LIGHT AND VEGETATION. 



197 



suiting in the formation of tubers. Such structures are occasionally 

 observed in plants grown thickly together, 



Vochting, by a number of most ingenious experiments, has suc- 

 ceeded 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 rotundi folia) has an upright stem twenty to sixty centi- 

 metres 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 dis- 

 tinct 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 pro- 

 duced, 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 situa- 

 tions. 



