380 Ewart. — The Action of Cold and of 
flora to extend, into deeper water than it otherwise could ; 
but in a shallow exposed pond it may occasionally cause 
a dangerously intense perpendicular illumination to be still 
further prolonged. Moreover, ordinary pond-water teeming 
with living and dead particles is only semi-transparent, while, 
as has in various places been shown l } an apparently trifling 
protection — such as a sheet of oiled tissue-paper, or the 
inclination of the leaf obliquely or parallel to the incident 
rays, afford — may suffice to adequately shield the sensitive 
tissues or organs from any injurious effects which prolonged 
and continuous exposure to direct sunlight might have 
produced. 
When a mass of green Algae has been floating on the 
surface of a pond exposed to direct sunlight, frequently the 
uppermost filaments may be entirely or largely bleached, 
and either still living or dead 2 . Whether this takes place 
or not, the filaments beneath will always be subjected to 
a very much diminished intensity of illumination. There 
can hardly be any doubt but that in an ordinary pond, 
even when comparatively shallow, the brighter the sunlight, 
and the more there is of it, the more active will be the 
growth of the vegetation in the pond as a whole. This is, 
however, simply because then the average amount of light 
which the vegetation as a whole receives, approaches nearer 
to, but does not surpass, the optimal intensity for continued 
assimilation. The very air or oxygen bubbles, with which an 
assimilating filament may become covered, cut off from it 
a certain amount of the light which would otherwise reach 
it. For all these reasons, when the illumination is com- 
paratively weak, green organisms can develop on the surface 
only. 
When, however, exposure to bright light induces a formation 
1 Assim. Inbib., 1 . c., 1895 and 1896; Ann. of Bot. Vol. xi, Sept., 1897. 
2 Partially bleached filaments of Spirogyra may remain living for a considerable 
time, and may show a feeble evolution of oxygen from the entire chlorophyll band 
or from parts of it only. If the bleaching is at all marked , a regeneration of chloro- 
phyll is possible only in the young cells. 
