Illumination and Vegetation. 273 
0-427, near the tree on the sunny side it fell to 0-299 ; (on the shady 
side it was only 0-023). 
Few plants are exposed to the full value of illumination, as 
they would be in a large plain with their leaves horizontal and 
all unshaded by one another, or by adjacent plants. In herbs the 
self-shading of parts is small, hut in trees it is very large and there 
results a hollow centre to the tree where light is insufficient, 
surrounded by a shell of illuminated leaves. Some trees have open 
crowns like Pinus, into which the light can penetrate, others have 
closed crowns. Cypress is an extreme case, where the green shell, 
20 cms. thick, surrounds a hollow space (about 72% of the whole 
volume) in which all leaves are dead. 
In Nature any given species tends to have only a restricted range 
of natural illumination and Wiesner started out to record these 
relations and as far as possible to give a physiological explanation 
of them. He introduced the term Lichtgenuss (literally, light-use 
or light-enjoyment; “ photic ration ” of some translators) to express 
the degree of illumination in which a plant is found in Nature. 
We may now proceed to define precisely the different light- 
relations to which quantitative values have to be given in this 
connection. 
There is first the degree of illumination prevailing on any more 
or less shaded individual plant. This may be expressed, relatively 
to the total light on a freely exposed spot, by a fraction, say 
Such a fraction would mean that if simultaneous exposures of 
sensitive paper were made in the two places, the paper in the shaded 
habitat would take seven times as long to reach the standard tint 
as the paper in the fully exposed spot. Such a comparison can be 
made at any time of day and might be called the individual 
Lichtgenuss of the plant. When a number of different individuals 
of a species in the same locality are measured up in this way the 
fractional illumination will not be precisely the same for all: there 
will be a maximal and a minimal Lichtgenuss. The range of the 
fractions (writing the numerator as 1) expresses the Lichtgenuss of 
the species which I propose to term the relative Light-range 
(Rel. L.R.). Thus Poa annua, Rel. L.R.=i—i means that 
individuals of this species were found in fully exposed spots and also 
in shaded spots where the light did not however fall below i of the 
full light in the open. 
In different latitudes and at different altitudes a given species 
does not exhibit the same Rel. L.R. Larger fractions of the light 
