212 THE FORMATION 



and seeds, and mobility reaches its highest expression in these. It is, on the 

 other hand, greatly restricted in offshoots and plant bodies, at least in ter- 

 restrial forms, though it will now and then attain a marked development in 

 these, as shown by the rosettes of Sempennvum and the tumbling plants of 

 Cycloloma. For the sake of convenience, in analyzing migration, all plants, 

 may be arranged in the following groups with reference to the organ or part 

 distributed. 



1. Spore-distributed, sporostrotes. This includes all plants possessing 

 structures which go by the name of spore, such as the acinetes of Nostoc 

 and Protococctis, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia, 

 ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and 

 the gemmae and spores proper of liverworts, mosses, and ferns. These are 

 almost always without especial contrivances for dissemination, but their ex- 

 treme minuteness results in great mobility. 



2. Seed-distributed, spermatostrotes. This group comprises all flowering 

 plants in which the seed is the part modified or at least disseminated. The 

 mobility of seeds is relatively small, except in the case of minute, winged or 

 comate seeds. 



3. Fruit-distributed, carpostrotes. The modifications of the fruit for dis- 

 tribution exceed in number and variety all other modifications of this sort. 

 All achenes, perigynia, utricles, etc., properly belong here. 



4. Offshoot-distributed, thallostrotes. To this class are referred those 

 plants, almost exclusively cormophytes, which produce lateral, branch-like 

 propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc. 

 Migration with such plants is extremely slow, but correspondingly effective, 

 since it is almost invariably followed by ecesis. 



5. Plant-distributed, phytostrotes. This group includes all plancton and 

 surface forms, whether motile or non-motile, and those terrestrial plants in 

 which the whole plant, or at least the aerial part, is distributed, as in tumble- 

 weeds and in many grasses. 



264. Contrivances for dissemination. Any investigation of migration to 

 be exact must confine itself to fixed forms. For these the degree of perfec- 

 tion shown by dissemination contrivances corresponds almost exactly to the 

 degree of mobility. Because of the difficulty of ascertaining the effect of 

 ecesis, it is impossible to determine the actual effectiveness in nature of dif- 

 ferent mo<lifications, and the best fliat can be done at present is to regfard 

 mobility, together with the occurrence and forcefulness of distributive agents, 

 as an approximate measure of migration. The general accuracy of such a 

 measure will be more or less evident from the following. Of 118 species 

 common to the foot-hill and sand-hill regions of Nebraska, regions which are 



