Chap. Ill] 



LIGHT 



63 



s > 



Fig. 40. .Schistotega osraundacea. Optical section of 

 a cell of the protonema in which the path of the rays 

 of light is traced. S' . . S' a ray, which is totally 

 reflected by the back wall of the cell /. P protoplasm. 

 c chlorophyll, v cell-sap. After F. Noll. 



in the neighbourhood of the optical axis are so refracted as to be con- 

 centrated on to the chlorophyll-corpuscles, which lie just in front of the focus 

 of the lens, near the optical axis, and are consequently intensely illuminated. 

 Each individual chlorophyll-corpuscle, owing to its stronger refractive index, 

 acts again as a small lens and causes the rays, that meet it and are already 

 converging, to converge still 

 more in its interior, so that 

 the intensity of the illumina- 

 tion on its posterior surface 

 is still further increased. The 

 result of the whole process 

 of refraction is a vivid illu- 

 mination of the chlorophyll- 

 apparatus, which is concen- 

 trated in the optical axis 

 near the focus ^ The lumin- 

 ous property is a physical 

 phenomena that necessarily 

 results, but is devoid of any 

 significance to the plant. 



Similar contrivances for 

 illuminating the chlorophyll-containing parts occur, if not in such perfection, 

 in other plants living in shady places. The papillae which cover the velvety 

 surface of many tropical herbs dwelling in shaded habitats serve to con- 

 centrate the rays of light -. But also in 

 nearly smooth leaves, as Fig. 41 shows, 

 similar adaptations may be noticed. 

 This figure represents a transverse 

 section of the leaf of Argostemma 

 montanum, an herb often found grow- 

 ing in the densest shade of the moun- 

 tain forests of Java. 



The bluish metallic lustre of many 

 plants that grow in deep shade remains 

 unexplained. It characterizes to a very 

 intense degree a few tropical species 

 of Selaginella and Trichomanes only, 



but is pretty widely spread in a less striking manner, and may be observed 

 even at home in Sambucus nigra. This remarkable feature is never seen 

 in sunny spots, and must therefore in some manner be connected with the 

 feeble illumination of ver}' shady places. 



Fig. 41. Argostemma montanum. A 

 Javanese plant. Transverse section through 

 a leaf growing in the deepest shade. Mag- 

 nified 200. 



' Noll, op. cit., p. 4S2 



2 Stahl, IV. 



