the activity of the plants thereon. All work which we do with our brains or muscles involves the consumption of food which has been formed by plants under the warm rays of the sun. So that man's thoughts and labour, as well as that of every living creature, is in the first instance rendered possible by sunshine. But the sunlight, besides this all-important function, affects plants in other ways. One of the most interesting of the early spring flowers is the Coltsfoot. On bare blackish and unsightly heaps of shale one may see quantities of its golden blossoms. Now if one looks at them on a fine sunny day, every single blossom will be widely opened and each will turn towards the sun.