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Pistia stratiotes, commonly cultivated by the Chinese to feed pigs, 

 the duck-weeds Lemna paucicostata, tenera and polyrhiza and 

 Wolfta 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 Utricularias, U. exoleta and U. stellaris, 

 ( Lent ibul arise) Enhydrias angustipetala and Blyxa malaccensis 

 ( Hydro char idex) Naias minor (Naiadaceae) two or more species of 

 Chara, and a very large number of Alga, minute cellular crypto- 

 gams which 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 Utricularias and Char as, 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 draws 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 



