INVASION 2i1 
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, Bacteriaceae, Oscillatoria, 
Volvocaceae, and Bacillariaceae, 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 offshoots of many Lili- 
aceae 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 Dingler, 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 modification, 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 Marchantia, the fruits and runners of Fragaria, etc. Special 
modifications have, as a rule, been developed in direct connection with spores 
