RESPONSE OF THE PLANT TO ITS SURROUNDINGS 273 



kind with those of another, between animals with animals, 

 or between plants and animals, as in the case of the clover 

 and bumblebee, and the yucca and pronuba. 



The occurrence of root tubercles on certain of the legu- 

 minosse (63) is a clear case of symbiosis, the microscopic 

 organisms in the tubercles getting their food 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. 



310. 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 

 external 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 du-ec- 

 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. 



311. Temperature may be even and steady, like that of 

 most oceanic regions, or it may be subject to sudden ca- 

 prices and variations, like the " heated terms " and " cold 

 snaps " that afflict our Eastern coast region every few years. 

 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 upon the distribution of plants over the globe; but it 

 can have little or no effect in evolving local difi'erences in 

 vegetation, because the temperature of any given locality, 

 except on the sides of high mountains, will ordinarily be the 

 same within a circuit of many miles. 



312. 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 



