40 INVASION 



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 deter- 

 mine the actual effectiveness in nature of different modifi- 

 cations, 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 

 foothill and sandhill regions of Nebraska, regions which are 

 suflBciently 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 foothill region, many are doubt- 

 less 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 foothill regions, 2 only, 

 Amorpha nana and Roripa nasturtium, are relatively immobile, 

 but the minute seeds of the latter, however, are readily dis- 

 tributed, and the former is altogether infrequent. 



The following groups of plants may be distinguished ac- 

 cording to the character of the contrivance by which dis- 

 semination is secured. 



1. Saccate, saccospores {Saccospwae, craKKos,o,sack, a-n-opa, 17, 

 seed, fruit). Here are to be placed a variety of fruits, all 

 of which agree, however, in having a membranous envelope, 



