SEED DISPERSAL. 



459 



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

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

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

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



Fig. 481. 

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



grains are also devoured, and while many such seeds are destroyed, others 

 are not injured, and finally are lodged in suitable places for growth, often 

 remote from the original locality. Thus animals willingly or unwillingly 

 become agents in the dissemination of plants over the earth. Man in the 

 development of commerce is often responsible for the wide distribution of 

 harmful 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 common 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 distance. In the impatiens, or touch-me-not as it is better 

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

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

 for several yards in all directions. During autumn, on dry days, the pods 

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

 apart, and the seeds are hurled away. 



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 



