164 PLANT EELATIONS. 



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. 



122. 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 12.2 Fahr. There are certain plants which can 

 work at higher temperatures, notably certain algas 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. 



While 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 

 it is uniformly distributed, and in the other great extremes 



