THE FACTORS OF DISTRIBUTION 5$ 



more back to marsh. If the streams that drain it are- 

 obstructed by sand-dunes, by trees which may be blown 

 down, or by beaver -dams, their running waters, now 

 stagnant, may be invaded by Sphagnum, more trees 

 will be undermined by the displaced water, and the 

 water-level being thus once more raised, forest will 

 become marsh or even lake. 



It is suggested that alternating floras attributed to 

 alternating wet and dry climatic periods following the 

 Glacial Epoch may find their true explanation in such a 

 sequence of events, which is, it will be seen, entirely 

 independent of climate. 



CHAPTER III 



ORGANIC ENVIRONMENT 



NEITHER the inorganic (climatic and edaphic) conditions 

 nor the vegetation that responds to them are stable. 

 Constant slight changes, with occasional violent changes, 

 are the order of nature. Thus the balance between 

 plants and their inorganic surroundings and between 

 one species of plant and another is being constantly 

 disturbed, so as to require readjustment. Even when 

 the surrounding conditions are temporarily stable, plants, 

 whether of the same or of different species, are com- 

 peting for water, food, and light. The growth of a plant 

 in any one spot depends, not only on whether the 

 climate and soil are suitable, but also upon whether the 

 ground is preoccupied, whether the roots of surrounding 

 plants spreading through the soil are draining it of 

 moisture, or the shade of their overhanging leaves is 

 excluding the sunlight. 



The dependence of plants growing together (symbionts) 

 varies greatly in degree. The benefits of the association 

 may be all on one side, as in most parasites, epiphytes, 

 and saprophytes; or there may be mutual advantages, 

 or the species may merely grow side by side competing 

 on more or less equal terms. Completely parasitic 

 plants, such as Dodders and Broom-rapes, obviously in 

 no way benefit their " host " plants. They are able to 

 dispense with the formation of chlorophyll, or even of 



no 



" 



