132 SUCCESSION 



marshes and swampS; but the quantity of decomposing 

 vegetation in many is not great enough to produce an 

 efBcient reaction. The characteristic plants of peat bogs 

 and humus swamps are xerophytes, which possess a super- 

 ficial root system : structurally, they are almost identical 

 with dissophytes. The occurrence of xerophytes in situ- 

 ations typically hydrophytic is no longer a puzzling contra- 

 diction of the unique value of water content in the control 

 of vegetation. It is readily explained by the reduction of 

 the available water, and by the lack of proper aeration in 

 the presence of humic acids. Formations of this type 

 usually start as freshwater swamps. The succession is con- 

 sequently xerotropic, but no thorough study of its stages 

 has as yet been made. 



(8) Succession by modifying atmospheric 

 factors. All layered formations, forests, thickets, many 

 meadows and wastes, etc. , show reactions of this nature, and 

 are in fact largely or exclusively determined by them. The 

 reaction is a complex one, though it is clear that light is 

 the most efficient of the modified factors, and that humidity, 

 temperature, and wind, while strongly affected, play subord- 

 inate parts. In normal successions, the effect of shade, 

 i.e., diffuse light, enters with the appearance of bushes or 

 shrubs, and becomes more and more pronounced in the 

 ultimate forest stages. The reaction is exerted chiefly by 

 the facies, but the effect of this is to cause increasing 

 diffuseness in each successively lower layer, in direct ratio 

 with the increased branching and leaf expansion of the 

 plants in the layer just above. In the ultimate stage of 

 many forests, especially where the facies are reduced to 

 one, the reaction of the primary layer is so intense as to 

 preclude all undergrowth. Anomalous successions often 

 owe their origin to the fact that certain trees react in such 

 a way as to cause conditions in which they produce seedlings 

 with increasing difficulty, and thus offer a field favorable to 

 the ecesis of those species capable of enduring the dense 

 shade. Successions of this kind are almost invariably 



