PLANT SOCIETIES: ECOLOGICAL FACTORS 
the same amount of water and the same water level, but 
if the substances dissolved in the water differ in certain 
particulars, two entirely distinct societies may result. 
125. Heat. The general temperature of an area is im- 
portant to consider, but it is evident that differences of 
temperature are not so local as differences in the water 
supply, and therefore this factor is not so important in the 
organization of the local associations of plants, called socie- 
ties, as is the water factor. In the distribution of plants 
over the surface of the earth, however, the heat factor is 
probably more important than the water factor. The range 
of temperature which the plant kingdom, as a whole, can 
endure during active work may be stated in a general way 
as from to 50 C. ; that is, from the freezing point of 
water to 122 Fahr. There are certain plants which can 
work at higher temperatures, notably certain algae growing 
in hot springs, but they may be regarded as exceptions. It 
must be remembered that the range of temperature given 
is for plants actively at work, and does not include the tem- 
perature which many plants are able to endure in a specially 
protected but very inactive condition. For example, many 
plants of the temperate regions endure a winter tempera- 
ture which is frequently lower than the freezing point of 
water, but it is a question of endurance and not of work. 
It must not be supposed that all plants can work equally 
well throughout the whole range of temperature given, for 
they differ widely in this regard. Tropical plants, for in- 
stance, accustomed to a certain limited range of high tem- 
perature, cannot work continuously at the lower tempera- 
tures. For each kind of plant there is what may be called 
a zero point, below which it is not in the habit of working. 
AVhile it is important to note the general temperature 
of an area throughout the year, it is also necessary to note 
its distribution. Two regions may have presumably the 
same amount of heat through the year, but if in the one case 
jt is uniformly distributed, and in the other great extremes 
