=264. Contrivances for dissemination.= Any investigation of migration to be exact must confine itself to fixed forms. For these the degree of perfection 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 different 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 sufficiently diverse to indicate that these common species must have entered either one by migration from the other, 83 exhibit modifications for dissemination, while 8 others, though without special contrivances, are readily distributed by water, and 4 more are mobile because of minuteness of spore or seed. Some degree of mobility is present in 73 per cent of the species common to these regions, while of the total number of species in which the mode of migration is evident, viz., 95, 66 per cent are wind-distributed, 20 per cent animal-distributed, and 14 per cent are water-distributed. It need hardly be noted that this accords fully with the prevalence and forcefulness of winds in these regions. Of the species peculiar to the foot-hill region, many are doubtless indigenous, though a majority have come from the montane regions to the westward. The number of mobile species is 121, or 60 per cent of the entire number, while the number of wind-distributed ones is 85, or 70 per cent of those that are mobile. Among the 25 species found in the widely separated wooded bluff and foot-hill regions, 2 only, _Amorpha nana_ and _Roripa nasturtium_, are relatively immobile, but the minute seeds of the latter, however, are readily distributed, and the former is altogether infrequent. The following groups of plants may be distinguished according to the character of the contrivance by which dissemination is secured: 1. Saccate, saccospores. Here are to be placed a variety of fruits, all of which agree, however, in having a membranous envelope or an impervious, air-containing pericarp. In _Ostrya_, _Physalis_, _Staphylea_, the modification is for wind-distribution, while in _Carex_, _Nymphaea_, etc., it is for water-transport. 2. Winged, pterospores. This group includes all winged, margined, and flattened fruits and seeds, such as are found in _Acer_, _Betula_, _Rumex_, many _Umbelliferae_, _Graminaceae_, etc. 3. Comate, comospores. To this group belong those fruits and seeds with long silky hairs, _Gossypium_, _Anemone_, _Asclepias_, etc., and those with straight capillary hairs or bristles not confined to one end, _Typha_, _Salix_, etc.