252 MISSOURI BOTANICAL GARDEN 



soil, being responsible for the absence of loess in certain 

 portions of the territory (cliffs), for the presence of colluvial 

 soil (at the base of slopes), for the presence of wide stretches 

 of sand (in the bottomlands of the Mississippi), etc. It 

 further governs the amount of soil water, being directly re- 

 sponsible for the xcrophytic character of the upper portions 

 of more recent ravines as contrasted with the mesophytic 

 conditions prevailing in other portions. In thus influencing 

 vegetation, physiography also modifies light and temperature, 

 thereby inducing still other changes in the plant associations. 

 By ascribing to physiography in its widest sense the 

 diversity shown by the flora in different regions, the issue 

 has merely been postponed, for it remains to be decided 

 which of the factors subordinate to physiography is of 

 greatest influence. The question is one difficult to answer, 

 for it would seem that now one factor, then another, is of 

 greatest importance. Besides, plants possess an immense 

 power of adaptation to their surroundings and are able to 

 support a great variation in the degree of any one factor. 

 This is best illustrated by greenhouse plants, originally grow- 

 ing under the most diverse conditions, which vegetate side 

 by side in uniform surroundings and even flourish when suf- 

 ficient space is given. But this is true for adult plants only. 

 Seedlings are far more exacting, especially in regard to proper 

 conditions of temperature and soil humidity. The chemical 

 condition of the soil seems to be of the slightest importance. 

 This statement applies to the majority of soils, for though 

 their composition may vary greatly, it is but seldom that 

 one or more substances are present which, because of their 

 presence or the amount in which they are present, directly 

 or indirectly either retard germination or so affect the seedling 

 as to severely injure it or prohibit its very existence. The 

 salty lands of sea-coast marshes and the alkali lands are ex- 

 ceptions to this rule. Comparatively few species can exist 

 here. While cultivated plants with a few notable exceptions 

 are affected by the salts of the alkali lands, there arc a num- 

 ber of native species which flourish on them.* Alkali lands, 



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* Hilgard, E. Soils. 430. 



