Chromatic Adaptation. 237 
illustrated, the level of the soil surface, the bottom-water-level, and 
probably the salt content, are the correlated physical features which 
determine the distribution of the vegetation. Similar conditions 
often obtain in fresh-water marshes where the bottom water level is 
close to the surface. 
CHROMATIC ADAPTATION. 
Facts and Theories Concerning the Adaptations of Plants 
to Differences of Illumination. 
By F. F. Blackman. 
A CERTAIN human interest attaches to the different adapta¬ 
tions by which plants attempt to rise superior to unfavour¬ 
able variations in their environment. 
The green land-plant, as a generalised structural type, is an 
expression of the necessity, under which it always is, of obtaining 
energy by the absorption of radiation from above. Nearly all the 
striking variations of this type which the plant-world displays are 
correlated, however, not with variations in illumination, but with 
special variations in the water supply of their particular habitat. 
The absence of striking adaptations to different intensities 
of natural illumination ceases to surprise, when one realises the 
fact that no plant under existing natural conditions can utilise 
more than a small fraction of the energy of the direct sunshine 
which may fall upon it. There may be enough chlorophyll in a leaf 
to absorb 20% to 25% of the sun’s radiation 1 in addition to the 
amount absorbed by the sap, cell-walls and protoplasm, but the 
actual C0 2 -assimilation performed cannot, under natural conditions, 
represent more than about £th of the available energy. The reason 
for this is that, in nature, assimilation is limited not by light but 
by the available C0 2 -supply, because sufficient C0 2 cannot diffuse 
into a leaf from an atmosphere containing only three parts of C0 2 
in 10,000. The writer has shown that an active leaf exposed both 
to the bright diffuse light and to the direct sunshine of a brilliant 
August day can absorb enough energy in a form available for CO 
.assimilation to produce a photosynthesis of six litres of C0 2 per 
square metre of area per hour. Now, from determinations made 
1 Timiriazeff. The Cosmical Function of the Green Plant. Proc. 
Roy. Soc., Vol. 72, p. 449. 
