PRINCIPLES 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 forty-one out of the seventy- 
two months, and they are most likely to be followed by ‘ain, most 
frequently by a heavy precipitation, when coming from the east, south, 
or southeast. The cool and dry 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 only 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 but 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 by its organl- 
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 
1p. TH. Mell, Climatology of Alabama, op. cit., pp. 59 to 63. 
