CURVATURES PRODUCED BY CHEMICAL STIMULI 87 



is owing to this fact that when all the products of digestion have been 

 absorbed the leaves of Drosera^ Dionaea^ and Aldrovanda re-expand, 

 although a mechanical excitation may still be exercised by the undigested 

 remains. Since, however, it is entirely a question of specific excitability 

 it is not surprising to find, as was shown by Darwin, that in the case of the 

 leaf of Pinguicula vulgaris a chemical excitation does not persist much 

 longer than a mechanical one. Conjoint excitation produces varying 

 results, and although the summation is usually positive, chemical stimulation 

 causes the leaf of Dionaea to be less responsive to mechanical stimuli 1 . 



Both mechanical and chemical stimuli are perceived by the heads 

 of the tentacles of Drosera^ whereas the hairs on the leaf of Dionaea are 

 especially responsive to mechanical stimuli and much less so to chemical 

 excitations. The small gland-hairs of Dionaea appear to be the special 

 receptive organs for chemical stimuli, but show little or no mechanical 

 excitability 2 . The motor-mechanism excited by both forms of stimulation 

 may, however, be of similar character, and this applies even when, as in the 

 leaf of Dionaea^ the movements induced by mechanical excitations are more 

 rapid than those produced by chemical stimuli. 



Every vital response produced as the result of the chemical quality 

 of an absorbed substance may be regarded as being due to a chemical 

 excitation. This applies to all chemonastic movements, whether induced 

 by the presentation of some special material or by a quantitative change 

 in the composition of a nutrient medium. The chemical excitation may 

 naturally be transitory in character and need not necessarily produce 

 a permanent alteration. Instances of the former are afforded by the 

 contraction of the stamens of Berberis and the closure of the stigmas of 

 Mimulus produced by sudden exposure to ammonia vapour, and also by 

 the contraction of the stamens of Berberis and Helianthemum induced by 

 a sufficiently rapid fall in the partial pressure of the oxygen in the sur- 

 rounding air 3 . Since, however, in these cases as well as in that of Mimosa 

 the movement may be excited by various internal disturbances, it is 

 impossible to say whether a chemical excitant directly awakens a special 

 chemical irritability, or acts indirectly by inducing internal disturbances 

 which operate as the immediate exciting agencies. 



The same stimulus may in many cases excite several dissimilar 

 responses simultaneously, and in carnivorous plants stimulation may not 

 only induce movement but may also awaken, modify, or accelerate the 



1 Darwin, 1875, 1. c. ; Munk, 1. c., p. 99. 2 Cf. Darwin, 1. c., pp. 267, 295. 



3 Correns, Flora, 1892, p. 151. It is worthy of note that it is not every plant which readily 

 responds to seismonic stimuli that can be stimulated in this way. As Correns showed, the excitation 

 is not due to the mechanical disturbances induced by the sudden evacuation of the air. It is further 

 to be expected that many substances may produce a response in Mimosa if only they penetrate with 

 sufficient rapidity to produce a ' shock-effect.' 



