496 



PEACTICAL BOTANY 



and green algae. Coniferous or hard-wood forests (Sect. 446) 

 contain varied assemblages of herbs, shrubs, and trees. The 

 plant life of the pond and that of the forest are good examples 

 of associations. A plant association is a set of plants, usually con- 

 sisting of several genera (perhaps comprising many species), of 

 somewhat similar aspect, living together under essentially simi- 

 lar conditions. It is important to notice that even a small area 



FIG. 370. A flowerless xerophyte (the lichen Usnea) growing on conifers 

 in the Maine woods 



may contain several associations. For instance, a rocky ledge 

 in a meadow may have an entirely different plant population 

 from that of the meadow around it. The aspect of an associa- 

 tion depends largely upon the kind of vegetation forms (hy- 

 drophytes, mesophytes, or xerophytes) which the station can 

 support. It is also influenced by other circumstances, such as, 

 in the case of aquatics, whether the plants are wholly or only 

 partially submerged ; in the case of land plants, whether trees, 

 shrubs, or herbs predominate. 



