INFRARED PHOTOGRAPHY APPLIED TO BOTANY AND PALAEONTOLOGY
One of the most noticeable characteristics of the leaves of plants and grass is their green color. This is due to the presence of the pigment chlorophyll. Chlorophyll plays a very importan part in the life of the plant. It facilitates the absorption of carbon dioxide from the air and enables the plant to form starch from it, by means of a mechanism which is not well understood. There are other pigments present in plants, the chief of them being carotene and xanthophyll. These generally accompany chlorophyll, and they are yellow in color. In some roots, flowers, and fruits these pigments may actually be present in complete absence of chlorophyll. When the green chlorophyll in leaves and fruit is destroyed by light or some other agency, the yellow pigments remain, and there is a rapid change of color from green to orange or yellow. It is not known whether these yellow pigments resemble chlorophyll in playing a direct part in assisting the assimilation of carbon dioxide.
When light falls on a leaf, part of it is reflected, part is absorbed by the materials of the leaf, and the rest is transmitted through it. A leaf has quite a complicated structure, and the problem of what happens to light when it passes through it is not nearly so simple as, say, in the case of a solution of a green dye in water. It has, however, been very much studied by biologists. The tissues cause the light entering it to be scattered. This is mainly due to reflection and refraction, and not to scattering by small particles such as occurs when light passes through a hazy atmosphere. The leaf pigments change the spectral quality of the light, an effect which is increased by the multiple reflections which the light undergoes inside the leaf.
A fair proportion of the radiation falling on a leaf is reflected at the surface. It ranges from about 2 to 15 per cent in the case