33 



features, try to isolate themselves from the all-surrounding 

 moisture. Nearly all the species that exhibit a water-asso- 

 ciation habit are bog plants, of the botanical textbooks, and it 

 is believed that they have resorted to the cliffs because of the 

 lessened competition and the lack of the more intense struggle 

 for existence which apparently overbalance the drawbacks at- 

 tendant on the new environment. 



The first noticeable feature of the flora is the remarkable 

 mingling of the forms of the colder and warmer latitudes, al- 

 though it is plainly manifest that there is a great preponder- 

 ance of species of northern regions. It perhaps ought to- be 

 stated that the driftless condition of this whole area is, in all 

 probability, the explanation in large part, for the first presence 

 of these species from diverse latitudes. (See School 

 Science, 1909, paper by the author.) Another marked 

 character is the luxuriance of growth in many 

 places, the rock soil seeming to afford very congenial habitation, 

 and one is forced to conclude that many forms derive a large 

 measure of their sustenance from the damp air surrounding. 

 A third feature, and the one that adds spice to the collector's 

 trips, is the exceeding scarcity of localities for many forms, 

 and the further fact that they are isolated examples of specie.s 

 found abundantly in other parts of North America. 



For convenience in studying the plants of the cliffs, it will 

 be well to divide them into five groups acording to the physical 

 features most predominant in their habitation: — 



1. The plant-association of dripping and well-lighted cliffs, 

 facing northerly. 



2. The plant-association of dry and well-lighted cliffs, 

 facing northerly. 



3. The plant-association of twilight cliffs, densely shaded. 



4. The plant-association of cliffs with southern exposure. 



5. The transition-cliffs association. 



The dripping cliffs almost invariably have an exposure to- 

 ward the north and east, are usually massive, thick-bedded and 

 towering, and very often have a most pronounced overhang, 

 due to erosive agency of the adjacent stream. The water is 



