THE ECOLOGY OP PLANTS. 447 



under consideration and absent from neighbouring areas, 

 and vice versa. To determine these, compare the physical 

 features of the different areas, and try to map out the 

 whole district into regions characterised by definite plant 

 associations, which will, of course, be found to merge into 

 one another at their edges. 



It need hardly be added that the subject of Plant Distri- 

 bution cannot be mastered by merely committing to memory 

 lists of plant-names. It must be approached in the spirit 

 of observation and inquiry, and the problems it presents 

 answered with reference to the student's knowledge of the 

 Structure and Physiology of Plants. 



420. Vegetation Survey Work, which has been carried 

 out in various parts of Britain, 1 should be attempted by the 

 student on a small scale at any rate. 



Get the Geological Survey Map of your immediate neigh- 

 bourhood, to learn the characters of the underlying rock, and 

 the sheets of the " six-inch " Ordnance Survey for the district, 

 giving the heights, etc. 



Visit the area at intervals throughout the year and make 

 lists of the plants found. The various Plant-Formations 

 and Plant- A ssociations can be shown on the " six-inch " map 

 by means of letters, numbers, signs, or lines, or by colouring 

 with pencils or water-colours. 



The smaller plant-communities are called associations, 

 while formations are larger units each consisting of a group 

 of associations. 



While these terms cannot always be sharply defined, Dr. 

 W. G. Smith 2 gives the following illustration: 



" A natural wood consisting entirely of oak-trees has one 

 dominant form the oak whose presence is determined by 

 the prevalent climatic, soil, and other conditions, while its 

 size and gregarious nature give it dominance ; in this wood 

 there may be other sub-dominant trees or shrubs (e.g. birch 

 or hazel), more or less isolated, but which, given the oppor- 

 tunity by removal of the oak, will become dominant forms ; 



1 See papers by Smith, Moss, Rankin, Lewis, and others, in various 

 periodicals (Geographical Journal, New Phytologist, Naturalist, etc.). 



2 New Phytologist, 1905. 



