216 THE FORMATION 
267. Agents of migration. In the last analysis, however, the possibility ot 
migration depends upon the action of distributive agents; in the absence of 
these, even the most perfect contrivance is valueless, while their presence 
brings about the distribution of the most immobile form. In short, migra- 
tion depends much more upon such agents than upon mobility, however per- 
fect the latter may be. It is, moreover, evident, that the amount and extent 
of migration will be determined primarily by the permanence and forceful- 
ness of the agent, as indicated by its ability to bring about transportation. 
Finally, as will 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. Correspending to these agents, there may be recog- 
nized the following groups: 
1. Water, hydrochores. These comprise all plants distributed exclusively 
by water, whether the latter acts as ocean currents, tides, streams, or surface 
run-off. In the case of streams and run-off, especially, mobility plays little 
part, provided the disseminules are impervious or little subject to injury by 
water. Motile plants, or those with motile cells, which belong entirely to 
this group, may be distinguished as autochores, which correspond closely to 
mastigospores. 
2. Wind, anemochores. This group includes the majority of all permo- 
bile terrestrial plants, i. e., those in which modifications for increasing sur- 
face have been carried to the extreme, or those which are already permobile 
by reason of the minuteness of the spore or seed. Saccate, winged, comate, 
parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits 
represent the various types of modifications for wind-distribution. 
3. Animals, zoochores. Among terrestrial plants, dissemination by at- 
tachment represents essentially the same degree of specialization as is found 
in wind-distributed plants. The three types of contrivances for this purpose 
are found in spinose, hooked, ‘and glandular fruits. Dissemination by 
deglutition and by carriage, either intentional or unintentional, though of 
less value, play a striking part on account of the great distance to which . 
the seeds may be carried. Dissemination by deglutition is characteristic 
of sarcospores, and distribution by carriage of creatospores. 
4. Man, brotochores. Dissemination by man has practically no connec- 
tion with mobility. It operates through great distances and over immense 
areas as well as near at hand. It may be intentional, as in the case of 
cultivated species, or unintentional, as in thousands of native or exotic 
species. No other disseminating agent is comparable with man in respect 
to universal and obvious migration, 
