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1 . Plant societies on uplands. 



2. Plant societies in lowlands. 



3. Plant societies along rivers and streams. 



4. Plant societies along the coast. 



Broadly speaking, the term lowland is here used to designate 

 depressions of all sorts — areas occupied by lakes, swamps, etc. 

 All other types of topography are included under uplands. In 

 the present paper attention is confined to upland successions. 

 By way of introduction a representative upland succession, such 

 as may be studied along the trap ridges in the vicinity of New 

 Haven, will be described. Then, with this as a background, 

 differences between successions on various substrata and in 

 different parts of the state will be considered. 



Probably nowhere in this region are the environmental con- 

 ditions to which vegetation is subjected more severe than on the 

 bare surface of an exposed rock. Insolation during the daytime 

 is intense, temperature changes are extreme, and water is absent 

 for long periods. Add to these the difficulty, where the surface 

 slopes and crevices are absent, of securing a foothold, and it is 

 evident that even among xerophytes comparatively few plants 

 are qualified to exist in such localities. Usually the first living 

 organisms to appear on a freshly exposed trap surface are crustose 

 lichens, e. g., Buellia petraea and Lecanora cinerea. These form 

 a black or grayish incrustation over the surface and adhere so 

 tightly to the rock as to be practically inseparable from it. 

 Immediately following these, but apparently dependent upon 

 them for a foothold, frequently comes Physcia trihacea, a foli- 

 aceous lichen whose rosette-shaped thallus for the most part 

 is closely adnate to the substratum. These three plants repre- 

 sent the pioneers of vegetation. Many crustose lichens are 

 said to secrete acids by means of which they effect to a certain 

 extent the disintegration of the rock on whose surface they 

 occur. In this way, as well as by their very presence, they tend 

 to create a substratum upon which it becomes possible for foliose 

 and fruticose lichens and certain mosses to secure a foothold. 

 And not only do the crustose lichens prepare the way for other 

 plants, but by so doing they pave the way for their own de- 



