Many species of plants are so plastic and capable of such ready response to variations in external conditions that they undergo distinct morphological changes in response to variations in the intensity of the light. The common potato is an example of this fact. The edible tubers are simply thickened stems, and the plant has the habit of storing starch in any stems not acted upon by the light. The branches arising from the base of the main stem are generally underneath the surface of the soil, and afford the proper conditions for tuber formation. Sugar is constructed in the leaves, carried down the length of the stem, and deposited in the underground branches as starch. Space is made for the accumulating store by the multiplication of the thin-walled cells of the pith. If any of the upper branches should become shaded, they become at once the focus of converging streams of sugar, and similar enlargement ensues, resulting in the formation of tubers. Such structures are occasionally observed in plants grown thickly together.
Vöchting, by a number of most ingenious experiments, has succeeded in producing tubers on any branch of a potato plant by simply inclosing the branch in a small dark chamber. As the result of one experiment the entire main stem springing from a sprouting tuber was converted into a new tuber nearly as large as the first. The entire plant at the close of the experiment had the form of a dumb-bell, with the old tuber as one ball and the new tuber as the other.
The same writer has described important results obtained from a study of the action of light upon the stems of cactus, consisting of a number of flattened internodes. When the growing tips of such plants were allowed to develop in a dark chamber the new internodes grown were cylindrical in form. Such behavior suggests that these plants were originally furnished with cylindrical stems and foliar leaves. The leaves at some time in the history of the plant were found unsuitable, and gradually atrophied, while the stems were flattened and extended to take up their functions.
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.