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 Sempervivum 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 Protococcus, 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 modifications, and the best that can be done at present is to regard: 
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 
