Plant Distribution in the Northern Hemisphere

Abstract

PLANT GEOGRAPHY is the study of the existing distribution of floras and plant communities, and the underlying causes of this distribution and its bearing on the evolution of plants. The plant geographer is particularly interested in how a particular plant or a particular plant community reached a certain area and how the present distributions of floras have come about. Plant geography is a very complex subject, because so many different factors are responsible for the distribution of plants. The subject is so large that I am confining my remarks to giving very shortly some of the main factors that affect the distribution of plants in the northern hemisphere, and illustrating it by showing how these factors apply in one genus, Paeonia, which is confined to the northern hemisphere. During the ages the geography of the world has continually changed, some? times quickly but more often slowly, hardly perceptibly. Meanwhile climate has also changed. Climate is one of the main factors determining the distribu? tion of plants. It is a complex factor, introducing not only heat and cold, but also humidity and the amount of sunshine. All species of plants have a certain tolerance of climate; for instance, certain plants such as Dryas octopetala, the dwarf birch, and Rubus arcticus will grow in Greenland, while other plants, like the olive, require a Mediterranean climate. The climate of the Mediterranean area is very different from that of central Europe. The hot dry summers and main rainfall in a mild winter encourage the growth of evergreen trees, like the olive, the evergreen oaks, and the myrtles, and also plants, such as most species of cyclamen and colchicum, which are dormant during the hot summers andflower with the autumn rains, so that they grow and ripen their seed during the mild winters. In central Europe on the other hand the trees are largely deciduous, like the apple, flowering late in spring and fruiting in the autumn. Other types of plants growing on high mountains, such as the lovely little Eritrichium nanum and the species $oldanella, require a long resting period under the snow, where they lie prdtected all the winter until, with the hot sun and damp ground conditions of summer, growth begins again. These are just a few examples of plant behaviour under different sorts of climate.