CHAP, xi Sensitivity and Stimulation 495 



whose primordia are radially arranged, is induced by the 

 action of gravity. 



The mechanism of the transmission of stimulation was 

 investigated by Pfeffer in 1873. He showed that it can 

 travel over chloroformed parts of stems, and hence he 

 sought to explain it by the pulsation of water in the tissues. 

 When a stem of Mimosa is cut, a stimulus is exercised only 

 if the section goes as deep as the vascular cylinder and 

 a drop of water escapes. Then the leaves are gradually 

 affected as the disturbance of the hydrostatic equilibrium 

 passes on. Hence he held that the transmission of the 

 stimulus is due to movements and changes of pressure, in 

 the water in the vascular bundles. Ziegler, in 1874, and 

 Batalin, in 1877, supported the view that the vessels form 

 the path for the transmission of stimuli. With the demon- 

 stration of the continuity of the protoplasm through the cell 

 walls of contiguous cells by Gardiner and by Russow in 1883, 

 the probability of a transmission resembling that of a nervous 

 impulse in animals was at once realized. This hypothesis 

 did not, however, at once meet with acceptance. In 1890 

 Haberlandt reaffirmed Pfeffer's view that dead regions of 

 both stem and leaf retain conductivity after being killed, 

 In 1896 he advanced the view that in Mimosa the conduction 

 is due to the tannin tubes (Schlauchzelleri) of the phloem, 

 which transmit positive or negative pressure waves to the 

 pulvini. These mechanical explanations seem, however, 

 insufficient to account for the rapidity with which a normal 

 response occurs, and in 1896 McDougal showed that varia- 

 tions of hydrostatic pressure, whether positive or negative, 

 when artificially produced do not constitute a stimulus in 

 Mimosa. 



In 1888 Oliver suggested that the path in Masdevallia 

 is a sheath of thin-walled parenchyma accompanying the 

 xylem. Czapek held that the path of transmission is not 

 the vascular tissue, but the fundamental parenchyma, as 



