Chap, xi Sensitivity and Stimulation 485 



was found to be differentiated from the contact irritability, 

 the differentiation being possibly most complete in Pingui- 

 cula. Darwin found that mechanical stimulation of the 

 leaves of the latter caused them to curl over at their edges 

 but did not affect the secretion. The application of car- 

 bonate of ammonia caused secretion but not movement. 

 The perceptive organs for the stimulus of contact in Dionaea 

 were found to be the large hairs upon the leaf-blades, 

 those for the appreciation of chemical substances the small 

 glandular hairs scattered all over the surface. 



Darwin's observations were not quite the earliest indi- 

 cating sensitiveness to chemical excitation. Pfeffer showed 

 in 1873 that the application of chloroform vapour causes 

 curvature of the pulvinus of Mimosa. This, however, may 

 be a local effect upon the protoplasm and need not indicate 

 a sensory mechanism. After Darwin's work appeared, 

 examples of the possession of this form of sensitiveness 

 by plants multiplied considerably. In 1881 Engelmann 

 described the effect of oxygen in causing movement of 

 Bacterium termo, and De Bary showed that chemical stimuli 

 aid in bringing together the sexual structures in Sapro- 

 legnia. Pfeffer's proof that solutions of malic acid and of 

 sugar lead the antherozoids of ferns and mosses respec- 

 tively to the open necks of the archegonia dates from 

 1883 ; in 1884 Molisch and De Bary independently showed 

 that sensitiveness of this kind is manifested in the approxi- 

 mation of the pollen-tube to the ovule, while Stahl and 

 Pfeffer separately described the behaviour of various Myxo- 

 mycetes under the attraction or repulsion of such chemical 

 substances as nitre, glycerin, sugar, and extract of tan. Two 

 years later Berthold showed that the ooblastema filaments 

 of Dudresnaya respond to chemical stimulation when they 

 fuse ; in 1888 Overton, and in 1890 Haberlandt, proved 

 that similar sensitiveness guides the approximation and 

 subsequent fusion of the filaments of Spirogyra. In l88g 



