242 
Aspects of Ecology. 
Photoharmose, or the response of the plant, functionally and 
structurally, to “ light-stimuli ” is treated in the same way as 
Hyd roharmose. 
The light-factor has not the dual relation that holds for water, 
so one might expect that a closer treatment would be possible, but 
this is not so, for reasons that will emerge later. Sun leaves, in 
unit intensity, and shade' leaves in intensity *002, show much 
difference in structure, but an undetermined part of this is corre¬ 
lated with transpiration. 
As regards the “adjustment” of photosynthetic response to 
different light-intensity, Dr. Clements states that the “efficient 
difference ” is very large with intense natural light: this we interpret 
as the operation of the C0 2 -limiting factor. At very low intensities 
it is suggested, however, that the efficient difference again becomes 
large, a quite isolated conception. 
Dr. Clements has some puzzling ideas as to the significance of 
leaf structure in photosynthesis. He says that “ the fundamental 
response of the chloroplast to light-stimuli is the production of 
chlorophyll.” All workers on the subject have held that light 
causes destruction of chlorophyll, which is replaced by metabolic 
change. Also, the absence of chloroplasts from the epidermis of 
sun leaves is attributed to the absence of C0 2 in that tissue; but 
this could hardly apply to the lower epidermis near the stomata. 
Of the light energy absorbed by the chloroplast 2 - 5% is said to be 
used in phytosynthesis : there can be no constant ratio like this, but 
all must depend upon the intensity of the light. The photo¬ 
synthetic activity of a leaf, it is proposed to measure by the number 
of plastids in a column of constant width taken right through 
the leaf. 
As an adaptation to light-stimuli, we find in sun leaves, thicker 
and smaller lamina, thicker cuticle, less air space and more 
developed palisade ; but it is not proposed to measure these 
precisely. Nordhausen has recently shown that this leaf-structure 
is partly due to past history and so to be found already in the 
folded bud, and partly due to the effect of the conditions in which 
the leaf develops. 
In conclusion of this section we may point out that the whole 
intensity of the light falling on a leaf is not available for photo¬ 
synthesis, any more than the whole water-content of the soil is 
available for the roots. The maximum utilisable amount of light is 
