86 



.101'KNAL OF THE ROYAL IIOKTICUI/ITItAL SOCIETY. 



PLANT COMMUNITIES. 



By J. W. Cabb, Af.A., F.L.S., Professor of Biology in University 

 College, Nottingham, 



Lecture delivered Afaroli 25, 1902.] 



PLANTS, like animals, tend to associate together into groups or com- 

 munities varying in character according to the diverse conditions pre- 

 sented l>y different areas of the earth's surface, those species which are 

 adapted for existence under the same general conditions of temperature, 

 moisture, soil, illumination, shelter, &C., being apt to live in company. 

 The general character of such a company or society is determined by the 

 nature of its more prominent members, or by that of the locality which 

 it occupies. Thus, there are forest communities, dominated by trees ; 

 meadow societies, presenting a densely matted carpet or turf of grasses 

 and low herbs ; swamp and pond societies, heath and moorland com- 

 munities ; the plants of sea cliffs, shingle beaches, fresh and salt water 

 marshes ; sand and rock societies, and so on. 



Just as closely allied plants frequently affect very different situations 

 (e.g. Ha nunc it/ us aquatiUs, submerged in ponds and streams; Ii. L'ukjhu 

 and Flammiiht, growing in marshes ; H. bidbosus and acris, in meadows ; 

 11. arrcnsis, in cornfields; Thalictrum jlacum, found by stream sides and 

 in wet meadows ; T. coliumm, on dry limestone rocks ; and T. alpinum, 

 on high mountains), so conversely we find widely different plants living 

 together under the same conditions, and consequently a plant society will 

 often comprise representatives from many groups of the vegetable kingdom 

 from the highest to the lowest. 



The organisation or composition of a plant society is determined by 

 the nature and permanence of the physical conditions to which the 

 particular area in which it occurs is subject. The water supply is one of 

 the most important factors in the composition of plant communities, and 

 while at ono extreme we find the plants of pond societies living entirely 

 and permanently immersed in water, at the other are the species inhabit- 

 ing dry sandy heaths and dunes, with a very limited and occasional supply 

 of water. 



Closely bound up with the water supply is the nature of the soil 

 Stiff heavy soils retain much of the rain which falls upon them, so that 

 it can only percolate downwards slowly and with difficulty. Loose sandy 

 soils, being very permeable, rapidly lose their water and become dry and 

 arid. The chemical composition of the soil, moreover, directly influences 

 the character of the plant groups growing upon it, for it determines the 

 nature and amount of the food materials Supplied to them ; clay, lime- 

 Stone, sandstone, peat, &0., all have their characteristic species. The 

 soil-covering of turf, dead Leaves, &C. is also of importance in relation to 

 the regulation of temperature and retention of water. 



Light is essential to the wellbeing of all green plants ; but while some 

 bask in the full blaze of direct sunlight, others shun intense light and 



