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Pistia stratiotes, commonly cultivated by the Chinese to feed pigs, 
the duck-weeds Lenina paucicostata, ten era and polyrhiza and 
Wolfia arrhiza , and Azolla. 
These plants do not in any way appear to be injurious to drinking 
water. 
The submerged plants however are distinctly objectionable. The 
commonest of those are the Utricular ias, U. exoleta and U. stellar is, 
f Lent ibid arise) Enhydrias angustipetala and Blyxa malaccensis 
( Hydro char ideas) Naias minor ( Naiadace.se ) two or more species of 
Char a, and a very large number of Alga, minute cellular crypto- 
gams vvhich grow in great numbers on the stems and leaves of the 
higher plants and in shallow places in the ponds and streams. 
Enhydrias, Naias, the U triad arias and Charas, are readily pro- 
pagated not only by their seeds, but also by small broken- off pieces 
of stem and grow with immense rapidity soon filling up a pond or 
tank. They grow more or less free from the bottom of the 
tank and chiefly in the shallower parts. Blyxa, a plant with a tuft of 
grassy leaves grows on the bottom in shallow water and is propaga- 
ted by seeds or stolons Ponds soon get full of these plants, the 
seed or small pieces of the stems being brought apparently on the 
feet of wading birds, such as snippets or carried by streams or rushes 
of rain, floods, etc. 
Tons of these weeds are removed annually from the Garden’s Lakes 
where they seem to have so thoroughly established themselves that 
eradication is impossible. So rapidly do they grow that it is neces- 
sary to clean them out of the lakes in cartloads two or three times 
a year. 
It is in the shallower parts of the ponds where the depth is not 
more than three or four feet that these plants grow most thickly, 
and this was found to be the case also in the impounding Reservoir, 
where all these plants occur, but fortunately not to so great an ex- 
tent as in the Garden’s Lakes. They seem unable to grow in deep 
water. 
To clear these plants out, use is made of a long stretch of wire 
netting fixed to two vertical sticks which is drawn or pushed through 
the water like a seine net and draw's the weeds to the side. Those 
that grow on the bottom have to be pulled up by hand when they 
float and are dragged in by the net. The coolies who do this work 
complain of irritation to the skin caused by handling these plants 
in this manner and cannot be kept at the work all day on account 
of this. 
On these water weeds grow a large number of minute Alga of all 
fresh water groups and the decomposition of these minute plants 
seems to have a great tendency to foul the water, and to give it an 
unpleasant smell and taste. 
The Utricularias by the aid of their small insect catching bladders 
destroy to a certain extent the rotifers, small Fly larvae and other 
aquatic animals and may be of some use in ridding the water of 
some of these animals but infusoria and animals of other orders 
