MIGRATION 33 



Mobility represents the inherent capacity of a plant for mi- 

 gration, and in its highest expression, motility, is in itself 

 productive of movement. As a general rule, however, 

 modifications for securing mobility are ineffective in the 

 absence of proper agents, and the effective operation of the 

 two will be profoundly influenced by distance and topo- 

 graphy. 



The historical development of the concept of migration 

 has been extremely fragmentary, although its recognition 

 dates back to the beginning of phytogeography. The need 

 for a systematic consideration of the subject has been 

 obscured by the greater interest taken in the origin and 

 naturalization of cultivated plants. Linne (1745:2) first 

 mentioned migration briefly as follows: "Migrarunt ad 

 floram nostram ab Exteris regionibus multae plantae, quae 

 suas in Suecia fixere sedes ad littora praesertim & oras 

 maritimas". In the PMlosopMa Botanica (J752:86), he has 

 made an excellent analysis of dissemination, distinguishing 

 wind-distributed parts (Volitantia) as pappose, comate, 

 winged and inflated, mechanically propelled seeds (Elasticae) 

 as contractile, mucronate, fibrillar and creeping, and finally, 

 parts with devices to secure attachment as those having 

 hamate calyx, pericarp or seeds. He indicated, moreover, 

 that animals distribute seeds by deglutition and mastication, 

 while streams, seas, tides, lakes and rains all aid in dissem- 

 ination. Humboldt (1805:22) was apparently the first to em- 

 ploy the terms migration and mobility, but he did this only 

 incidentally in connection with the enforced migration of 

 cultivated plants. Mirbel (J8J5:348) treated briefly of dis- 

 semination by elasticity of the pericarp and by winds, 

 water, birds, mammals and man, drawing the larger 

 number of his illustrations from Linn6. He also 

 made a detailed classification of seeds and fruits 

 (593,796) according to their form and surface. DeCandolIe 

 (1820:45) recognized the primary importance of natural 

 barriers (barri^res) in gradual migration, considering at some 

 length the obstructive effect of large bodies of water, 

 deserts, mountain ranges and radically different vegetation. 



