PKINCIPLES OF PLANT DISTRIBUTION. 27 



winds in Alabama come from the south, southeast, and southwest. 

 According to the meteorological summary of the observations collated 

 by the State service, covering a period of six successive years, the 

 winds from these directions prevailed in fortj^-one out of the seventj^- 

 two months, and they are most likely to be followed by rain, most 

 frequently l)y a heavy precipitation, when coming from the east, south, 

 or southeast. The cool and dr}" winds from the north and northwest 

 are least likely to be followed by rain. The western and northwest- 

 ern currents prevailed during the period stated in twenty-three months, 

 the north and northeasterly onl}^ in eight months. From the wind 

 chart showing the average direction of the wind in Alabama from 

 1884 to 1889, it appears that southeastern winds prevailed almost 

 exclusively during the winter and earlier part of the spring, but with 

 some northwestern winds; that winds from the northwest and north- 

 east predominated in spring and in summer, and that in autumn the 

 winds were from the south, southeast, or southwest, and more rarely 

 from a northern direction. The differences in mean annual directions 

 of the wind are l)ut slight. In their rush toward centers of depres- 

 sion, the warm winds from the south, charged with moisture, imping- 

 ing upon the cold currents from the north, produce a whirlpool, 

 resulting in electrical storms, generally with a heavy rainfall, often 

 assuming the force of a tornado. These tornadoes, moving generally 

 in a northeasterly direction, are most frequent in the north-central 

 part of the State, and happen most often in the latter part of the winter 

 or in the spring. 



CLOUDINESS. 



South and southwestern winds are generally followed by a sunny 

 sky, those coming from the east and northeast by a veil of clouds 

 which strong blasts from the north are apt to rend and disperse. 

 According to the meteorological summary quoted,^ in a succession of 

 six years the number of clear days per year averaged 120, of fair days 

 119, and of cloudy days 116. No data are at hand for the deduction 

 of the average duration of sunshine during this period. 



GENERAL PRINCIPLES OF THE DISTRIBUTION OF PLANTS. 



Wherever life finds support plant life thrives and is reproduced, but 

 no one plant, except perhaps a few of the lowest forms, is found dis- 

 persed over every part of the globe. Every one of the multitude of 

 forms which constitute the plant covering of the earth is bv its organi- 

 zation restricted within certain limits. It grows and reproduces its 

 kind in those places where conditions of climate and soil are most 

 favorable for its particular needs. If all plants could adapt themselves 



'P. H. Mell, Climatology of Alabama, op. cit., pp. 59 to 63. 



