MARYLAND WEATHER SERVICE 45 



We thus see that the topographic features of an area, taken 

 together with the physical character of its soils, determine the vary- 

 ing amounts of water available to plants for absorption over such 

 small areas as possess uniform conditions of temperature, light, at- 

 mospheric humidity and wind. The discrepancy which exists be- 

 tween the uniformity of the atmospheric conditions to which the 

 aerial parts of all the plants of a given locality of such size as a 

 county are subjected, and the diversity of the soil conditions over the 

 same area, differing with topography, is at the basis of the differences 

 in vegetation which are so manifest between nearby areas of different 

 topographic character. 



The science of physiography lias for its subject matter not only 

 the description of present topography but the study of the changes 

 now going on in the topography. Such study enables the physi- 

 ographer to unravel the past history of topographic features and to 

 predict their fate. To the student of plant-life the most important 

 phase of physiography is the relation of present topography to the 

 physical conditions of the soil. However, it is seen to be true that 

 the relation of vegetation to topography persists throughout changes 

 which take place in the history of topographic features. It there- 

 fore follows that the vegetation of different topographic situations 

 undergoes slow changes parallel to those in the topography and de- 

 pendent upon them. In its relation to Ecological Plant Geography 

 physiography is a study of the progressive changes in plant habitats, 

 and is therefore of fundamental importance in interpreting the 

 present changes in plant associations due to changing conditions, and 

 in tracing the immediate past and future history of plant habitats.* 

 The constant changes of topography which physiography recognises 

 are distinguishable first as those which arc operative in reducing 

 the land masses of the earth toward sea-level, and second those 

 which are tending to straighten the shore-line of the sea and its 

 arms. Rainfall and stream action are the principal agents in the 

 tarrying on of the first of these processes, waves and along-shore 

 currents the second ; the first tend in their action toward a vertical 



♦Cowles, H. C, The Physiographic Ecology of Chicago and Vicinity; a 

 study of the Origin. Development and Classification of Plant Societies. 

 Botanical Gazette, Vol. XXXI.. 1901, pp. 73-108 and 145-182. 



