2j6 Notes on Recent Physiological Literature. 
to note that in Nature one does not find, generally, in sub-minimal 
light, etiolated specimens but simply no specimens at all. This 
is interpreted as the result of the struggle for existence with 
more acclimatized competitors. Wiesner holds, from his experiments 
on growing Hepatica in different illuminations, that reduced leaf- 
structures occur in excessive as well as in subminimal light. He 
therefore regards the natural light-range as corresponding to an 
“ optimal light-need.” 
It must be pointed out in criticism that the terms optimum 
and maximum are used by Wiesner in a quite uncritical w r ay. 
There is no evidence for most plants that the full open light of 
nature is a maximum for them in the physiological sense of a 
cardinal point. It may be the maximum light occurring naturally, 
just as 0 03% is the maximum amount of C0 2 occurring naturally, 
but neither is a maximum in Sachs’ sense. Also, for plants which 
seem to flourish best in full open light, the optimum illumination is 
said to be identical with the cardinal maximum, a very unsatifactory 
conception. 
Critical experiments have never yet been made to try whether 
a plant which normally grows fully exposed to sun plus diffuse light 
utilises in any way all the light that falls upon it. 
It is clear that it cannot use the full intensity for assimilation^ 
as the supply of C0 2 is not adequate, but possibly the full intensity 
might not be wasted in forwarding some important but minute 
metabolic changes. Then again the form of the plant is to a certain 
extent a response to the stimulus of light, and the full sum of light 
might be significant in determining small points of form that have 
biological importance. 
Experiments have been made to compare cultures grown in full 
and diminished illumination, but no one has yet proposed to cutout 
direct sunshine, while retaining all the diffuse light of the sky. 
It a plant is grown on the north side of a wall so high that no 
sun reaches it in summer, then this wall will necessarily stop out, 
and that continually, the diffuse light from all parts of the sky that 
the sun traverses. This involves a reduction of the diffuse light by 
at least a third. 
Before explaining how direct insolation could be extinguished 
without decreasing seriously the diffuse light we may consider such 
experiments in this direction as are at present recorded. 
Wiesner experimented on small Beech trees in pots placed in 
