DISPERSAL OF SEEDS lOl 



special adaptation to secure the dispersal of their seeds to 

 longer, or shorter, distances. 



The chief agents at work in the transport of the seeds are 

 wind, water, and animals. 



In some instances the pericarps of the fruits when ripe are 

 subject to spring-like tensions, and at the time of dehiscence 

 open, more or less violently, and scatter the seeds in all directions, 

 often to a distance of several feet. The ripe pods of many legu- 

 minous plants, such as peas, beans, and bird's-foot trefoil, 

 disperse their seeds in this manner, and the valves of the pods 

 after the opening of the fruit twist or curl up suddenly. 



Fruits, which scatter their seeds by the sudden released 

 mechanical strains when dehiscence .takes place, are also met 

 with on the bitter-cresses {Cardamine hirsuta L. and C. 

 impatiens L.) several species of cranesbill {Geranium) and 

 many balsams {Impatiens). 



The wind is, however, the most powerful and most obvious 

 agency at work in the distribution of seeds, and an enormous 

 number of modifications are noticeable among plants to secure 

 dispersal by this means. 



In the orchises, poppies, and other plants, the seeds are small 

 enough to be readily blown considerable distances in the air as 

 soon as they escape from their capsules. Some seeds are smooth 

 and round, and easily roll along the ground. More commonly, 

 however, the adjoining bracts or some portion of the flower, fruit 

 or seed, is modified in such a manner that it presents a large and 

 light surface to the air, and the whole structure is thus rendered 

 buoyant. 



In many plants of the Corapositae (Chap, xxxiii.), such as 

 thistles, groundsel, and dandelion (Fig. 148), the calyx is repre- 

 sented by a tuft of long delicate hairs which act as a parachute 

 capable of preventing the rapid fall of the fruit when once the latter 

 is taken up by the wind. Even in a moderate breeze the fruits 

 of such plants are carried long distances before they finally drop. 



