F. F. Blackman. 
238 
by Horace Brown' of the gain in weight 2 of actively assimilating, 
leaves, it would seem that a leaf in the open air is limited to- 
acquiring about SOOcc. of CO., per square metre of area per hour. 
It is clear then that the energy available for assimilation in 
sunshine is so much in excess of the supply of raw material 
available for the leaf to work up, that comparatively feeble light is- 
adequate for all assimilatory possibilities. 
We should not expect, then, to find adaptations for making the 
most of the light that reaches a leaf, except when the natural 
illumination falls below a certain low level. Confirmed “ shade- 
plants ” show a certain amount of structural adaptation in this 
direction, but the differences between sun-leaves and shade-leaves 
are also partly correlated with the function of transpiration. The 
most efficient illumination-adaptations seem to take the form of 
lens-like arrangements of cell-walls which focus upon the chroma- 
tophores such light as is available. It is very striking that, in no 
case, does a land-plant heighten its assimilating power by developing 
in its chromatophores a pigment which will absorb some of the rays 
of light which chlorophyll transmits unutilised. 
With plants growing submerged in various depths of water the 
state of things is quite different. Water is a blue liquid, and the 
deeper the water the more the rays of the red end of the spectrum; 
are absorbed. Spectroscopic observation has shown that at fourteen 
metres below the surface of the sea the light that penetrates is 
mainly composed of green and blue, feeble in yellow, and lacks red 
entirely. 3 These red rays are just those most efficient in CO.,- 
assimilation and this loss, combined with loss of light of all kinds 
by reflection from the troubled surface of the sea, and by absorp¬ 
tion by fine suspended particles brings the efficiency of the light so 
low that it falls below the intensity corresponding to the quantity of 
C0 2 available for assimilation. 
An adaptation is then profitable to promote the assimilating 
activity of sea-weeds at these depths, and we find that this takes the 
form of a brilliant red colouration, which provides an efficient absorber 
of the green and blue rays that preponderate in these depths. 
Between the red deep-water algae and the superficial algae which 
Horace Brown. Presidential Address, Chemical Section of 
British Association, 1899. 
Sachs deduced from one of his experiments that 1400 cc. might 
be absorbed, but there are good reasons for regarding this 
number as too high. 
Engelmann. Coulcur et Assimilation. Arch. Neerlandaises. 
Tome xviiii. 1SS3. 
