April, 1911.] 
The Ancient Vegetation of Ohio. 
3 2 9 
wild rosemary (Andromeda polifolia) occur in moist ravines and 
rich woods, while leather leaf (Chamaedaphne calyculata) , the 
buck bean (Menyanthes trifoliata) and Labrador tea (Ledimi 
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 physiological 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 evolutionary 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 by which a bog or a desert is char- 
acterized, except where the influence of external conditions has 
produced irreversible changes in a hereditary line. The struc- 
tural alterations in roots and shoots of bog plants can not be 
looked upon as of comparatively recent origin. The phenonemon 
of xeromorphv 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. 
