SEED DISPERSAL. 



459 



893. Many plants pos ess attractive devices, and offer a substantial 

 reward, as a piice for the distribution of their seeds. Fruits and berries are 

 devoured by birds and otlier animals ; the seeds within, often passing un- 

 harmed, may be carried louy distances. Starchy and albuminous seeds and 



Seeds of geum showing tlic booklets where the end of tlie style is kneed. 



grains are also devoured. an<l while many such seeds are destro\'ed. others 

 are not injured, anel finally are lodged in suitable places for growth, i.tften 

 remote from the etriginal localit}'. Thus animals willingly or unwillingly 

 become agents in the (.lissemination of plants over the earth. Man in the 

 development of coiumerce is often responsible fur the wide distribution of 

 narmful as well as beneficial species. 



894. Other plants are more independent, and mechanisms are employed 

 for violently ejecting seeds from the pod or fruit. The unequal tension of 

 the pods of the comm.on vetch (Vicia sativa) when drying causes the valves 

 to contract unequally, and on a dry summer day the valves twist and pull in 

 opposite directions until they suddenly snap apart, and the seeds are thrown 

 forcibly for some <;li>tance. In the impatiens, or t< aich-me-not as it is better 

 known, when the pods are ripe, often the least touch, or a pinch, or jar, sets 

 the iive valves free, they coil up suddenly, and the small seeds are thrown 

 fo-r several yards in all directions. During autumn, on dr\' days, the pods 

 of the witch hazel contract unequally, and the valves are suddenly spread 

 apart, and the seeds are hurled awa>'. 



Other plants have seeds provided with tufts of pappus, or hair-like 

 masses, or wing-like outgrowth.= which serve to buoy them up as they 



