44 



Heredity and Environment 



Fig. 20. Dionaca 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. 



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

 sensitive hairs on the leaf of Dionaca, 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 



