560 KEPORT OF STATE GEOLOGIST. 



PLANT SOCIETIES. 



Plants are living things, and their occurrence and persistence in any 

 given region is not a matter of chance, but is determined by surround- 

 ing conditions. There is a constant struggle in nature among plants, 

 not merely to fill unoccupied territory, but also to maintain positions 

 already held. Because of this, plants having similar vital necessities 

 are found associated, forming plant societies or unions, and these plant 

 societies stand as the sign of the capacity of that particular region, in 

 the sustaining of plant life. Plant societies are usually made up of 

 many different kinds of. plants, since, if those of the same form or of 

 closely related forms were thus associated, the vital necessities would 

 be absolutely identical, and the struggle for existence would become 

 too intense. An ability to determine the significance of these plant so- 

 cieties, to interpret the factors which make them possible, would 

 serve to save much costly experimentation and to prevent many seri- 

 ous failures. No surer sign can be found of the agricultural capacity 

 of a soil than is furnished by the native plants there growing. But the 

 sign to have significance must be interpreted. 



The chief factors controlling plant distribution are light, heat, water, 

 soil, both as regards its chemical composition and physical character, 

 and wind. 



Light, because many of the most important functions of the plant, 

 such as transpiration and carbon fixation, are largely or wholly depen- 

 dent upon its presence. 



Heat, because plants have certain temperature requirements. These 

 requirements differ in a considrable degree in different species, but, 

 as a rule, plants work only at a temperature between 32° to 122° Fahr. 



Water, since all soil derived foods must be taken in the form of a 

 watery solution, and its presence in a certain amount is imperative. 

 In this, also, there exists a wide range, as is shown by the fact that 

 some plants live totally submerged in water, while others are able to 

 endure long continued drought. 



Soil, for the reason that its chemical composition determines the 

 food materials carried by the water, while its physical character de- 

 termines both its ability to receive and retain moisture. Sand and clay 

 illustrate reverse conditions in the matter of the reception and reten- 

 tion of water. 



Winds, where strong and long continued, because of their drying 

 effect upon the atmosphere and the consequent abstraction of water 

 from the plant by the thirsty air. In our own State this factor is, per- 

 haps, less apparent in its influence than the others, although, in certain 



