AGGREGATION AND MIGRATION 241 



etc. The value of mobility to the plant is dependent upon the 

 presence of proper agents for causing movement, and the opera- 

 tion of these two factors is much affected by distance and 

 topography. 



255. Mobility. Mobility indicates the power of the plant for- 

 movement. It depends primarily upon devices for bringing 

 about dissemination, though the number of seeds is also an im- 

 portant factor in it. Mobility is most marked in those plants 

 which are themselves motile, bacteria, diatoms, vol vox, etc.,. 

 or possess motile spores, such as most of the green algae. On the- 

 other hand, it is little or not at all developed in those flowerings 

 plants with large, heavy seeds or fruits. By far the greater number- 

 of plants exhibit some degree of mobility. The range is extreme, 

 from the almost immobile offshoots of lilies, which move by growth, 

 to the non-motile but very mobile spores of fungi. There is no- 

 necessary correspondence between mobility and motility. The 

 latter is practically absent in terrestrial plants, and, in spite of 

 its importance among the algae, it plays a relatively small part 

 in migration. The degree of mobility is determined chiefly by 

 the nature of the device used in dissemination, but the number of 

 seeds or spores produced has an important effect in increasing or 

 decreasing it. A third factor of much influence is the position 

 of seed or spore with reference to the action of the distributive 

 agent. 



256. Organs of dissemination. Plants differ much with respect 

 to the organ modified or utilized for dissemination. Such modi- 

 fication, while it usually affects the fruit or seed alone, may act 

 upon any organ, or upon the entire plant body. Special modifica- 

 tions are usually developed in connection with spores and seeds,, 

 and mobility is most marked in species of this sort. It is much- 

 reduced in the case of offshoots and plant bodies, at least in terres- 

 trial plants, notwithstanding a few striking exceptions, such as 

 the tumble-weeds. The following indicates the grouping of plants 

 with reference to the part distributed. 



1. Spore-distributed. This group includes all plants possessing 

 structures which are called spores, viz., algae, fungi, liverworts, 

 mosses, and ferns. Spores rarely have special devices for dis- 

 semination, but their minute size makes them extremely mobile. 



2. Seed-distributed. This group comprises all flowering plants 

 in which the seed is the part modified or disseminated. Seeds are.- 



