VEGETATION OF RUNNING WATER 



39 



vertical habit better than the net-veined leaf of the dicotyledon ; 

 a large number of the plants growing in or by running water are 

 monocotyledonous. Where the running water is very shallow, 

 and the plants are to some extent submerged in it, the leaves are 

 not vertical ; the plant is on the whole creeping. In most rivulets 

 and brooks the Brook - Lime, the Apium, and the Water - Cress 

 are to be found. The 

 Brook-Lime is one of the 

 Veronicas, and has the 

 characteristic bright blue 

 flowers of that genus ; in 

 this species they are not 

 as large as in the Ger- 

 mander Speedwell, a com- 

 mon weed that flowers 

 early in spring on road- 

 sides and hedgebanks. It 

 is a smooth shining plant 

 with succulent stems and 

 oval glossy leaves. Like 

 the Brook - Lime, the 

 Apium or Marshwort has 

 a creeping stem and erect 

 flowering branches. The 

 whole plant is smooth 

 as water plants generally 

 are. Each leaf consists of 

 from three to ten pairs of 

 leaflets; the flowers are 

 white. This is a very 



. , , , . . . FIG. 14. Marshwort (Apium nodiftorum}. 



variable plant ; in certain 



situations it may be several feet in length, but it is usually 

 very much shorter. The Water-Cress (Nasturtium officinale) is 

 a cruciferous plant, now very much cultivated, especially in the 

 southern counties. Great care is now taken that the water in 

 which it is grown should not be contaminated by sewage. It 

 is important that the beds should be near the source of the 

 water supply, in order that the temperature may vary as little 



