Pethybridge & PRAV.GB-R—'Vefjetatwn South of Dublin. 125 



as possible of the various species of plants which inhabit the area 

 studied, in classifying the plants thus listed according to their 

 taxonomic relationships, and of indicating in greater or less detail tlie 

 particular localities in which each species is to be found growing. A 

 comprehensive example of such a florislic study of the distribution of 

 plants is to be found in Mr. Colgan's recently-published "Flora of 

 the County Dublin," which covers, from this point of view, the 

 greater ])ortion of the country dealt with in the present paper, in 

 addition to a large portion with which this paper does not deal. 

 Floristic distribution of plants does not lend itself well to delineation 

 by means of cartography, seeing that practically each species requires 

 a separate map to indicate its distribution in the area. 



The second method, on the contrary, is essentially a synthetical one. 

 1^0 very profound botanical study of a district is necessary before it 

 becomes evident that the species of plants growing in it are naturally 

 aggregated into a number of vegetation-types or synthetic plant- 

 groups, which recur within the area wherever similar conditions of 

 environment exist. The species of plants found associated together 

 in these groups have not necessarily any floristic relationships with 

 one another ; they are bound together, however, in common comrade- 

 ships or societies by similar requirements as to the necessities for 

 existence, or even by dissimilar but mutually complementary require- 

 ments in this respect. Such more or less well-defined groups 

 of plants are termed Plant Associations, and it is the structural and 

 physiological features of the species composing them rather than their 

 systematic relationships which not only largely determine the com- 

 position of these associations, but also influence their geographical 

 distribution. Different conditions of climate, soil, water-supply, 

 and the various other environmental factors are evidenced by the 

 •existence of different associations, so that the distribution of the 

 vegetation from this — the "ecological" — point of view is closely 

 bound up with the (/eographj of the area in its widest sense, and can 

 be represented m some detail, and with considerable accuracy, on a map 

 of the area under consideration. 



In any well-marked association some gregarious or social species 

 of plant is the predominating feature, and gives the character to the 

 association. Other gregarious species may also be present, but their 

 •spread is held in check by that of the dominant plant. Should the 

 ■conditions change, one of these secondaiy social species may become 

 the dominating one. l^ot less characteristic of each association, 

 although in a merely superficial glance at the association less obviously 



J/ 2 



