238 ECOLOGY 



from the plant and at the same time enabling it to get 

 food for itself from the air in a way that it could not 

 otherwise do. 



341. Relations with Inanimate Nature. — But it is to the 

 relations of plants with inanimate nature, and their group- 

 ing into societies under the influence of such conditions, 

 that the term "ecology" is more strictly applied. The ex- 

 ternal conditions that lead to the grouping are called 

 ecological factors. The most important of these are tem- 

 perature, moisture, soil, light, and air, including the direc- 

 tion and character of the prevailing winds. Each of these 

 factors is complicated with the others and with conditions 

 of its own in a way that often makes it difficult to determine 

 just what effect any one of them may have in the formation 

 of a given plant society. 



342. Temperature, for instance, may be even and steady 

 like that of most oceanic regions, or it may be subject to 

 sudden caprices and variations like the " heated terms " 

 and "cold snaps" that afflict our northern and southern 

 States respectively every few years. We must remember, 

 too, that it is not the average temperature of a climate but 

 its extremes, especially of cold, that limit the character of 

 vegetation. 



Temperature probably has more influence than any other 

 factor in the distribution of plants over the globe, but it 

 can have little or no effect in evolving local differences in 

 vegetation because the temperature of any given locality, 

 except on the sides of high mountains, will ordinarily be 

 practically the same within a circuit of many miles. 



343. Moisture, again, may be of all degrees, from the 

 superabundance of lakes and rivers and standing swamps, 

 to the arid dryness of the desert, and the water may be 

 still and sluggish, or in rapid motion. It may exist more 

 or less permanently in the atmosphere, as in moist climates 

 like those of England and Ireland, where vegetation is 

 characterized by great verdure, or it may come irregularly 



