REGIONAL DISTRIBUTION 495 



higher groups with which the region is covered. In the 

 latter case the student considers mainly the ecological groups 

 which are present; that is, whether the plants are water or 

 land forms, and in regard to the land plants whether they are 

 xerophytes or mesophytes. In studying plant distribution 

 from the floristic side a very important topic is the considera- 

 tion of the history of the flora. This deals, for instance, with 

 such questions as the investigation of the center or centers 

 from which the plants were derived, the course which they 

 took in migrating outward from those centers, and the time 

 required to cover the territory which they now occupy. 



451. Ecological plant geography. For most purposes the 

 ecological characters of the principal floras of the earth are 

 more important than their systematic relations ; that is, we 

 are especially concerned to know that arctic vegetation is 

 scanty and of dwarf forms, that vast grassy meadows and 

 prairies and extensive hard-wood forests, often of few spe- 

 cies, are characteristic of temperate regions, and that tropical 

 forests (in the rainy areas) are extremely dense, interwoven 

 with lianas, and burdened with epiphytes. Such facts are of 

 more general interest than the knowledge of the proportion 

 of the floras of the different zones constituted by each family 

 represented. 



The most important unit for beginners in ecological plant 

 geography to consider is the association. 1 In order to see what 

 this term means it will be necessary to recall some of the 

 things which most observing people already know without 

 having studied botany. In a pond like that shown in Fig. 358 

 one is likely to find white pond lilies, yellow pond lilies, pick- 

 erel weed, arrowhead, pondweed (Potamogetori), water smart- 

 weeds, rushes, and perhaps a good many other flowering plants. 

 Besides these there may be dozens of species of blue-green 



1 The plant formation is a larger unit, made up of associations. Forma- 

 tions may consist of many families, genera, and species, but must comprise 

 such vegetation forms as are able to thrive in the habitat where the forma- 

 tion occurs. 



