340 



TEXTBOOK OF PLANT PHYSIOLOGY 



lability of animals. It may be suppressed either by a lowering 

 of temperature to below 15° C, or by various anaesthetics, espe- 

 cially the vapors of ether or chloroform. 



What seismonastic sensitiveness really consists of has not yet 

 been discovered. The only thing so far that has been established 

 is that it is unequally distributed through the plant and that the 

 most sensitive part is the lower surface of the pulvinus, the swollen 

 base of the petiole. The mechanism of the movement consists of a 

 change of turgor in the upper and lower halves of the pulvinus 



which supports the leaves of Mimosa 

 (Fig. 141). An irritation suddenly 

 alters the permeability of the pro- 

 toplasm in the lower half of the 

 pulvinus, A. The cell wall, which 

 A— il H was formerly in a condition of ten- 



sion, not being distended by the 

 protoplasm, contracts. At the same 

 time part of the cell sap is pressing 

 out into the intercellular spaces. This 

 results in a rapid decrease of turgor 

 in the lower half of the pulvinus. 

 As the upper half B preserves all of 



Fig. 141. --The pulvinus of a leaf itg tension, the petiole will bend 

 peduncle of Mimosa. On the left, ' 



showing external view. On the downwards and the leaf droops. 1 his 

 right, A, section of the lower half extrusion of sap from the cells into 



of the pulvinus; B, section of .... 



upper half (after Transeau). the intercellular spaces is indicated 



by the lower half of the pulvinus 

 becoming transparent. If a shock is produced in a leaf that has 

 been cut off close to the stem and left to recover in a humid 

 atmosphere, then it may be observed that the pulvinus curves 

 and that a drop of liquid appears at its cut end. As the reaction 

 passes, the liquid is absorbed by the leaf and the pulvinus again 

 straightens out. 



The transmission of stimulus in Mimosa occurs with such rapid- 

 ity that it cannot be explained as being due to diffusion of a 

 hormone, which after its formation in the irritated cells, must be 

 moved through the parenchyma tissues of the stem. Moreover, it 

 has been observed for some time that the excitation may be trans- 

 mitted through dead sections of the stem, and that, even if the 

 stem has been cut across and then joined together by a glass 



