62 



HEREDITY AND ENVIRONMENT 



Fig. -20. Dionaea muscipula (Venus' Fly-trap). Three 

 leaves snowing marginal teeth and sensitive hairs (SH). The 

 leaf at the left is fully expanded, the one at the right is closed. 



is called forth, but if they be stroked a second 

 time within three minutes the leaf instantly 

 closes. If a longer period than three minutes 

 elapses after the first stimulus and before the 

 second no visible response follows, i.e., two 

 successive stimuli are necessary to cause the 

 leaves to close, and the two must not be more 



