218 [Proc. B.N.F.C , 



Mr. Praeger, in the course of his lecture, said that almost 

 the whole of the land surface of the globe, and, in addition, 

 the shallower waters of the sea, were covered with a varied 

 growth of plant life. The plant was a complicated organism, 

 composed of a number of parts possessing different functions, 

 and each part was adapted to the peculiar conditions under 

 which it existed, and to the peculiar functions which it had to 

 fulfil. It followed that the plant as a whole was a highly- 

 complicated machine, delicately adjusted to a large variety 

 of environmental factors and internal conditions. The most 

 important factors in the environment — cological factors, as 

 they were now commonly called — were water, light, heat, and 

 soil. Viewing the vegetation of the earth as a whole, heat is 

 the most important factor in determining the character of the 

 vegetation; but in any area of moderate size, such as our own 

 Island, the small range of temperature caused the heat factor 

 to occupy but a secondary place, and the question of water 

 supply became the dominating influence. The number of 

 possible combinations of the principal factors being limited, 

 it followed that similar sets of conditions occurred over and 

 over again, resulting in similar vegetation in places separated 

 from each other. These groups of plants living in association 

 on account of similar requirements were termed plant socie- 

 ties, or plant associations. Some species, being more com- 

 pletely adapted to prevailing conditions than others, were 

 able to hold rivals in check, and hence they usually got in any 

 plant association one or two dominant forms, with others 

 holding subsidiary places. For instance, on our mountains 

 the heather was usually completely dominant, but as they 

 descended towards the lower grounds changing conditions 

 allowed powerful rivals to intervene, till they got the whin 

 or bracken or an association of grasses or trees dominant. 

 The special adaptations by which plants rendered themselves 

 fitted for the conditions under which they live were very 

 varied and highly interesting. Plants had to secure a suffi- 

 ciency of water, of heat, of light, of food, and also to guard 

 against an excess of all these. Taking the factor of water, 



