AGGREGATION AND MIGRATION 243 



the part usually modified into a long, plume-like organ, producing 

 a high degree of mobility, as in Clematis, Pulsatilla, and Sieversia. 



7. Awned (acospores). These are nearly all grasses, in which 

 the awns serve for distribution by wind, water, or animals, and 

 even by certain creeping movements. The degree of mobility 

 in many cases is great. 



8. Spiny (centrospores). This group contains a few species in 

 which distribution of the spiny fruits is brought about by attach- 

 ment, as Cenchrus and Tribulus. The mobility is fairly high. 



9. Hooked (oncospores). The members of this group are ex- 

 tremely numerous, and the degree of mobility as a rule is very 

 high. All agree in the possession of hooks and barbs, which serve 

 for attachment, though the number, size, and position of the 

 hooks vary greatly. 



10. Viscid (gloeospores). In these the inflorescence is more or 

 less covered with a viscid substance, as in species of Silene, or the 

 fruit is beset with sticky hairs, as in Cerastium, Salvia, etc. 



11. Fleshy (sarcospores). These are fleshy fruits which are 

 scattered in consequence of being swallowed, especially by birds. 

 The seeds are usually protected by a stony envelope which enables 

 them to resist digestion. The mobility varies greatly, but the 

 area over which migration may be effected is large. 



12. Flagellate (mast igosp ores). These are plants with ciliate 

 or flagellate spores, as in CEdogonium, Ulothrix, Vaucheria, etc., 

 or with plant bodies similarly motile, Bacteriacece, and Volvocacece. 



258. Influence of seed production. The chances of migration 

 depend in a large degree upon the number of fruits, seeds, or spores 

 produced. A large seed production increases the movement of a 

 mobile species. In the case of two species with equally good 

 devices for distribution, the one with the largest number of seeds 

 is the more mobile. Even in immobile plants, seed production 

 increases the few chances of movement. 



Two kinds of seed production are distinguished upon the basis 

 of the relation between number of seeds and of flowers. In one 

 species the flowers are many, but the seeds few or single in each, 

 as in composites, grasses, sedges, etc. In the other the lumiljer 

 of seeds in each flower is large, as in lilies, orchids, violets, etc. 

 In so far as the actual number of .seeds produced is conccnie<l, 

 some species of one type do not differ greatly from some of the 

 other. As a rule, however, species with many flowers are more 



