PLANTS 43 



feature that promotes migration and a considerable group have be- 

 come highly specialized in this respect. Such specialization has af- 

 fected the fruit as a rule, by virtue of the available material in the 

 wall; seeds are much less frequently modified, hairs and wings con- 

 stituting the usual devices. In fruits, the modification may assume 

 the form of a sac, wings, hairs, parachute, chaff, plumes, awns, spines, 

 hooks, or a fleshy pulp. Most of these are adapted to distribution by 

 wind, but awns, spines, and hooks serve for carriage by attachment, 

 and fleshy fruits are distributed in consequence of their use as food. 



The i)rincipal agents in dissemination are wind, animals, and man. 

 Although many disseminules are carried by water, especially in ocean 

 currents, few of these remain viable after long immersion, except those 

 of water plants. It is obvious that carriage by the wind and by 

 attachment to animals depends upon the development of a suitable 

 modification. This is true, in part, of fruits and seeds used for food, 

 notably the fleshy ones, but a large number of these are scattered 

 incidentally by animals that seek them. From these interrelations 

 springs a vast group of seed and fruit coactions; though these have 

 received much general attention, their comprehensive and detailed 

 study in various communities awaits further recognition of their 

 importance. 



Community Relations. It is hardly necessary to emphasize the 

 point that the life history of an isolated individual differs in a number 

 of respects from that of similar individuals in their proper community 

 setting. Hence, complete and detailed life histories can be observed 

 only in the natural habitat, even though garden or other control 

 methods can be profitably employed for the major phases of germina- 

 tion, growth, propagation, and reproduction. In the first place, com- 

 petition is the outcome of the number and spacing of individuals, 

 whether of the same or different species. It depends primarily upon 

 the reaction of these on the habitat, a process that leads generally 

 to a limited supply of some essential factor. When this effect is 

 marked, growth may be seriously hindered or entirely inhibited at 

 any stage in the life cycle. Thus, competition may lead to the sup- 

 pression of branches or propagules; it may prevent the formation of 

 tillers or the production of flowers. On the other hand, it may pro- 

 foundly modify the size, number, or structure of organs or parts, with 

 a more or less corresponding effect upon reaction or coaction. 



Because of the primary relations of plants and animals in the 

 community, coaction is always an important and often a controlling 

 process in the life cycle. This is particularly true of food coactions 

 and especially of those concerned in pollination. Grazing animals 



