738 Water in Relation to Health and Disease. 
The importance to be attached to the presence of flowering 
plants in our ponds and streams is centred in the facts that 
“ the vital phenomena are the same for green water plants 
as for green land plants,” and the process of assimilation is far 
more energetic in the daytime than that of respiration. They 
must constantly be giving off oxygen into the surrounding 
water, and their influence on its purity must be uniformly 
favourable. “Moreover,” says Mr. Bennett, “ none of this class 
of plants are able to withstand the injurious effects of a large 
amount of ffecal or other pathogenic organic impurities in the 
water ; and their presence, if in a healthy, flourishing condition, 
must be regarded, not only as beneficial, but as a sign of 
comparative purity, of the water.” 
Quite the reverse are the indications of aquatic fungi. Ex- 
cluding bacteria, the same author remarks : — “ The number of 
aquatic fungi is small, and of these the only one which prac- 
tically comes into consideration in relation to the purity of water 
supply is the so-called ‘ sewage fungus,’ Beggiatoa alba. Since 
all fungi which are not parasitic are saprophytic, i.e., derive 
their nutriment from organic bodies in a state of decomposition, 
their occurrence is in itself sufficient evidence of the presence, 
in the water in which they grow, of putrid or decomposing 
organic substances ; and since they do not assimilate in the 
proper sense of the term,- — i.e., do not decompose carbon dioxide 
and produce carbohydrates or other similar compounds, — they 
can exercise no purifying influence on the water by the dis- 
engagement of oxygen. The appearance presented by the sewage 
fungus is that of a dirty-looking jelly-like layer, covering the 
bottom and sides of the watercourse in which it occurs. It is 
found plentifully in drains which carry off the effluent water 
of sewage-farms, and is especially abundant where the current 
is slow and the watercourse toi'tuous. Wherever it occurs it is 
“ an unfailing sign of the presence in the water of a large 
quantity of deleterious decomposing organic compounds.” 
Microscopically examined, it is found to consist of delicate soft 
filaments interlacing each other in all directions. 
The algae of our ponds and streams form a large and inter- 
esting group of aquatic plants, several of which would appear to 
exercise a marked influence on the purity or impurity of water, 
and by their presence to indicate in some degree the extent of 
its organic contamination. Algae are best known to the 
ordinary observer in that form termed “ blanket weed.” This, 
however, is only one of numerous examples which abound in 
our rivers and ponds. 
The higher algae consist of closely packed masses of long 
