LAWS OF RESPONSIVE GROWTH-CURVATURES 525 



has all the characteristics of the tip of the organ, while the 

 peduncle below often represents the area of quickest growth, 

 and functions as the responding region. Another advantage 

 of the unopened flower-bud, again, lies in the fact that its 

 upper end is not covered over with appendages like that of 

 the stem. The unopened buds, with peduncles, of Crocus^ 

 then, will be found suitable for this investigation. As 

 specimens of the roots, again, the long straight water-roots of 

 Bindweed are very suitable for these experiments. 



The records of the responsive movements are taken by 

 means of the recording microscope fully described in a 

 subsequent chapter. The quiescence of the organ, before the 

 application of stimulus, is tested by the fact that the record 

 is a horizontal line. The occurrence of an up curve in the 

 record represents responsive movement towards, while a 

 down curve means movement away from, the stimulus. In 

 this investigation, as in the preceding, various forms of 

 stimulation have been employed. Mechanical stimulus is 

 applied by friction of emery-paper. The jar produced by 

 this causes a temporary disturbance of the image in the field 

 of view of the microscope ; but this soon subsides, and the 

 excitatory movement commences after a short latent period, 

 increasing steadily until the effect of stimulus is exhausted. 

 The chemical form of stimulation has the advantage of 

 producing no mechanical jar. The stimulation produced by 

 light, which is the most perfect, will be described in the 

 subsequent chapter on heliotropism. Thermal stimulation is 

 effected by holding a platinum wire, heated by the electrical 

 current, in more or less proximity to the point in the tissue 

 which is to be excited. 



I. Mechanical response to unilateral stimulation of the 

 tips of shoot and root : {a) Moderate j-Z/w/^/z/j.— Applying a 

 single mechanical stimulus of emery-paper friction, of mode- 

 rate intensity, unilaterally to the bud of Crocus, a movement 

 was induced in the responding organ, which carried the tip 

 away from the source of stimulation. This movement 

 persisted for four minutes, after the application of this single 



