246 HISTORY OF BOTANY 



been conveyed or transmitted from the original point of 

 application to all these regions of response, and, second, 

 that there must exist some mechanism by which the 

 movements are carried out ; in other words, the plant 

 must possess something analogous at least to nerves and 

 contractile tissue. Whatever the nature of the trans- 

 mitting agency may be, we have to admit that it is nothing 

 like so efficient as the corresponding agency in an animal. 

 In the human nerve the rate of transmission has been 

 estimated at about 50 metres (160 feet) per second, 

 while that in the " Sensitive Plant " is variously estimated 

 at from 1-5 cm. to 10 cm., i.e. at most 4 inches per second. 

 You will remember that Tangl, Gardiner, and others 

 long ago demonstrated the existence of exceedingly 

 delicate protoplasmic threads connecting the protoplasts 

 of adjacent cells, and many authorities, notably Stras- 

 burger, Czapek, Hill, and Haberlandt, consider that the 

 excitation may be transmitted along such intercellular 

 protoplasmic fibrillae. Nemec, in the beginning of the 

 present century, went a step further and claimed to have 

 demonstrated very fine fibrillae or longitudinal proto- 

 plasmic tracts in many elongated cells, extending from 

 one transverse wall to the opposite one, but Haberlandt 

 failed to find conductors in the sensitive organs he 

 examined, with the solitary exception of Mimosa, the 

 "Sensitive Plant." In this much - investigated plant 

 Haberlandt considers the transmitting elements to be 

 certain elongated tubular cells with delicate pitted walls 

 associated with the bast region of the vascular bundles. 

 Haberlandt explains the transmission of the excitation 

 in the following words : 



" When the pulvinus of a pinnule moves upwards 

 in response to a shock, pressure is exerted upon the 

 highly turgescent transmitting cells, partly owing to 

 changes in the form and volume of the relaxed half 

 of the pulvinus, and partly owing to the mechanical 

 effects of the curvature ; the local rise of pressure pro- 



