April, 1911.] The Ancient Vegetation of Ohio. 329 



wild rosemary (Andromeda polifolia) occur in moist ravines and 

 rich woods, while leather leaf (Chamaedaphne calyculata), the 

 buck bean (Menyanthes trifoliata) and Labrador tea (Ledum 

 groenlandicum) are found along slow streams. The majority of 

 these plants occur in Europe and Asia, in habitats of similar 

 conditions. They are bog plants only in the southern part of 

 their range. This departure is in no sense an adaptation to 

 climatic influences but is an equilibrium relation or balance 

 between the absorbing organs, the conducting shoots and the 

 transpiration surface against drought conditions common to 

 either habitat. The structures and distribution habits are 

 induced by physiological aridity or poverty of available water; 

 morphological limitations in the conduction of water do not play 

 a role. The j^hysiological water relation alone must be taken into 

 account for the form and habits of bog and swamp xerophytes, 

 even if the plants inhabit regions of pronounced rainfall and 

 milder temperatures. The appearance of such differentiation can 

 not be taken as one of rapid and notable evolutionar\' develop- 

 ment or as one of the most important in the history of plants; nor 

 would it be safe to assume that bog and desert floras owe their 

 origin to gradual adaptations resulting from the action of climatic 

 changes. The possibilities of survival are very great for forms 

 thrown into the complex conditions of a locality where the func- 

 tional and structural capacities are suitable for the limiting 

 physico-chemical factors encountered in the habitat. The plants 

 are functionally fitted to occupy the place in a zone with its sys- 

 tem of factors. The qualities of growth which enable competition 

 and the crowding out of other forms are not of primary importance 

 in the struggle and selection where physiological capacities have 

 the survival value for activity during drier seasons. Invaders 

 would not exclude the forms b\' which a bog or a desert is char- 

 acterized, except where the influence of external conditions has 

 produced irre\'crsible changes in a hereditary line. The struc- 

 tural alterations in roots and shoots of bog plants can not be 

 looked u}3on as of coinparativeh' recent origin. The phenonemon 

 of xeromorphy has exhibited itself too generally in a variety of 

 plants of conditions in space and time; as such it is the general 

 response in plants to minimize or balance disturbed physiological 

 water relations. 



Ohio State University, Columbus. 



