42 FRESH-WATER AQUARIA. 



serve a double purpose. They render the rockwork 

 built around and over their external surfaces (except 

 where the edges are broken at the bottom to form a 

 tunnel, and the usual opening in the middle of the 

 inverted base) lighter than it would be if it were 

 heaped up, one solid piece on another. And we 

 have already seen that the colder bottom and 

 warmer surface waters will set up a feeble vertical 

 current action. 



If aquatic plants are intended to form a part of the 

 stock contents of the tank, the best plan is to procure 

 them when young from some dyke or pond, and plant 

 them in flower-pots. These flower-pots may be hidden 

 among the rockwork ; nay, the latter may be loosely 

 fastened around them by means of Portland cement so 

 as to completely conceal them. Such species as the 

 water violet, water plantain, water soldier, and arrow- 

 head grow best when thus treated ; and as their flower- 

 ing spikes ascend above the water whilst their leaves 

 are mostly either submerged or floating, they form very 

 pretty accessories to the larger fresh-water aquaria.* 

 In the arrow-head, water ranunculus, and several 

 others, the floating or surface leaves are of a different 

 size and shape to the submerged leaves. All the plants 

 just mentioned require a good depth of soil to be 

 planted in, and their transference to flower-pots prevents 



* Such aquatic plants ought not to be kept where gas is lighted at 

 night, as they are then unduly forced, and pine away from not obtaining 

 their necessary repose. 



