886 PLANT COMMUNITIES AND FLORAS. 



that the associated individuals may support one another in the conduct of their 

 lives, and that they may even be dependent upon one another. 



A knowledge of the communities which exist within the realm of plants is of 

 gi-eat importance in many ways. It throws a strong light, not on]}^ on the mutual 

 relations of the different species which are associated by common or similar needs, 

 but also on the connection of plant-life with local and climatic conditions and with 

 the nature of the soil. It may fairly be said that in the various zones and regions 

 of our earth no kind of phenomenon so thoroughly gives expression to the climate 

 and the constitution of the soil as the presence of particular plant-commvmities 

 which prevail, and, accordingly, the determination and description of such communi- 

 ties constitutes an important part of geography. Hitherto, it is true, only a few 

 investigators have paid attention to this subject, and even they have given it but 

 moderate study, the reason being probably that for the determination and description 

 of plant-societies comprehensive data concerning all the species which flourish in the 

 district under investigation are requisite, and the acquisition of such data has been 

 gi-eatly neglected of late years owing to the paramount attractions of other depart- 

 ments of Botany. The small progress of our knowledge in that direction is due also 

 in part to the circumstance that a uniform method of investigating, describing, and 

 classifying plant-commiudties has not up to the present time been successfiilly 

 instituted. 



The first thing that strikes anyone who takes up this subject is the fact that the 

 different species of plants play very unequal parts in the formation of communities. 

 Certain species predominate in respect of the number of their individuals. They 

 determine the general character of the community, and form the groundwork of the 

 vegetation as a whole, whilst the rest only make their appearance here and there, 

 and look as though they were merely intercalated in the groimdwork. It stands to 

 reason that such dominant species, as they are called, belong chiefly to those which 

 by nature grow together in numbers, and that those in particular are the most 

 conspicuous which are aggregated together on a large scale. 



Having regard to the dependence of plants upon soil and climate, the nature of 

 which has been fully set forth in the first volume of this work, it might be expected 

 that all plants living under identical conditions would have a common aspect or 

 physiognomy. But this idea is only confirmed in the case of dominant species. 

 The subordinate species may differ from the dominant ones, and also amongst 

 themselves. One of the most usual causes of such differences of form is that the 

 subordinate species of a community pass through the processes of budding, flowering, 

 and fruiting at different seasons relatively, and that one species is adapted to the 

 conditions which prevail m the spring, another to those of summer, and a third to 

 those of autumn. It may also happen that certain reciprocal advantages accrue to 

 neighbouring members of a community from the variety in the forms of their stems, 

 foliage, and flowers. If one species affords at the right moment the shade required 

 by another, or serves as a support for it to climb up, or protects it from high winds, 

 such assistance not only does no harm to the community, but, on the contrary, con- 



