Illumination and Vegetation. 275 
The minimum values observed for the light within the crowns 
of different trees near Vienna are very different (being of course a 
direct measure of the density of the foliage). 
Buxus or 0012. Populus alba or 0-085. 
Fagus or 0-015. Larix \ or 0-250. 
Dimly lighted as are the interior leaves of umbrageous trees 
yet plants are found to grow in the shade beneath them. In a pine 
wood the prevailing light on the ground may fall to ^ or -g^. 
Under the trees in fa Vaccinium myrtillus may yet be found, in g l ff 
Pteris aquilina and in Oxalis acetosella : in less light than this 
nothing occurs, and a “ dead ” shade results. In tropics the 
“ dead ” shade comes at about which represents nearly the same 
absolute light-intensity as ^ in Europe. 
For all shade-bearing' Phanerogams Aspidistra elatior seems to 
hold the record, for it struggles on in the darker parts of many a 
dwelling room where the light is about of total outside daylight. 
Detailed data of the Lichtgenuss of a number of plants are to 
be found in Wiesner’s book: an interesting systematic study of a 
district by these photometric methods forms a part of Hesselmann’s 
paper on “ Schwedischer Laubwiesen ” ; see Botan. Centrlbl. 
Beihefte, Bd. XVII., 1904. 
III. 
We now have to face the fundamental question—What is the 
physiological significance of the different ranges of illumination in 
which plants exist ? 
This is not at present definitely answerable. To a preliminary 
physiological analysis of Lichtgenuss Wiesner devotes a chapter of 
his book, and he concludes that the limits of the light-range of a plant 
are determined by the interaction of numerous physiological factors 
of which assimilation of C0 2 takes a place in the first line. This 
is held to be broadly due to effective assimilation ceasing at the 
lower limit of the light-range ; a view inadequately based on the 
observation that at mid-day in summer an Acer or Fagus shows 
abundant starch in its outer leaves and diminishing quantities in 
the inner leaves till the innermost show little or none. 
Secondarily the natural light-range is held to be determined by 
the well known fact that plants grown in light below the natural 
minimum do not attain the normal form, but show more or less of 
etiolation. Now, one of the characteristics of etiolation is a less 
resistance to external conditions—rain, fungi, etc., and it is interesting 
