2jo Notes on Recent Physiological Literature. 
NOTES ON RECENT PHYSIOLOGICAL LITERATURE 
II.— Illumination and Vegetation. 
J. Wiesner. — Der Lichtgenuss der Pflanzen, 322 and viii. pp., with 
twenty-five figures. Leipzig, 1907. 
OR green vegetation illumination of some sort is clearly 
essential. When we come to make a scientific study of the 
relation between the intensity of this illumination and the vigour 
of vegetation we find that the matter is by no means SO' simple 
as might be anticipated. 
A mere estimation of the intensity of natural illumination in 
any given habitat is by no means easy, while the working out of 
the effects of light upon all the separate vital processes of the 
plant has provided occupation already for many generations of 
investigators, without finality being reached. The algebraical 
summation of these effects to give the numerical relation between 
the illumination and the vigouiLof any one plant, as a whole, is as 
yet quite beyond us. 
Preliminarily, this last relation may be approached empirically 
and to this aspect of our problem no one has devoted more work 
than Professor Wiesner. In the last fifteen years he has published 
a score of papers on the subject of Lichtgenuss, and just recently he 
has embodied the essentials of his work in the very readable book 
which has suggested this article. 
I. 
Let us first consider the illumination of the earth’s surface, 
and methods of measuring it. 
The sun illumines our planet partly by its direct rays which 
penetrate through our atmosphere and partly, indirectly, by the 
scattered diffused light radiating in all directions from the heavens. 
This light is a temporary deduction from the sun's direct 
rays by the atmosphere, not entirely lost to us, but eventually 
straggling down in the form of a mild diffuse radiance. When 
direct insolation is blotted out by clouds or mist, diffuse light, 
provides a considerable light-intensity. Both these components 
