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 cleistogamic (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 alge, 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 : 
Ruppia maritima (tassled pondweed), which has pol’en 
very similar to Zostera ; Zannichellia pedunculata ; and a 
form of the water-crowfoot, Ranunculus Baudotit. 
Seaweeds can only grow in abundance on a rocky coast. 
They are attached to the substratum by a small flat dise, 
and if a plant became fixed 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 
