FORMATIVE INFLUENCES 583 



warmth is attainable. Nor is light uniform, for though the total sea- 

 sonal 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 be- 

 tween 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 atmos- 

 phere, 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 con- 

 tinue 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 in- 

 fluence 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 quan- 

 tity of light which reaches them. Stem and leaves reach their ordi- 

 nary dimensions only under ordinary illumination. There are struc- 

 tural 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, mois- 

 ture, 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 



