tion, or the power of assuming this position by motor or impassive wilting movements.
Among the plants of the temperate zone the so-called compass plants are examples of similar adaptations. The compass plants include, among others, the wild lettuce (Lactuca scariola) and rosin weed (Silphium laciniatum). These plants place the leaves in a vertical position with the tips pointing north and south in such manner that the direct rays of the morning and evening sun only may strike the surfaces at right angles, while the edges are presented to the fierce rays at noonday. That this arrangement is an adaptation against the intense light is evident when it is seen that specimens growing in shaded locations or in diffuse light place the leaves in the typical horizontal position. To meet the functional conditions, both sides of the compass leaves are almost equally provided with palisade cells for food formation and stomata for transpiration. The estimation of the light striking a compass leaf shows that it receives approximately the same amount of light as a horizontal leaf during the course of a day, but the two maxima of intensity, morning and evening, are much below that of the noon of horizontal leaves.
The influence of light upon plants may be briefly summed as follows:
Light is necessary for the formation of food substances by green plants, and it is an important factor in distribution in land and marine forms.
Growth and reproduction are generally retarded by the action of the blue-violet rays.
Light is fatal to certain bacteria and other low forms of vegetable life.
Many plants have the power of accelerated growth of stems in diminished light as an adaptation for lifting the leaves above "shading" objects.
The growth of many leaves and of the perianth of flowers is hindered in diminished light.
The outward form of many organs, particularly leaves, is dependent upon the intensity of the light received.
The internal structure of bilateral or dorsiventral organs is largely determined by the direction of the rays.
Plants have the power of movement to adjust their surfaces to a proper angle with impinging light rays, as a protective adaptation.
