26 ALFRED DACHNOWSKI 



related to the system of degradation processes which has given 

 character to Ohio topography, and as to the highways of dis- 

 persal from centers of plant distribution, the continuity of dis- 

 persal and of environment, and the divergence and degree of 

 individual variability of plants in structure and function with 

 departure from localized foci of dispersal. Glacial influences 

 obliterated the older system of surface drainage which was north- 

 ward across the state; the new drainage now marked by streams 

 flowing in the opposite direction, is still in an undeveloped stage, 

 and the plant and animal life of today is still in the process of 

 northward dispersal. 



Along a line of elevation which separates the waters of the 

 Ohio River from those of Lake Erie are striking instances of 

 peat-depositing water basins and finger lakes. Many of the 

 larger glacial lakes have long since become filled with peat. 



Since the postglacial migration of plants the following major 

 successions of vegetation have led to the establishment of the 

 existing flora on peat depositing lakes, and in shaping the par- 

 ticular associations: (1) Open water succession; (2) Marginal 

 succession; (3) Shore succession; (4) Bog succession; (5) Meso- 

 phytic succession. There are a number of secondary successions 

 which have been brought about by the destruction (fires, drain- 

 age), and removal (cultivation) of the original vegetation. As here 

 listed this sequence of groups indicates the structural and his- 

 torical relationships. The more genetically related groups and 

 their associations were formerly not isolated from one another 

 by any considerable space; they occupied the region more con- 

 tinuously than found today. 



The first three groups are members of a related series in a 

 larger vegetation type — the hydrophytic formation. It culmin- 

 ates in a stable association so far as the free water is concerned. 

 The fourth group is pronouncedly xeropytic in response to phys- 

 iological drouth conditions. The members resemble in many 

 points of form and structure the vegetation of rainless regions 

 and bare rock surfaces, — representative types of the xerophytic 

 formation. With the complete filling of the water basin the 

 accumulation of vegetable debris above the water level tends to 



