38 INVASION 



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 re- 

 lation to mobility. In analysing 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 wiU be 

 possible to establish a coefiScient of mobility, derived from 

 terms of position, weight, resistant surface, and trajectory 

 for definite wind velocities or for particular propulsive 

 mechanisms. 



Plants exhibit considerable diversity with reference to the 

 part or organ modified, or at least utilized, for dissemination. 

 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 

 and seeds, and mobility reaches its highest expression in 

 these. It is, on the other hand, greatly restricted in off- 

 shoots and plant bodies, at least in terrestrial forms, though 

 it will now and then attain a marked development in these, 

 as shown by the rosettes of Sempervivum and the tumbling 

 plants of Cycloloma. For the sake of convenience, in analys- 

 ing migration, all plants may be arranged in the following 

 groups with reference to the organ or part distributed. 



