140 On Living Organisms [March, 
at first. The result did not admit of the slightest doubt : 
the water-cress planted in sewage not merely lived and grew, 
but far surpassed the other three lots in luxuriance and 
vigour, and continued so to do till the experiment was 
stopped by the frosts in the beginning of winter. Hence it 
may surely be concluded that the presence of water-cress in 
any stream or pond affords no proof of the purity of the 
water. It must be understood, however, that the sewage in 
question, though rich in foecal matters, blood from slaughter- 
houses, &c., was perfectly free from manufacturing refuse. 
I have never heard of any instance of water-cress being 
found growing in any stream which receives the waste waters 
of chemical or dye-works, woollen-faCtories, &c. But as 
the sewage in question contained very much less free oxygen 
in solution than the potable water or the river water, it is 
plain that the growth of higher vegetation does not follow 
step by step the rising or falling proportion of oxygen. I 
may ask, indeed, why should it ? It is almost needless to 
remind the reader that all plants which develope chlorophyll 
in their tissues liberate oxygen on exposure to sunlight. 
Hence aquatic vegetation is a cause rather than an effeCt of 
ihe presence of free oxygen in water, and its absence in 
highly polluted streams is due not to the deficiency of that 
particular element, but to some positive injurious agency, 
such as acids, alkalies, metallic salts, &c. In the absence 
of such plant-destroyers green vegetation is very efficient in 
freeing water from foecal impurities. There are, of course, 
here certain limits ; solutions of putrescent organic matter 
may be too concentrated to admit of vegetable life, just as 
liquid manures may be applied in too strong a dose. But 
this is a point rarely reached in any stream, and when 
chlorophyllaceous vegetation is absent we may generally seek 
the cause in the waste waters of some manufacturing estab- 
lishment. Town-sewage when treated by any process which 
leaves in it a large proportion of any compound of lime, is 
also unfriendly to aquatic vegetation. The sanitary authori- 
ties of a large manufacturing town were advised by the late 
Mr. Smee to plant Elodea canadensis in the outer tank of 
their sewage-works, and to attempt the growth of reeds and 
sedges in the shallow waters along its margin. The advice 
was excellent, inasmuch as such vegetation would have com- 
pleted the purification of the sewage. But as the process 
adopted involved the daily use of about fifteen tons of lime, 
it was not to be wondered at if the plants failed to survive. 
As far as I have observed, natural streams of very hard 
water are poor in aquatic vegetation. 
