INVASION 211 



in every instance of migration, as for example in the simple elongation of a 

 rootstalk, but in the great majority of cases each plays its proper part. Mo- 

 bility represents the inherent capacity of a plant for migration, and in its 

 highest expression, motility, is in itself productive of movement. As a gen- 

 eral rule, however, modifications for securing mobility are ineffective in the 

 absence of proper agents, and the effective operation of the two will be pro- 

 foundly influenced by distance and topography. 



262. Mobility denotes potentiality of migration as represented by modi- 

 fications for this purpose. It corresponds, in a sense, to dissemination, 

 though seed production also enters into it. Its most perfect expression is 

 found in those plants which are themselves motile, Bactcriaceae, Oscillatoria, 

 Volvocaceae, and Bacillanaccae, or possess motile propagules, such as most 

 Phycophyta. On the other hand, it is entirely undeveloped in many plants 

 with heavy unspecialized seeds and fruits. Between these two extremes lie 

 by far the greater number of plants, exhibiting the most various degrees of 

 mobility, from the motile though almost immobile oft'shoots of many Lili- 

 accac to the immotile but very mobile spores of fungi. It is thus seen that 

 motility plays a relatively small part in migration, being practically absent 

 in terrestrial forms, and that it bears a very uncertain relation to mobility. 

 In analyzing the latter, contrivances for dissemination are seen to determine 

 primarily the degree of mobility, while the number of seeds produced will 

 have an important effect in increasing or decreasing it. A third factor of 

 considerable importance is also involved, namely, position with reference to 

 the distributive agent, but any exact knowledge of its importance must await 

 systematic experiment somewhat after the methods of Dingier, but with air-, 

 currents, etc., of known velocity and direction. The time is not distant when 

 by such methods it will be possible to establish a coefficient of mobility, de- 

 rived from terms of position, weight, resistant surface, and trajectory for 

 definite wind velocities or for particular propulsive mechanisms. 



263. Organs for dissemination. Plants exhibit considerable diversity 

 with reference to the part or organ modified, or at least utilized, for dissem- 

 ination. This modificatiori, though usually affecting the particular product 

 of reproduction, may, in fact, operate on any part of the plant, and in certain 

 cases upon the entire plant itself. In the majority of plants characterized 

 by alternation of generations, the same" individual may be disseminated in 

 one generation by a reproductive body, and in the other by a propagative 

 one, as is the case in the oogones and conidia of Peronospora, the spores and 

 gemmae of MarchanUa, the fruits and rimners of Fragaria, etc. Special 

 modifications have, as a rule, been developed in direct connection with spores 



