276 BRITISH PLANTS 



even if the flowers could adjust themselves to different 

 levels, the waves and surf would keep them perpetually 

 wet. The pollen-grains of a marine aquatic, therefore, 

 must either "be adapted for dispersal under water or the 

 flowers must be oleistogamib (p. 165). At the same time 

 the plant has to adjust its absorbing structures to suit 

 the salt water. The only genus of British flowering plants 

 which has succeeded in adapting itself to this mode of 

 life is Zostera (grass-wrack), of which two species have 

 been found in this country — Z. marina and Z. nana. 

 They occur all round the coast in muddy estuaries of 

 rivers, often growing where they are left uncovered by 

 the receding tide. The pollen-grains are thread-like, and 

 have the same specific gravity as sea-water, so they can 

 float at any depth in the water, and be carried to the large 

 stigmas. The plant is rooted in the mud, and its long, 

 narrow, strap-shaped leaves offer little resistance to 

 currents. 



The reproductive cells of the algse, on the other hand, 

 are always adapted for life under water. They possess, 

 as a rule, small hair-like structures, which by their move- 

 ment propel the cells through the water. 



In the brackish water which collects in ditches on salt- 

 marshes the following aquatic flowering plants are found : 

 Buppia maritima (tassled pondweed), which has pollen 

 very similar to Zostera ; Zannichellia pedunculata ; and a 

 form of the water-crowfoot, Ranunculus Baudotii. 



Seaweeds can only grow in abundance on a rocky coast. 

 They are attached to the substratum by a small flat disc, 

 and if a plant became flxed to loose sand or mud the first 

 wave which came along would wash away plant and sand 

 together. Almost the only exception to this is Chorda 

 filum, which possesses a very long, thin, cord-like frond, 

 the lower part of which may become embedded sufficiently 

 deep in sand or mud to prevent the plant from being 

 torn away. 



We haveseen in Chapter V., p. 53, that the depth at which 

 seaweeds grow is dependent on the presence of colouring 

 matter in their fronds — the red seaweeds growing in deep 

 water, the brown in shallower water, and the green ones 

 quite near the surface. The brown seaweed is usually 

 the dominant form, and these exhibit a zoning amongst 

 themselves. The depth at which they grow depends on a 



