A. Dachnowshi — Problem of Xeromorphy. 39 



{Menyanthes trifoliata) and Labrador tea {Ledum groenlandi- 

 cum) are found along slow streams. The majority of these 

 plants occur in Europe and Asia, in habitats of similar condi- 

 tions. 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 physiolog- 

 ical aridity or poverty of available water ; morphological limi- 

 tations in the absorption or 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 xero- 

 phytes, even if the plants inhabit regions of pronounced rainfall 

 and milder temperatures. The appearance of such differ- 

 entiation cannot be takeu as one of rapid and notable evolu- 

 tionary development 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 sur- 

 vival are very great for forms thrown into the complex con- 

 ditions of a locality where the functional 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 system of factors. The 

 qualities of growth which enable competition in the crowding 

 out of other forms are not of primary importance in the 

 struggle and selection where physiological capacities and 

 structural modifications have the survival value for activity 

 during drier seasons. Invaders would not exclude the forms 

 by which a bog or desert is characterized, except where the 

 influence of external conditions has produced irreversible 

 changes in a hereditary line. The structural alterations in 

 roots and shoots of bog plants cannot be looked, upon as of 

 comparatively recent origin. The phenomenon of xeromorphy 

 has exhibited itself too generally in a variety of plants and of 

 conditions in space and time. The transition to xeromorphy 

 has been made not once but several times along different lines 

 of descent. It arose in the highest plants independently from 

 that of the ferns and cycads, and the xeromorphy exhibited 

 here and there among families in a genus probably arose inde- 

 pendently along a minor line of development ; as such it is the 

 general response in plants to minimize or balance disturbed 

 physiological water relations. 



Ohio State University, 

 Columbus, Ohio. 



