294 



ECOLOG Y. 



the development of commerce is often responsible for the wide 

 distribution of harmful as well as beneficial species. 



460. Mechanisms for ejecting seeds. Other plants are more 

 independent, and mechanisms are employed for violently eject- 

 ing 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 



Fig. 248. 



Touch-me-not (Impatiens fulva) ; side and front view of flower below ; above unopened 

 pod, and opening to scatter the seed. 



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 whisked 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, when the seeds, as 

 from a catapult, are hurled away. 



Other plants have learned how useful the " wind " may be if 



