Some very striking adaptations of form of organs to the intensity of the light have been analyzed by Goebel. The common harebell (_Campanula rotundifolia_) has an upright stem twenty to sixty centimetres in height. The upper part of the stem bears sessile lanceolate leaves, decreasing in size from the base to the summit. The first leaves formed by the stem on its emergence from the soil are entirely different in construction, showing a heart-shaped lamina with a distinct petiole. These leaves are formed at the actual surface of the soil, are generally more or less shaded or covered by fallen leaves, and in fact are not known or seen by many collectors or observers of the plant. Goebel found that similar leaves might be formed on any part of the plant if it were shaded from the full glare of the sun's rays. The cordate leaves at the base of the stem were always produced, however, no matter to what intensity of illumination that part of the plant was subjected. It is therefore safe to conclude that the cordate leaves are inherited forms, and that the lanceolate organs are adaptations to light which may be shown by any individual of the species. In general it is to be said that the leaves of sun-loving species have a thick epidermis, entirely free from chlorophyll, with stomata on the lower side only, a firm consistence due to the formation of woody tissues, and are often provided with a coating of hairs. The leaves of shade-loving plants, on the other hand, have a thin-walled epidermis often containing chlorophyll, stomata on both sides, and are not so plentifully provided with hairs as those in exposed situations. The variations in external form described above are due to the intensity of the illumination. At the same time the structure and arrangement of the cells depend on the direction from which the light rays come. Thus, an organ receiving light from one side only will exhibit a structure different from an organ of the same kind receiving direct rays from two or more sides. Light, then, is a cause of dorsiventrality--that is, of the fact that the upper and lower sides of organs are not alike in structure. The leaf affords a splendid example of dorsiventrality as a result of the exposure of one side only to direct light. The upper side of a horizontal leaf, such as the oak, beech, or maple, contains one or two layers of cylindrical cells with their long axes perpendicular to the surface. In vertical leaves, such as the iris, these _palisade_ cells, as they are termed, are not so well defined, and in all leaves grown in darkness this tissue is very much reduced. If a young leaf not yet unfolded from the bud is fastened in such a position that the under side is uppermost, palisade cells will be formed on the side exposed to the direct rays of the sun. The influence of light upon the sporophylls, or reproductive organs of the seed-forming plants, is quite as well defined as upon the vegetative organs.