DUCKWEEDS. 43 



the necessity of filling the bottom of the tank with 

 the depth of soil or mud in which they require to grow ; 

 and as these vertically-growing plants might be placed 

 around the sides or at the ends of the tanks, more 

 room would then be left for the evolutions of fishes or 

 other aquatic animals. Hence, rockwork in the centre 

 of fresh-water aquaria is to be shunned, as it both inter- 

 feres with the movements of the objects and prevents 

 us witnessing them. The surface of the water should, 

 if possible, be partly covered with vegetation, for it 

 keeps the water cool and forms a retreat for the smaller 

 inhabitants, and also to some degree prevents undue 

 evaporation. One or two of the many species of 

 duckweed (Lemna) are useful in this respect, for none 



Fig. 6. Fig. 7. 



Ivy-leaved Duckweed (Lemna trisulca). 

 Lesser leaved Duckweed (Lemna minor). 



of them need any soil. They derive what nutriment 

 they require from the water in which they float, suck- 

 ing it by means of the slender rootlets which may be 

 seen let down from the layer of green frond-like 



