272 OHIO BIOLOGICAL SURVEY 



formed by its strong roots, and gave every evidence that it would con- 

 tinue to grow and become a large tree. 



On account of the character of the root system of the plant, the 

 hemlock forest has small opportunity to reproduce itself when once 

 it is cut off. The removal of the trees changes the ravines from the 

 coolest to the hottest parts of the country and the shallow-rooted seed- 

 lings have small chance of survival. It is fortunate indeed, therefore, 

 from a practical point of view, that the conunercial value of hemlock 

 timber is less than that of the trees with which reforestation may occur. 



The cause of the non-occurrence of the hemlock forest in the north- 

 ern section of our area is apparently due to a slight difference in physi- 

 ography. On account of the greater thickness of the sandstone to the 

 southward, the valleys are younger in a physiographic sense; the can- 

 yon walls are higher and more nearly continuous along the larger 

 streams, and in the smaller ravines the waterfalls are higher and more 

 numerous than further north. In these deeper ravines conditions are 

 more extremely mesophytic, if the term be permitted, than elsewhere. 

 That is to say, conditions here more nearly resemble those in the most 

 typical of all mesoi^hytie formations, the tropical rain-forest, than any- 

 where else in our area. The atmosphere is kept continually near the 

 point of saturation, while the shade in the deepest portions of the forest 

 is so intense as to absolutely prohibit the growth of plants other than 

 the forest trees themselves. 



When the valleys have become somewhat older and developed suf- 

 ficiently to have a mantle of soil on the bottom and u]) the sides, the 

 hemlock sooner or later gives way to the Liriodendron forest. Under 

 natural conditions this may not happen for thousands of years as, for 

 instance, in the Queer Creek canyon, where a typical hendock forest 

 occupies a deep bottom land soil and probably would continue to do 

 so for a long time to come. T>ut even here the bank of the stream is 

 occupied by various deciduous trees which would gradually but cer- 

 tainly beat back the ancient hemlock forest. In the typical Lirioden- 

 dron forest, as seen further north, the hemlocks and birches are limited 

 to the locks forming the upper rims of the ravines, while the whole of 

 the soil-covered valley is dominated by the deciduous forest about to 

 be discussed. 



The Liriodendron Forest. The Liriodendron forest flourishes in 

 conditions but little different fiom the hemlock forest which it is gradu- 

 ally replacing. Its most typical development occurs in the characteristic 



