Acclimatization in Arizona. 17 



it would assist in explaining the distribution of the native species 

 over the country, both as concerns vegetation forms and related 

 plant groups. 



Not until one makes a study of the fundamental causes 

 underlying plant distribution does he come to a realization of the 

 effects of the various climatic and other factors on the compo- 

 sition of the flora of a region; and this is especially true when 

 plants are included that have come into possession of the land 

 only recently, and hence owe their success to inherent qualities 

 by virtue of which they thrive under the conditions obtaining, 

 and, in the case of weeds, encroach upon the growth of other less 

 energetic plants. It is partly on this account that the introduced 

 plants of a country are invariably an interesting group, those 

 that are poorly suited to the new environments disappearing as 

 quickly as they appear. 



It need hardly be said to be appreciated that of the large 

 number of new varieties 'of plants brought into this country 

 every year for planting only a few become established and con- 

 tinue to flourish. Some fail because of poorly aerated or drained 

 soils, lack of humus or nitrogen in the land, or on account of 

 an excess of alkali present. Others suff"er from extremes of 

 summer or winter temperatures or from too strong light. And, 

 finally, even with an abundant and continuous supply of soil 

 moisture, extreme dryness of the air in connection with heat 

 may be so pronounced as to scald or wither the foliage, or 

 to force the plant to give off more water from its leaves through 

 transpiration, than can possibly be taken up by its roots in the 

 same time, with resultant severe wilting during the heat of the 

 day. Both these conditions check normal growth, and if long 

 continued weaken or kill the plant. 



With maximum temperatures ranging from 105 to 112 de- 

 grees F. during many summer days, usually with extreme aridity, 

 many trees and shrubs are subject to severe leaf-scald, and though 

 they continue an existence from year to year they make at best 

 a slow, unsatisfactory growth. Among these may be mentioned 

 tulip tree {Liriodendron tnlipijera), common catalpa {Caialpa 

 biynonioides) , sycamore (Platanus occidentalis) , American linden 

 or basswood {Tilia americana), Kentucky coffee tree {Gymno- 

 cladus canadensis) ,l^orway maple {Acer platanoides) , sycamore 

 maple (Acer Pseudo- Platanus), vixnged e\ni{Ulmiis a/a/a) American 



