PLANTS FOR THE AQUARIUM 87 



common form is L. minor ; the larger ivy -leaved duckweed 

 (L. sulca), with fronds like miniature ivy leaves, is less common 

 in Britain. 



The duckweed illustrates some interesting general points in 

 regard to water plants. It will be noted first that it is not 

 attached to any substratum. This is associated with the fact 

 that it lives in stagnant water. Water plants living in running 

 water are furnished with roots which attach them to the bottom, 

 but these roots are often not well developed, and their sole 

 function is often attachment, the water plant when completely 

 submerged being usually capable of absorbing its food over its 

 whole surface, so that the roots have not the same significance 

 as in a land plant. Again, Lemna buds freely, and thus repro- 

 duces vegetatively. Water plants have nothing to fear from 

 drought, can usually obtain abundant food, and we find that 

 their vegetative growth is rapid, and they can frequently repro- 

 duce by separating buds or branches. Again, the fronds of 

 Lemna float, and it will be found interesting to compare these 

 little structures with the leaves of a big water-lily, for example, 

 and to note that the mechanism for floating is almost every- 

 where the same in water plants. Note specially the flattened 

 shape, the difficulty with which the floating organs are wetted, 

 and so on. On the other hand, when the leaves are completely 

 submerged, they tend to be either slender and grass-like, or 

 much divided, as in water-crowfoot. 



Another point is of importance in enabling us to recognise 

 the species of a water plant. In some cases, as in the water- 

 starwort (Callitriche) family and the Naiadacese, all the members 

 of the family are aquatic. In these cases the adaptations to 

 aquatic existence are very perfect. In other cases, one or a few 

 members of a terrestrial family have taken to- the water; 

 examples are the water-crowfoot among buttercups, the water- 

 violet among the primroses, the water-lobelia among the cam- 

 panulas, and so on. In this case the adaptations to the aquatic 

 life are much less perfect, and the structure of the leaves, and 

 especially of the flowers, is of great assistance in identification. 

 In naming water plants recourse should be had to a Flora, such 

 as Bentham and Hooker's British Flora. The following account 



