FACTORS AFFECTING PLANT DISTRIBUTION , 31 



under the edaphic factors. The climate and the soil are distinct 

 enough in themselves ; the difficulty of classification lies in the de- 

 gree of modification which they undergo in reactions with each other 

 and with the organic universe. For the present discussion the writer 

 believes that he has carried the resolution of factors far enough. 

 More rigid separation would destroy the continuity of the story of 

 the facts in relation to one another. 



5. Examples of Succession 



Now that the factors contributing to succession have been thus 

 briefly reviewed, some examples of successions which show how the 

 vegetation gradually builds up soils and alters conditions for plant 

 growth, should be included. For this reason two extreme areas 

 have been chosen in order that the effect of plant growth in 

 each case may be brought out. The two areas are a swamp and a 

 dry rock surface. The effect of the changing conditions induced by 

 the vegetation can be noted in the change in plant population shown 

 in the following diagrams. 



In the swamp series the open water surface is partly covered 

 by floating aquatics which gradually deposit debris on the bottom 

 of the swamp. This material slowly decomposes into the soil, which 

 in time reaches near the surface. Here and at the shallower mar- 

 gins, rushes and sedges enter and occupy the soil until the shrubs 

 typical of a button bush swamp, Cephalanthus, Rosa Carolina, and 

 Alnus rugosa, enter. These are succeeded by a willow stage, a red 

 maple stage, a swamp oak stage, and finally an oak-hickory forest, 

 in which beech and hard maple gradually appear. There has been 

 a tendency toward drier conditions continually during this succes- 

 sion. The swamp gradually becomes filled and drained as the vege- 

 tation accumulates. The last stage of the swamp is a forest, with 

 all but a few undrained patches occupied by a beech-maple forest. 

 Wherever the conditions remained but slightly changed by the vege- 

 tation, the vegetation is itself much as it was formerly. There is, 

 in other words, reaction between the vegetation and the environ- 

 ment. 



In the cliff series we start with bare rocks on which only 

 lichens grow. The action of these plants is to decompose slightly 

 the rock surface and to add their own remains to the finely parted 

 mineral material. Some of this soil sifts into crevices and here the 

 action of the larger plants becomes effective in widening and deep- 



