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THE AMERICAN NATURALIST [Vol. XLIH 



observations were made by him, especially that the gland 

 at the tip of a tentacle is exclusively the sensitive region 

 and that a stimulus can be propagated down the cen- 

 tral tentacles and along to and up the adjacent ones. 

 Although he made most careful and repeated examination 

 of the protoplasmic behavior as the stimulus progressed, 

 and noted some curious changes which take place in the 

 protoplasm of the responding tentacles, yet he was un- 

 able to advance any explanation of the physics of the 

 process. Indeed, none has been offered since and much 

 that Darwin said is still the last word on the subject. 



Extending his work from the effect of mechanical 

 stimuli to chemical ones, Darwin showed that many sub- 

 stances of very various kinds could effect a very vigorous 

 response. He was, indeed, the first to make an en- 

 deavor at a close study of the insectivorous or rather 

 carnivorous plants. Not only were his observations on 

 the inflection of the tentacles of Drosera and the trapping 

 of insects by this and other forms largely new; but also, 

 and what was more important, perhaps, his investigations 

 into the digestive action of their secretions were really 

 the first in the field. His experiments along this line 

 were especially exhaustive; and, with the exception of 

 some details later writers have done little by way of 

 correction or amendment. He did not it is true, isolate 

 any specific enzymes but nevertheless we came to the con- 

 clusion that a pepsin-like enzyme was excreted by these 

 plants, which enables them to digest external proteid 

 material. He also noted the rennet-like action of the ex- 

 cretion in Drosera and arrived at the just conclusion that 

 the curdling of milk due to the latter could not come from 

 the acidity of the excretion alone, though at that time little 

 or nothing was known concerning vegetable rennets. It 

 is not to be wondered, when one considers the lesser 

 acquaintance which was then possessed of ferments in 

 general and plant enzymes in particular, that Darwin did 

 not further discuss or attempt to investigate the enzymes 

 which he had discovered and to determine whether they 



