.240 
Aspects of Ecology. 
We are surprised not to find during the consideration of 
light-intensity more definite recognition of the diffuse light of the 
sky as an entity, measured in intensity for each position of the sun 
when the sky is clear and much greater, though no longer calculable 
when the sky is hazy or filled with white cumuli. The total light 
ifrom a sky full of white cumuli reflecting on to the earth may be 
;several times as intense as that of the direct sun alone , as Weber 
has shown. So with the momentary combination of white clouds 
and the sun shining between, a total light of intensity exceeding 
Dr. Clements’ unity may may be easily arrived at, though he 
regards unity as the maximum possible. 
Plants grow in habitats of various illumination from unity (at 
noon) to ’002 in the deepest shade-habitats. Dr. Clements has 
tried to obtain an idea of the “ effective differerence ” of light- 
intensity required to produce structural modification. He finds it 
>to be very large, and that transferring a plant from a habitat of full 
insolation (unity) to habitat of 0.1 illumination may produce no 
modification. It is interesting to note in this connection that 
photosynthesis is not reduced by diminishing illumination till the 
limiting value due to the paucity of C0 2 is reached. Dr. Clements 
correlates structural change of the leaf (to a shade-form) with 
intensity of photosynthesis and it may well be that if this is so, 
•CO, as a limiting factor is responsible for this apparent insensi¬ 
bility to change at high intensity of illumination : lower in the scale 
the leaf might be more sensitive. The Plant (Chapter III.) in its 
functional and structural aspects now claims our attention for a 
short discussion. 
Even if the factors of the habitat cannot be quantitatively 
correlated with measured functional and structural response, which 
endeavour Dr. Clements holds to be the only justification for 
exactly determining them, it seems to us that considerable value 
attaches to such precise knowledge of habitat in relation to the 
general investigation of the correlated formation as a whole. 
Function and Structure of thf Plant. 
The difficulties are here more subtle than with the habitat 
because the value of structure and the correct measurement of it 
depend upon a precise knowledge of its function, and as to details 
of this sort our knowledge is not well established. The leaf is the 
only organ which exhibits a wide range of obvious grades of 
structure in correlation with varying habitat. The extremes of this 
