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- 
minose (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 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. 
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 differences 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 
