Chapter X — 141 — Drosera 



or of a human hair 8/1000 of an inch in length and weighing only 

 1/78740 of a grain (.000822 milligram) or particles of precipitated 

 chalk, after resting for a short time on a gland, should induce some 

 change in its cells, exciting them to transmit a motor impulse through- 

 out the whole length of the pedicel, consisting of about 20 cells, to 

 near its base, causing this part to bend, and the tentacle to sweep 

 through an angle of above 80 degrees". 



It was generally conceded by both Nitschke and Darwin that 

 dead bodies do not provoke so much response as hving and therefore 

 moving bodies. This was explained by Pfeffer by pointing out that 

 mere constant contact does not produce response, but that there must 

 be both direct contact with the gland and friction on its surface. The 

 mucilaginous drop can prevent direct contact as in the case of rain or 

 quicksilver (which Pfeffer tried) or even particles suspended in it un- 

 less by their weight they fall against the sensitive surface. That the 

 minute particles of hair used by Darwin should produce the results 

 observed may be understood better when, as Pfeffer showed, vibra- 

 tion of the table or floor causes movements of such particles on the 

 surface of the gland sufficient to stimulate it. 



In addition to non-living substances, Darwin tested the reactions 

 of the tentacles to a large variety of organic materials with the purpose 

 of determining what digestive juice or juices are secreted by the leaves 

 of Drosera. His contribution to the problem of digestion will more 

 suitably be considered under the appropriate caption beyond. Here 

 it will be mentioned that he seemed to regard the movements of the 

 tentacles and the length of time they remain inflected as evidence of 

 the nutritional value to the plant of the material exposed to them. 

 But he himself records a various behavior of the tentacles in this re- 

 gard. He says in conclusion "The substances which are digested by 

 Drosera act on the leaves very differently. Some cause much more 

 energetic and rapid inflection of the tentacles and keep them inflected 

 for a much longer time, than do others. We are thus led to believe 



that the former are more nutritious than the latter " This 



generalization can hardly hold. Robinson found that pure creatin was 

 digested but caused no bending of the tentacles. As Schmid points 

 out, Darwin's work, rightly or wrongly, led emphasis to be too strongly 

 placed on the Drosera mechanism being an adaptation for the obtain- 

 ing of protein nutrition. While it is true that, to quote Darwin again, 



" inorganic substances, or such substances as are not attacked by 



the secretion, act much less quickly and efficiently than organic sub- 

 stances yielding soluble matter which is absorbed" it is also true that 

 some nitrogenous bodies equally do not, and therefore it is impossible 

 to formulate a rule. Darwin himself records the failure of urea to 

 procure movements. What explanation serves when HCl, boric acid, 

 malic acid and camphor stimulate to movement when Ca, Mg and K 

 salts generally do not? And ammonium phosphate was found more 

 energetic than other ammonium salts though containing less nitrogen. 

 But because potassium phosphate is taken up Darwin argued a need 

 for phosphorus. Schmid, considering this phase of the insectivory 

 problem, himself tested the action of pure salts and concluded that the 

 movements of tentacles alone cannot lead to any real index of the 

 value of insectivory from the nutritional-ecological point of view. 



