The Sun a Source of Energy.--We all know the sun is a source of most of the energy that is released on this earth in the form of heat or light. Every boy knows the power of a "burning glass." Solar engines have not come into any great use as yet, because fuel is cheaper, but some day we undoubtedly will directly harness the energy of the sun in everyday work. Actual experiments have shown that vast amounts of energy are given to the earth. When the sun is highest in the sky, energy equivalent to one hundred horse power is received by a plot of land twenty-five by one hundred feet, the size of a city lot. Plants receive and use much of this energy by means of their leaves. Effect of Light on Plants.--In young plants which have been grown in total darkness, no green color is found in either stems or leaves, the latter often being reduced to mere scales. The stems are long and more or less reclining. We can explain the changed condition of the seedling grown in the dark only by assuming that light has some effect on the protoplasm of the seedling and induces the growth of the green part of the plant. If seedlings have been growing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem and leaves of the seedlings incline in the direction from which the light comes. The experiment pictured shows this effect of light very plainly. A hole was cut in one end of a cigar box and barriers were erected in the interior of the box so that the seeds planted in the sawdust received their light by an indirect course. The young seedling in this case responded to the influence of the stimulus of light so as to grow out finally through the hole in the box into the open air. This growth of the stem to the light is of very great importance to a growing plant, because, as we shall see later, food making depends largely on the amount of sunlight the leaves receive. [Illustration: Two stages in an experiment to show that green plants grow toward the light.]