A. Dachnowski — Problem of Xeromorphy. 37 



gas which green plants can endure are conflicting* and call for 

 further work in the held and in the laboratory. 



The consideration of these facts leads to another point, the 

 inevitable conclusion that the form characters and the funda- 

 mental resistance to drought and desiccation distinctive of 

 xerophytic plants, whether in bogs or deserts, must have made 

 their appearance within early geologic time. They are not of 

 recent development^ The climate of northern America has 

 undergone oscillations between periods of maximum aridity 

 and maximum precipitation and humidity, with extreme varia- 

 tions in temperature during and following the several glacial 

 periods ; the amplitude occupying periods of perhaps many 

 thousands of years. Variations in climate so wide apart indi- 

 cate an almost complete change in the character of the flora 

 during the geologic periods. The xerophytic features which 

 characterize bogs and deserts are not to be taken, therefore, 

 as having come about by a direct and continuously increasing 

 edaphic or climatic aridity. Aside from the question as to the 

 methods and the activating conditions in evolutionary devel- 

 opment, it seems certain that the origin of xerophytic forms 

 is not one of recent development in the vegetable kingdom, but 

 must have been concomitant with the diastrophic and grada- 

 tion processes of the great geologic periods. The great floral 

 evolutions of geologic history were principally one of growth- 

 form, physiognomy, and functional behavior, and not of floral 

 structure alone. Water has always been the most important 

 of all the life relations in the environment of plants. In the 

 early types of gametophytic vegetation it remained necessarily 

 of greatest importance for the movements of gametes in 

 effecting fertilization and for dissemination. The luxurious 

 development of these forms in the ancient areas of low-lying 

 land became checked in the stress of aridity encountered with 

 the accumulation of their debris. With the origin and the 

 development of the sporophytic types of vegetation, which 

 were from the first less dependent upon free water, the pro- 

 longation of vegetation activity enabled the plants to occupy 

 the areas with greater habit reactions. The effects of desicca- 

 tion in the physiologically arid habitats resulted in greater 

 differentiation of organs, in protective and resistance features;}: 

 and in a greater range of dispersal. The vegetation had now 

 developed to forms capable of occupying dry land, and able to 

 maintain themselves as bog or desert vegetation in localities 



*Czapek, F., Die Ernahrungsphysiologie der Pflanzen seit 1896. Pro- 

 gressus Eei Botanicse, i, 468, 1907. 



f McDougal, D. T., Influence of aridity upon the evolutionary develop- 

 ment of plants. The Plant World, xii, 217-230, 1909. 



| Dachnowski, A., Physiologically arid hahitats and drought resistance 

 in plants. Bot. Gaz., xlix, 435-340, 1910. 



