THE FACTORS OF DISTRIBUTION 57 



such as the Pines of northern Europe and America, 

 Heather, meadow-grasses, etc., seldom consist exclusively 

 of one species, but one species is often markedly 

 dominant. Such plants are mostly perennial, repro- 

 ducing themselves largely by such vegetative methods 

 as long, slender rhizomes or stolons, as in the Coltsfoot 

 (Tussilago Farfara L.), Butter-bur (Petasiles hybridus 

 Gart.), and Marram-grass (Ammophila arenaria Link). 

 Many of them are shade-enduring, such as the Woodruff 

 (Asperula odorata L.) and most Ferns, whilst others, 

 such as the Beech, have the power of suppressing other 

 species by their own shade. 



Whilst it is difficult to exaggerate the intensity of that 

 struggle for existence by which the vast majority of the 

 spores, seeds, and seedlings produced are exterminated, 

 it must be remembered that there are cases of association 

 or commensalism that cannot be considered competitive. 

 In the sparse or " open " vegetation of saltings, and 

 many stretches of barren sand or acid bog, the scattered 

 plants of the few species that can live under such con- 

 ditions are far apart, and cannot be said to compete. 

 Or, again, two plants living side by side, such as the 

 carpet of moss in a pine -forest, may even be advan- 

 tageous to one another. The moss protects the soil from 

 desiccation whilst it benefits by the shade of the Pine. 

 Such associations are termed complementary. Another 

 case is where species live at different depths in the soil, 

 growing to the surface at different seasons. A common 

 example of this seasonally complementary association is 

 that of Holcus mollis L., growing near the surface of 

 sandy soil, with the rhizomes of Bracken (Pteris aquilina 

 L.) deeper down, and the bulbs of the Wild Hyacinth 

 (Endymion non-scriptum Garcke) yet deeper. (Fig. 3.) 



NEW GROUND. The beginnings of competition are 

 seen in the successive occupation of newly-formed open 

 ground. Shore-deposits, whether sandy or clayey, mud- 

 banks in river estuaries, abandoned river-beds, moraine- 

 heaps pushed forward by a glacier, the constantly 

 slipping talus or " screes " on a mountain side, the 

 freshly cooled lava-flows or accumulated " tuff " or ash 

 of a volcano, ground laid bare by a forest or prairie fire, 

 heath ground that has been cleared and pared, whether 

 for fuel or in preparation for cultivation, or the sites of 

 demolished buildings, are examples of such new ground. 



