244 PLANT PHYSIOLOGY AND ECOLOGY 



highly speciahzed for migration, and are consequently more mobile. 

 The number of fertile seeds is also much greater, a fact of much 

 significance, since the movement of abortive seeds is of no benefit 

 to the species. This fact taken in connection with their great 

 mobility partly explains the supremacy of composites and grasses- 



259. Position of disseminules. The position on the plant of 

 the part disseminated, i.e., its exposure to the distributing agent, 

 plays a part in mobility. In the majority of flowering plants, the 

 position of the inflorescence gives a maximum of exposure, but 

 in many cases special modifications are developed to place spores 

 or seeds in a more exposed position. The height of the inflorescence 

 from the ground or above the surrounding plants aids in increasing 

 the distance to which the spores or seeds are carried in the first 

 flight. 



The most perfect device of this kind is found in such com- 

 posites as the dandelion, in which the stalk stretches up after 

 the head closes finally. By the time the involucre expands to 

 release the parachute fruits, the flower stalk has grown to several 

 times its original length. The carpotropic movements of various 

 plants often serve to place seeds and fruits in a better position 

 for dissemination. In certain composites the involucral scales 

 are reflexed at maturity, thus loosening and lifting up the achenes. 

 A somewhat similar result is obtained in such grasses as Stipa 

 and Aristida by the twisting of the awns. In many mosses, liver- 

 worts, and puffballs, the spores are sifted out through slits or teeth, 

 or the whole spore mass is elevated and held apart by the mass 

 of elaters or threads. In most cup-fungi the spores are driven 

 out of the cup by tensions within. 



260. The agents of migration. The possibility of migration 

 depends primarily upon the action of distributing agents. In the 

 absence of these even the most perfect modification is without 

 value, while their presence often brings about the movement of 

 the most immobile plant. The amount and extent of migration 

 are determined chiefly by the permanence and forcefulness of the 

 agent concerned. Furthermore, the direction and rapidity of 

 migration depend upon the direction and intensity of the agent. 



]\Iigration results when spores, seeds, fruits, offshoots, or plants 

 are moved out of their homes by water, wind, animals, man, 

 gravity, glaciers, growth, or mechanical propulsion. In accord- 

 ance the following groups are distinguished: 



