Chapter XIV 



235 



The Utricularia Trap 



found entomostraca in the traps of U. deistogama. Prompted by 

 this she made a careful examination to see if she could observe the 

 method of capture. She thought in 1875 that the animals "open the 

 door and walk in", agreeing with Darwin and with Cohn that prey 

 push in the door, which then closes and prevents escape. In her 1876 

 account she re\ased her conclusions, for she then found, in U . purpurea, 

 that prey is suddenly engulfed, as if drawn into a "partial vacuum". 

 Not seeing that the trap walls change their posture, she was ignorant 

 as to how the vacuum could be achieved; yet her idea foreshadowed 

 the discovery to be made in 191 1 by Brocher. Mrs. Treat learned 

 through Dr. Asa Gray, in correspondence with Charles Darwin, 

 that the latter was making similar studies; so that it is of interest to 

 see that Cohn, Darwin and Mrs. Treat, whom Darwin later quoted, 

 were arriving at similar conclusions at the same time independently. 

 She further saw evidence that larvae were digested in the course of 

 48 hours. '"'I was forced to the conclusion that these httle bladders 

 are in truth like so many stomachs, digesting|and assimilating animal 

 food", she remarked. 



Fig. 7. — Copies of the original drawings of Cohn (left) and of Brocher, of the en- 

 trance of the trap of Utricularia vulgaris. 



Cohn's and Darwin's conceptions of how the trap works were 

 identical as is shown by their descriptions. Cohn said that "the 

 valve is held against the threshold by a pressure of water within the 

 trap, but that it is easy to open by pushing it inwards. This ar- 

 rangement makes it understandable that living water animals, en- 

 tering the peristome, lift the valve and without difficulty enter into 

 the hollow cavity of the bladder, whence they cannot escape since 

 the valve opens only inwardly, not outwardly." And Darwin spoke 

 in the same manner, saying that "animals enter merely by forcing 

 their way through the slit-Hke orifice; their heads serving as a wedge." 

 GoEBEL accepted this explanation, as did Meierhofer and Luet- 

 ZELBURG. An impressive drawing by Goebel as well as that by 

 Cohn (Text fig. 7), though incorrect, are still used as illustrations. 

 It is clear that up to this time the trap was regarded as a passive 

 mechanism, the animal caught having to do the work of forcing en- 

 trance. We must add however that it was thought that the door 

 was either forced against the threshold by a "m a tergo", the water 



