SEED DISPERSAL 



205 



341. Dispersal of seeds by expulsion. The seeds of many 

 plants are thrown for short distances by the sudden explosion of 

 the capsules, or by pressure which squeezes them out in such a 

 way that they are suddenly released. The valves of the pods in 

 the common vetch (one of the wild peas), as they dry, are brought 

 into a spiral tension so that they suddenly split apart and curl, 



Fig. 1 68. 



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

 pod, and opening to scatter the seed. 



thus throwing out the peas. In the " touch-me-not " (Impa- 

 tiens) the valves of the pod are also in tension, and when touched 

 or jarred they split and curl with sudden force, scattering the 

 seeds. In others, as in the witch hazel and violet, the pod splits 

 open and the valves squeeze from behind in such a way as to 

 force the seeds out in much the same manner that many a child 

 shoots watermelon or apple seeds by squeezing them from between 

 the thumb and finger. The " squirting cucumber " is so called 

 because by the absorption of water a pressure is produced which 

 squirts out the seeds. 



