44 



Heredity and Environment 



very different from that to the first. Macfarlane found that if the 

 sensitive hairs on the leaf of Dionaea, the Venus fly-trap (Fig. 20, 

 SH), be stroked once no visible response is called forth, but if 

 they be stroked a second time within three minutes the leaf in- 

 stantly closes. If a longer period than three minutes elapses after 

 the first stimulus and before the second no visible response fol- 

 lows, i.e., two successive stimuli are necessary to cause the 

 leaves to close, and the two must not be more than three minutes 

 apart; the effects of the first stimulus are in some way stored or 

 registered in the leaf for this brief time. This kind of phenome- 

 non is widespread among living things and is known as "summa- 

 tion of stimuli." In all such cases the effects of a former stimu- 

 lus are in some way stored up for a longer or shorter time in the 

 protoplasm. It is possible that this is the result of the formation 



FIG. 20. Dionaea muscipula (VENUS' FLY-TRAP). Three leaves showing 

 marginal teeth and sensitive hairs (SH). The leaf at the left is fully 

 expanded, the one at the right is closed. 



