GEOGRAPHICAL DISTRIBUTION 143 
more local variation than light. Plant functions depend on 
temperature (p. 35), and thus the range of temperature in a 
locality becomes an important factor in determining the 
species of plants inhabiting it, for some can stand high, 
some low, temperatures best ; others prefer uniformity of 
temperature rather than great range. Plants which contain 
much water are less able to stand extreme cold than those 
which contain little. High temperatures increase transpira- 
tion, and plants with insufficient protection against this will 
only be able to live in hot countries where there is plenty of 
moisture. 
Atmospheric Moisture, Rain, Snow, &c. Of the 
three geographical agents in determining distribution, this 
is perhaps the chief, water being of such very great import- 
ance in plant life. The effects of variation in the degree of 
saturation of the air are chiefly visible in the transpiration 
(p. 33) ; plants transpire the more the drier the air is. A plant 
must also regulate its transpiration according to the amount 
of water available for absorption by the roots. Plants show 
extreme structural variety corresponding to their rates of 
transpiration ; those living in very dry climates often differ 
extremely from those in wet (see below, Xerophytes, Shore- 
plants, Epiphytes, Water-plants, &c.). 
If the temperature of the air sink sufficiently, precipita- 
tion of some of the vapour occurs, as clouds, mist, fog, 
rain, hail, snow, dew, &c. Upon the form and amount of 
this precipitation the flora of a region largely depends. 
Plants living where there is long-continued mist, e.g. many 
alpines, must be able to do without direct sunlight, and to 
stand a saturated atmosphere. Wiesner divides plants into 
ombrophiles , which can undergo without injury long-continued 
rain, and ombrophobes , whose leaves soon decay or fall off 
under such circumstances; the plants of the wet tropics 
belong mostly to the former, those of deserts to the latter, 
group. A leaf, for the proper performance of its functions, 
must keep its surface dry. In wet regions adaptations for 
this purpose occur ; the most frequent is the drip-tip 
(Traufelspitze) or acuminate leaf-apex, a long fine point 
from which water rapidly drips off. It only occurs on leaves 
whose surfaces are easily wetted ; many leaves of w r et climates 
have polished surfaces from wffiich water at once runs off, e.g. 
