Sometimes seeds germinate under a barn floor for instance, and the puny pale little plantling reaches out slender stems, all of which turn, as a compass turns to the north, to perhaps a crack of light in one corner of the building. We have already seen how the search for light will carry the slender rattan palms of India hundreds of feet to the topmost leaves of the forest. Individual plants, and, as we shall see later, whole forests make desperate efforts to get to the light. We know already, that the struggle for light is just as bitter as the struggle for food by roots. And finally if, as we have many times proved by experiments, plants die when grown in a dark room, what is it that light does for plants and how is a process carried on that everything leads us to think is of the greatest possible importance? Quite obviously it is not the mere beauty of sunshine dancing upon the landscape, as entrancing a picture as that may be any summer afternoon, with the play of sunshine and shadow on the tracery of foliage. That green color of the foliage, the almost universal green of so much of the earth’s vegetation, restful to tired eyes, providing us with the most pleasant shade, has wrapped up within it the secret of just what sunshine does for plants. For under the magic of light acting upon this greenery one of the most important industries in the world, the manufacture of food, is constantly going on. LEAVES AS FACTORIES FOR THE MAKING OF FOOD It must be clear enough from the start that to call a leaf a factory for the making of food forces us to decide at once whether this is a mere way of speaking, or whether, incredible as it may seem, anything as thin as a leaf _can_ really produce food. As we eat lettuce, and millions of cattle graze every day, leaves as food producers win handily on that score. But to understand how food is produced in such a tiny factory demands that we walk about in it for a bit, study the inside of it and especially its many small chambers within which is not only the machinery, but some of the finished product stored up for later use. Unlike modern factories there are many entrances, from any one of which we can begin our tour of inspection. On the under side of nearly all leaves and on the upper side of some there are scores or even hundreds of small pores called _stoma_, so small that only with a microscope can they be seen. These entrances through the factory wall, are carefully guarded by a pair of watchmen whose business it is to see neither too much dry air gets in nor too much of the product of the factory gets out. They see to it, also, that waste products are thrown out at the proper time. These watchmen, or _guard cells_, as they are called, are constantly on the job, work almost automatically, but their chief function is connected with the proper ventilation of the place, and will be discussed later under “How Plants Breathe.”