126 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



(2) Climatic, involving different temperature, humidity, light, and 

 other conditions. Owing to the close dependence of plants on 

 climatic conditions, zones of vegetation and climate tend to cor- 

 respond with one another, the climate commonly determining the 

 general limits of a plant's distribution. A change of climate, such 

 as a migrating plant is apt to find in a new land, often constitutes a 

 very real or insuperable barrier — not only as a whole but, very 

 often, as to one or other of the climate's component factors, which 

 may react in a particular way on the plant's physiological make-up or 

 a vital part thereof. Moreover any condition, including lapse of time, 

 which proves lethal to disseminules may constitute a major barrier, 

 lack of viability being an important factor militating against migration. 



(3) Edaphic, due to features of the soil. These are again various, 

 involving physical structure, chemical composition, moisture con- 

 tent, temperature conditions, or even content of living organisms, 

 any one of which alone can prevent a disseminule from establishing 

 a plant in a new area, even if it germinates quite successfully. 

 Either in combination or separately, edaphic conditions tend to 

 limit the distribution of plants (and, of course, vegetation) rather 

 drastically within the main climatic belts — commonly to particular 

 habitats, which may be narrowly prescribed in their type and of 

 very limited extent. Absence of the suitable habitat, or at least of 

 the required conditions, is apt to constitute an insuperable barrier 

 to successful migration. 



(4) Biotic, due to living organisms, including other plants. The 

 competition for space, light, water, etc., of other plants already 

 established in an area and growing in reasonable equilibrium with 

 local conditions, is apt to constitute an insuperable barrier to the 

 successful establishment of newcomers, as is grazing or other dis- 

 turbance by animals (including Man). Consequently widespread 

 immigration is largely limited to more or less ' open ' areas, such as 

 cliffs, sands, and disturbed ground — where other sets of barriers 

 come into play. If they did not, virtually every scrap of soil or ray 

 of light would probably be utilized ; for the struggle for existence 

 is a very real and desperate one, taking place chiefly between organ- 

 isms where the general conditions for life are good, and predominantly 

 with the physical factors of the environment where conditions are bad. 



The magnificent perseverence and virtual ubiquity of plant life- 

 are vividlv exemplified to the author by the myriad Diatoms which 

 may so impressively if variously colour ice-floes on arctic seas. 

 During long flights over the North Polar Basin and elsewhere he 



