248 PLANT PHYSIOLOGY AND ECOLOGY 



persistent involucre or calyx that the latter serves as a kind of 

 mortar for the projection of the seeds when the stem is sharply 

 bent to one side by any force such as the wind or some animal. 



261. The work of migration agents. Two or more agents some- 

 times act upon the same disseminule, usually in succession. The 

 possibility of such action in nature is great, but actual instances 

 of it are not frequent, except when the activities of man enter 

 into the question. Some parts, such as awned inflorescences, are 

 carried almost equally well by wind or animals, and are often 

 scattered by the action of both. Seeds and fruits are frequently 

 blown by the wind into streams by which they are carried away. 

 As a rule, however, parts adapted to wdnd-distribution are injured 

 by immersion in the water, and the number of plants capable of 

 being scattered by the successive action of wind and water is 

 small. As a general rule, plants growing in or near the water, 

 if modified for migration at all, are adapted to water-carriage. 

 Species that grow in exposed grassy or barren habitats are for 

 the most part wind-carried. Those found in the shelter of forests 

 and thickets are usually scattered by animals, though the taller 

 trees and shrubs are generally wind-distributed by reason of 

 exposure to the upper air-currents. There is seen to be a certain 

 amount of correspondence, since hydrophytes are usually water- 

 carried, shade plants are borne by animals, and the majority of 

 sun mesophytes and xerophytes are wind -distributed. In each 

 group, however, are numerous exceptions to the rule, owing to 

 migration into various types of habitats. 



With respect to their action, agents are constant or intermittent. 

 The former include currents, streams, winds, gravity, growth, 

 and propulsion; the latter, animals, including man. In the case 

 of constant agents migration takes place more or less continuously 

 from year to year, and usually in a definite direction. With 

 intermittent agents dissemination is largely accidental; it is 

 indeterminate in direction, and recurs only at irregular intervals, 

 if at all. Migration is most effective when it is continuous, and 

 least when it is intermittent. In the one case the migration is an 

 annual one with the probability of the gradual adjustment of the 

 seedling. In the other, species are usually carried not only out 

 of their particular habitat, but often far beyond their native 

 region, and establishment may become difficult or impossible. 

 The rapidity of migration is usually greatest for intermittent 



