2i 4 TROPIC MOVEMENTS 



established the application of Weber's law to plants, and its extension to the 

 chemotropism of fungi and pollen-tubes was shown by Miyoshi, to phototropism by 

 Massart, and to geotropism by Czapek 1 . 



Pfeffer placed freely motile organisms in water or in solutions of stimulatory 

 materials, and determined the excess concentration required in capillary tubes to 

 produce a chemotactic attraction. In the case of the sperms of Ferns 2 , the liquid 

 in the tube must contain thirty times as much malic acid as that outside, and in the 

 case of Bacterium termo 3 , about three to four times as much meat-extract as in that 

 outside. Thus o-ooi per cent, of meat outside requires at least 0-003 percent, inside, 

 and i per cent, outside needs 3 per cent, inside the tube to produce a chemotactic 

 attraction of the bacterium used. Miyoshi found that a five times greater concentra- 

 tion was required to attract pollen-tubes, and a ten times greater concentration to pro- 

 duce a chemotactic attraction in the case of Saprolegnia. 



Massart 4 placed the sporangiophores of Phy corny ces between two constant sources 

 of illumination, and determined at what relative distances from the two sources 

 a curvature was just produced. Since the intensity of the light is inversely pro- 

 portional to the square of the distance, it is easy to calculate how much more 

 strongly one side must be illuminated than the other to produce a heliotropic 

 curvature. A difference of illumination of one-fifth was found to be necessary ; 

 so that plants are less sensitive than man, who is able to detect a difference of 

 illumination of one-hundredth. We are, however, only able by our sense of touch 

 to detect increases or decreases of weight of one-third, and similar relationships 

 hold good in regard to our sense of smell and of warmth. It must, however, be 

 remembered that in the case of the plant our only evidence of perception is an 

 actual response, and that a feeble stimulus might be perceived but not be able to 

 excite any curvature. 



Not only may the diffuse action of light or of chemical substances weaken 

 the tropic irritability, but also the performance of a response may have the same 

 effect. This is shown by the fact that as a tropic stimulus increases in intensity 

 the time of reaction is at first rapidly but subsequently slowly shortened. Thus 

 Czapek 5 found that the time of reaction of a root of Lupinus exposed to centrifugal 

 action equivalent to o-ooi and to i g. fell from six hours to one-and-a-half hours, but 

 only decreased to forty-five minutes when the centrifugal force rose to 40 g. The 

 times of induction afford, in fact, an indication of the relationship between the 

 excitation and the intensity of the stimulus. Diffuse and tropic actions probably 

 do not influence the excitability in precisely the same way ; but no investigations have 



1 Miyoshi, Bot. Ztg., 1894, p. 21 ; Flora, 1894, p. 81 ; Massart, La loi de Weber, etc. Bull, 

 de 1'Acad. royale de Belgique, 1888, 3" se>., T. XVI, No. 12 ; Czapek, Jahrb. f. wiss. Bot., 1898, 

 Bd. xxxn, p. 191 ; 1895, Bd. xxvn, p. 305. 



a Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 397. 



3 Pfeffer, 1. c., 1888, Bd. n, p. 634. The fact that the stimulation of bacteria is due to phobo- 

 chemotaxis is immaterial. 



* Id. Massart used the light reflected from a single lamp by a pair of mirrors at varying 

 distances. 



5 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 191 ; 1895, Bd. XXVII, p. 305. 



