Sunlight upon Aquatic Plants . 379 
the case in Gloeocapsa , Nostoc , G/oeocystis, &c. These plants 
can therefore also resist great extremes of cold. Navicula 
and other Diatoms do not appear when in the vegetative 
condition to be so resistant, and actively vegetating Desmids 
are still less resistant. As regards Selenotila and Rhaphido - 
nema , no data are at present available. 
The observations recorded by Messrs. West ( 1 . c., p. 35) 
that plants of Closterium striolatum and Cylindrocystis Bre- 
bissonii were found growing in the water derived from 
melting snow at a (probable) temperature of i°C. to 2° C. is 
interesting but not surprising, and has moreover but little 
scientific value, since the actual temperature of the water 
was not determined. 
With regard to the effects of direct sunlight upon fresh- 
water Algae, Messrs. West have apparently here fallen into 
the error of supposing that because the surface of a shallow 
pond is exposed to direct sunlight, that therefore the plants 
growing in it are exposed to the same intensity of sunlight 
as well ( 1 . c., pp. 35 and 36). It is perhaps hardly necessary 
to mention the fact that unless the sun is directly overhead, 
a larger or smaller percentage of the incident rays fail to 
penetrate the water, which is an optically denser medium, 
and at the surface of which both refraction and reflection 
take place. As is welt known, when the incident angle 
is an acute one, a very large part of the light, or almost 
the whole of it, may be reflected without penetrating below 
the surface ; and since the shadow of a landscape upon a 
still surface of water may appear on a photographic plate 
to have almost the same intensity as the original picture, 
it follows that the more refrangible rays with which we are 
immediately concerned must be reflected in as great if not 
greater amount than the less refrangible rays are. The 
oblique light-rays which actually penetrate are bent towards 
the perpendicular, and hence traverse a shorter distance to 
reach the bottom of a pond. This may be of considerable 
importance in the sea by enabling oblique light-rays to 
penetrate to a greater depth, and thus allow the marine 
