cells composing it are larger than any of the other cells. Its upper 

 surface has a curious structure which has led us to refer to it 

 somewhat facetiously as the doormat (fig. 2) ; of this more be- 

 yond. 



A close examination of the door reveals its rather complex 

 and entirely surprising nature. It must be studied in situ in order 

 to understand its functioning for its shape when injured is very 

 different from that of the living door. It is attached to the walls 

 of the bladder along perhaps two thirds of its periphery — making 

 a hinge that takes the form of a wide arch composed of the top 

 edge and the upper curves of the lateral edges. The doorstep, 

 it will be remembered, merges into the lateral walls and its upper 

 limits are found to coincide with the lower limits of this hinge. 

 The edge of the door is free, then, for approximately the length 

 of the doorstep. The door is not a plane structure but presents 

 a convex face to the outside. This convexity is communicated to 

 the free edge and results in the rather curious path followed by it. 

 Let us consider this in some detail for it is here that recently 

 discovered facts throw light on a mechanism that has intrigued 

 botanists for seventy-five years. 



We have first to describe the general mechanics of the trap. 

 It is easy to see that the bladders of a plant differ in shape one from 

 another and we may distinguish three distinct states. Thus, in 

 figure 4 the same trap is shown as it appears at different times. 

 The middle photograph represents it with the lateral walls show- 

 ing a slight dimpling. In the course of half-an-hour or so this 

 concavity had increased considerably giving the trap the shape 

 figured to the left. The remaining photograph was taken after 

 the bladder wall had been pricked with a fine needle. Czaja (1922- 

 4) and Merl (1922) noted these changes in form. 



It is wise before considering the significance of these changes 

 to continue observation of bladders under natural conditions. The 

 small animals that may be found in the pools inhabited by 

 Utricidaria frequently enter the vestibules of the bladder and move 

 about in it in a manner that suggests the presence of some at- 

 tractive substance. If the trap is in the markedly dimpled condi- 

 tion a surprising sequence of events follows. There is a momentary 

 commotion and then we may see one or more animals within the 

 trap, their escape prevented by the closed door, and the walls of 



