4 THE BOOK OF NATURE STUDY 



the level of the mud rose and only high tides could reach it, other 

 plants were able to gain a footing ; the marsh in time got solid 

 enough for animals to be turned in on it, and then the stage is 

 reached in which man's influence becomes the dominant factor. 

 In ecological botany the term Plant Formation is now used 

 to denote the total number of Plant associations which succeed 

 each other in a rapid succession in the way just described ; as 

 long as the plants of one association are giving place to another, 

 the association is said to be open ; when the final stage is 

 reached and the vegetation is fixed or stable, the association is 

 closed, and there may be intermediate stages. Thus the Sali- 

 cornia and Glyceria associations are open, the Juncus associa- 

 tion is intermediate, whilst that of the salt marsh pasture, showing 

 as it does the influence of man, has been called a " substituted " 

 association. 



The influence of a tidal river on vegetation is by no means 

 limited to the formation of salt marshes with their characteristic 

 plants at its estuary. The Severn, for instance, affects the 

 vegetation of Gloucestershire not only at, or near, its mouth, but 

 to a considerable distance inland, with the result that many 

 plants usually found on seashores occur almost in the middle of 

 what is practically an inland county. The following may be 

 mentioned : 



The Sea Milkwort at Beachley Point, 6 miles from Portskewet 

 at the mouth. 



The Sea Scurvy grass at Lydney, 14 miles from Portskewet. 



The Sea Spurrey at Slimbridge, 16 miles from Portskewet. 



The Sea Plantain at Longny, 20 miles from Portskewet. 



The Sea Starwort at Newnham on Severn, 20 miles from 

 Portskewet. 



The Severn is a particularly good instance of a tidal river, 

 and has a tidal bore, so that the effect on vegetation would be 

 more marked than in many others. 



THE VEGETATION OF SANDY SEASHORES. It will happen 

 that some classes may be able to make observations without 

 much difficulty on the vegetation of sandy seashores. It has 



