396 SCIENCE PROGRESS 



fortunately the innumerable investigations in this field have 

 shown more and more clearly that it is not possible to formulate 

 a general mathematical law which strictly fixes the relations 

 of the intensity of the stimulus and the intensity of the re- 

 sponse " {Irritability, New Haven, 191 3, p. 48). 



The problems of contact stimulation and stimulation by 

 wounding have been attacked by Stark. In regard to the 

 former it is shown {Jahrb. f. wiss. Bot. 57, 189-320, 1916 ; 

 Ber. deut. bot. Ges. 35, 266-91, 191 7) that haptotropism, the 

 contact irritability manifested by climbing plants, is widely 

 distributed although slightly developed throughout the plant 

 kingdom, being most evident among non-climbers, in Gra- 

 minese, Cruciferae, and Caryophyllacese. With the exception 

 of the side roots of Phaseolus, haptotropism is apparently 

 non-existent in roots. It is in the principal growing region 

 that curvatures begin, and it is in this regftm that they are 

 most readily induced. The reaction time and the magnitude 

 of the curvature depend upon the species, the age of the organ, 

 and the strength of the stimulus. Continued stimulation 

 results in over-excitation with the consequent failure of curva- 

 ture. In some seedlings {e.g. sunflower, flax, oat) curvatures 

 may appear as a result of wilting if the air is rather dry and if 

 the surface is rubbed with a rather rough surface. This cur- 

 vature as a result of wilting is simply due to removal of the 

 wax from the cuticle which renders it more permeable to water. 

 If, however, the seedlings are stroked with a smooth surface 

 and iri a damp chamber, a curvature due to contact results 

 which is often accompanied by greatly increased growth. 



Stimulation due to wounding was effected in various ways 

 by Stark {Ber. deut. bot. Ges. 34, 492-508, 1916 ; Jahrb. f. wiss. 

 Bot. 57, 461-552, 191 7), as for example by incisions, by burning, 

 or by corrosion with caustic substances such as silver nitrate. 

 In all cases the axis curves towards the wounded side, and the 

 reaction occurs in the dark as well as in the light, and under 

 water as well as in air. In a variety of instances it was shown 

 that the reaction region and perception region are different. 

 For example, in Graminese if the coleoptile is stimulated by 

 local cauterisation with silver nitrate the reaction occurs and 

 is confined to the apex of the hypocotyl. On wounding an 

 organ narcotised by ether no reaction takes place, but narcosis 

 after stimulation by wounding does not prevent the reaction, 



