2 4 o PLANT PHYSIOLOGY 



(/) Thermotropism. 



True thermotropic movements are not common, though they 

 may be demonstrated experimentally (DETMER, 478), but there 

 are some prominent thermonastic responses, as in the opening 

 and closing of some flowers, and in the alteration of position of 

 Rhododendron and other leaves. These are quite distinct in 

 kind from Thermometric movements, one of which I have myself 

 described in Annals of Botany, 18, 1904, 631. All of these 

 matters, however, are difficult of experiment, but the student 

 should follow them through the literature. 



(g) Other Tropisms. 



In addition to the more important forms of irritability above 

 mentioned, there are many others of minor importance, some 

 of which are only modifications or substitutions of those already 

 described. The more important are: Traumotropism, or re- 

 sponse to injury, which may be demonstrated by methods given 

 by DARWIN in his " Power of Movement in Plants " (compare also 

 REED, Journal of Applied Microscopy, 6, 2466, and SPALD- 

 ING'S discussion in Annals of Botany, 8, 1894, 423); Rheotropism, 

 response to direction of water-currents, which is proving simply 

 a phase of Thigmotropism (compare NEWCOMBE in Botanical 

 Gazette, 33, 1902, 177); Electrotropism and Galvanotropism, or 

 response to electrical currents, of some special interest, since 

 their responses could not have been adaptively acquired and 

 must be simply coincidental; responses to mechanical strains, 

 resulting in development of strengthening tissues; responses 

 in varying thickness of cuticle, etc., in desert plants in relation to 

 degrees of dryness; autotropic responses, or those determined 

 by form conditions of the organism itself. 



Finally the student should make a classification of the dif- 

 ferent known forms of Irritability, giving their names, physical 

 character of the stimulus, mode of its impress upon the proto- 

 plasm, place of the same, directions of the known responses, 

 nature of the responding mechanism, and ecological significance. 



