158 Biology in America 



migrants from Europe witness the Russian sow thistles, the 

 wild radish and the barnyard grass. 



To discuss in any detail the past movements and distribu- 

 tion of plants and animals in North America would in itself 

 fill a more than ample volume. In the preceding chapter 

 some of the great movements of animal life in North Amer- 

 ica have been briefly mentioned, and under the present head- 

 ing it must suffice to consider briefly some of the facts and 

 problems of present day distribution and of the relations of 

 plant and animal societies to one another and to their sur- 

 roundings. 



The study of plant ecology, that is, the study of the plant 

 at home, as an individual and as a member of society, owes 

 its inception in America to the work of Pound and Clements, 

 who in their ' ' Phytogeography of Nebraska" in 1897 de- 

 scribed the plant societies of that state and developed new 

 and more accurate methods for their study. This was fol- 

 lowed in 1905 by Clements' "Research Methods in Ecology," 

 in which the whole field of plant ecology and the methods for 

 its investigation were presented. Following the appearance 

 of these works came a host of papers dealing with various 

 phases of plant ecology, the most comprehensive of which is 

 that of Clements on " Plant Succession." 



"When an area of land is denuded of its plant covering, as 

 happens far too often in our fire-swept forests, or as a result 

 of floods or landslides ; or when a new area appears as in the 

 case of the drying up of a lake or the shifting of a river, 

 there is an inrush of plant settlers to occupy the virgin soil. 

 The character of the new settlers depends upon many fac- 

 tors the character of the new soil, the relative proximity of 

 adjacent species of plants, the ease with which the seeds or 

 runners of these plants can reach the new territory and their 

 readiness to establish themselves there after their arrival. 

 The arrival of the new settlers will depend upon all the many 

 factors which determine plant dispersal strength and direc- 

 tion of wind currents, presence of streams and rainfall, drain- 

 age which may carry seeds on to the new area, and the abun- 

 dance and movements of seed-bearing animals. 



In the broken chasms of mountain fastnesses, where the 

 shattered peaks that were, now lie in a mass of tumbled ruin, 

 or upon the sheer slopes of granite cleft by some contortion 

 of the earth, the humble lichen finds its home. In some small 

 chink or crevice of the rock where a few drops of water lin- 

 ger from the winter's snow, its filaments take hold, and when 

 the breath of summer dries its niche it lies dormant waiting 

 for the rain or melting snow. Through the acids secreted 

 by these lichens, and by the hardy, drouth-resistant mosses 



