44 INVASION 



production will be the more mobile. On the basis of the re- 

 lation of seeds to flower, two groups of plants may be dis- 

 tinguished', one, Polyanthae, in which the flowers are many 

 and the seeds few or single, as in Compositae, and the other, 

 Polyspermatae, Portulaca, Yucca, etc., in which the number 

 of seeds to each flower is large. So far as the actual number 

 of seeds produced is concerned, polyanthous plants may not 

 differ from polyspermatous ones, but, as a rule, they are 

 much more highly specialised for dissemination and are 

 more mobile. The number of fertile seeds is also much 

 greater, a fact which is of great importance in ecesis, and 

 which, taken in connection with mobility, partially explains 

 the supremacy of the composites. Among the fungi and 

 algae, the amount of spore-production in a large degree de- 

 termines the mobility, since these forms are intrinsically 

 permobile. 



In the last analysis, however, the possibility of migration 

 depends upon the action of distributive agents: in the 

 absence of these, even the most perfect contrivance is value- 

 less, while their presence brings about the distribution of 

 the most immobile form. In short, migration depends much 

 more upon such agents than upon mobility, however perfect 

 the latter may be. It is, moreover, evident that the amount 

 and extent of migration will be determined primarily by the 

 permanence and forcefulness of the agent, as indicated by 

 its ability to bring about transportation. Finally, as wiU 

 be shown later, the direction and rapidity of migration de- 

 pend directly upon the direction and intensity of the agent. 



Migration results when spores, seeds, fruits, offshoots, or 

 plants are moved out of their home by water, wind, animals, 

 man, gravity, glaciers, growth, or mechanical propulsion. 

 Corresponding to these agents, there may be recognized the 

 following groups. 



1. Water, hydrochores {Hydrochorae, vhtap, vSaros, to, vSpo-, 



water, x<"P''<"> spread abroad). These comprise all plants 



distributed exclusively by water, whether the latter acts as 



ocean currents, tides, streams, or surf ace run-off. In the 



'Pound and Clements Phytogeography of Nebraska 84 1898. 



