278 Notes on Recent Physiological Literature. 
sufficiently large shadow on the spot where the plant was placed, 
while yet the shadow-producing plate was sufficiently remote not to 
interfere with the diffuse light from the sky. This might he done 
by having a very long pole fastened to the earth at one end by a 
pivot so that it could be rotated in any plane diurnally, and also be 
tilted each day so that its rotation might be in the same plane as 
that of the sun’s apparent motion. 
If a shadow-board were fixed at right angles, at the free end of 
the pole, then it would always cast a shadow along the direction 
of the pole, and plants in pots ranged along its axis would be 
perpetually shaded from sunshine, but exposed to practically full 
diffuse light if placed near the centre of rotation and far from the 
shadow-board. 
Plants grown under these conditions could be compared with 
fully insolated plants as regards fresh and dry weight and general 
development. 
Then the question would have to be attacked as to how far 
sunlight can be replaced by dark heat-radiation. There are several 
facts which point to the importance of the heating effect of radiation. 
In arctic regions shade-plants hardly exist, and in all cases where a 
species has a wide distribution from North to South it is found that 
it has a bigger minimum of Rel. L.R. in its Northern habitats. 
Often, indeed, Wiesner remarks that sunlight, added to good 
diffuse light, hastens rather than modifies the development and 
further adds that broadly speaking it is on diffuse light that plants 
depend for their illumination, and that the illuminative value of the 
sun works rather through increasing this than by direct insolation. 
Regarding Lichtgenuss with a quite open and sceptical mind, 
it does not then seem quite certain that the position of the plant 
in the observed range of lights is really directly determined by the 
light itself. In most climates both temperature and transpiration 
march parallel with illumination, and either of these might have an 
important influence in determining the exact range of comfortable 
existence for a given plant. 
Further, light when it is the vera causa need not exhibit a 
uniform relation between its intensity and the magnitude of the 
effect. 
Thus in assimilation of C0 2 a limit is set bytheC0 2 naturally 
available, and when that amount is attained, further increase of 
light should not cause increase of assimilation, All available exact 
