I9II-] STEVEXSOX— FORMATION OF COAL BEDS. 601 



places in Rhode Island and Massachusetts. But in every case, the 

 quantity is insignificant as compared with the mass of other vege- 

 table material and this algal contribution must be wholly unimpor- 

 tant. At the same time, one can conceive of conditions which could 

 render it important. 



Shaler expressed the prevailing opinion when he asserted that 

 the presence of moisture determines the distribution of plant life 

 in swamp areas. Advance of sw-amp destroys the forest. He had 

 seen many places on the coast of Maine as well as in northern ]\Iich- 

 igan and Wisconsin, where invasion by Sphagnnui made the surface 

 so wet that even the most water-loving trees of those regions could 

 not maintain themselves. Davis, in his work on Michigan peats, 

 has discussed the causes leading to the succession of vegetation in 

 swampy areas. The shrubs growing at the water level are drought 

 plants, though living where water is abundant ; their leaves are 

 linear or even scale-like ; the cuticle is dense and the leaves are pro- 

 tected by a waxy or at times resinous coating — all contrived to 

 prevent too rapid evaporation. The explanation of the condition is 

 complex, but it depends mostly on the difficulty with which moisture 

 can be extracted from peat. Once thoroughly air-dried, peat is 

 almost impervious to water, so that plants growing on peat or a 

 peaty soil suffer more from drought than those on other soils. Even 

 when wet, it has little water for plants growing on it. A noteworthy 

 fact in this connection is that some plants, growing near water level 

 in southern Michigan, are found growing only on dry soils in north- 

 ern Michigan. They find their drought-resisting ability equally 

 essential in both regions. The distribution of these plants is ex- 

 plained by the fact that they have fleshy fruits, which birds eat 

 during their southward migration and the seeds are scattered over 

 moist areas. \Miile the plants must be able to resist drought, they 

 must be able to endure excess of moisture in some localities. Davis 

 saw Bctula piiiiiilla and some willows living in places where their 

 roots had been covered with one foot of water for several years. 



The conditions of advance described by Davis are familiar in 

 other states. They exist even on high swamp areas, as appears 



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