Page:Popular Science Monthly Volume 74.djvu/587

Rh warmth is attainable. Nor is light uniform, for though the total seasonal light-fall may be, like the total seasonal rainfall, a moderately uniform quantity, yet we know that the light which reaches our eyes and plays on field and forest varies almost from moment to moment, as a wisp of cloud, a trail of smoke, or a bird or butterfly, passes between us and the sun. Yet with all this variation in quantity from day to night, and even from moment to moment, there is no variation in quality. The composition of sunlight, as it reaches the earth's atmosphere, is the same age after age; its red, yellow, blue and other rays fall upon animal and plant in similar proportions. Until the source of light changes, until the composition of the sun becomes altered by the exhaustion of this or that substance, the quality of light must continue the same.

The leaves, stems and flowers of our household plants turn toward the window. Plants growing under a hedge turn out to one side, if they are able to bear the shade long enough to get out of it. In the early morning, or toward sunset, one can often see the leaves of weeds all turned eastward or westward, according to the source of light. These are familiar instances of the directive influence of light, an influence which changes with the direction and the intensity of the light and is dependent upon some, not all, of the rays of ordinary daylight.

The formative influence of light is no less real and definite, although not so generally recognized. It determines whether a plant shall be stocky or straggling, short or long. Greenhouse men speak of spindly plants unduly shaded as "drawn." The ordinary broad, brown bean —Windsor, or Horse, or Spanish—sown in quantity in the vegetable gardens of those parts of the west where the paths of the padres lay— correspond in height quite as much with the light they receive as with richness of soil. If sowed too closely, each plant over-shading its neighbor, they grow in the same length of time to nearly double the height of others solitary. Young pines in too close stands are tall, slender, sparingly branched. The low stature of some of the plants of mountain-tops is due not merely to crushing snow, brief growing time, and chilly nights, but also to the greater brightness of the light which falls on them than on the floor of the valleys below.

Thus the vegetative parts are affected quantitatively by the quantity of light which reaches them. Stem and leaves reach their ordinary dimensions only under ordinary illumination. There are structural differences between the sunned and shaded leaves of wild plants. The beech offers the best known case.

Eastern greenhouse men spend anxious days before Easter lest their lilies bloom too late. They can control the temperature, moisture, soil, in their houses, but beyond certain narrow limits they can not control the light. The same plants bloom at a decidedly lower temperature in California than in the Mississippi Valley and on the