GROWTH SUBSTANCES FOR TRAUMATIC CURVATURES 217 



Ricca (1916, 1926) observed that a stimulus caused by burning 

 the lower part of the stem can be conducted to the younger leaves 

 through a killed zone or even through water in a short tube. An 

 extract from stem tissue was shown to bring about closure of the 

 leaflets when the petiole of a detached leaf was dipped in 

 the solution. From these experiments, Ricca concluded that 

 the response in Mimosa must be due to a stimulating substance. 

 Movement of the stimulus through the transpiration stream 

 and other rapid means of conduction have been postulated on the 

 basis of experimental data by Snow (19246, 19256, 1925c), Dixon 

 (1925), Ball (1927), Bose and Das (1925), and others. Linsbauer 

 (1908) and Umrath (1925a, 19256) found that the rapidity of 

 stimulus conduction varies with the intensity and the type of 

 stimulation and the parts of the plant concerned. Carrying 

 forward these investigations with a technique proposed by Bose 

 (1926), Umrath (1928, 1929) was able to demonstrate several 

 rates of conduction by measuring the electrical changes accom- 

 panying excitation. 



The conclusions of the majority of previous investigators are 

 summarized, and the results of new experiments are presented, 

 in the recent paper of Houwink (1935). This author studied the 

 transfer of stimuli in Mimosa as evidenced by outward move- 

 ments of leaves and internal fluctuations of electrical potential. 

 For example, stimulation without womiding by the application 

 of a drop of water at less than 10°C. brought about an excitation 

 which was conducted by the living cells. The rate of conduction 

 of the "action currents" depended upon the temperature. Con- 

 duction did not occur through a killed part of the petiole or 

 through a zone cooled to 5°C. Wounding or burnmg led to the 

 formation of a stimulating substance which caused a change in 

 potential. This substance passed through dead zones and 

 through parts of the plant that did not produce it. It was taken 

 in from a wound by the negative pressure in the vessels and trans- 

 ported in the transpiration stream. Excitation brought about 

 by cutting a leaflet could be conducted (1) by action of the cells 

 ("action currents"), (2) by the transport of the wound substance, 

 and (3) by a very rapid mechanism of conduction affecting only 

 the main pulvinus. This last phenomenon was not accompanied 

 by changes in electrical potential and was not conducted through 

 a killed zone but passed through a cooled portion of the petiole. 



