CONDITIONS FAVOURABLE TO MECHANICAL KESPONSE 49 



apt to consider it as insensitive. But on applying the test 

 of electrical response, we discover that, though there is 

 no mechanical indication, excitation is nevertheless present. 

 Again, if the stimulus be sufficiently strong, the wave of 

 excitation will pass through the old leaf, without producing 

 any visible effect, and on reaching the younger will be 

 manifested by the conspicuous motile response of their 

 leaflets. 



Again, we have seen that one of the conditions for the 

 production of the responsive movement was the expulsion 

 of water from the excited tissue. Hence, if this expulsion 

 of water be in any way impeded, mechanical response may 

 not take place. This may be seen in the following experi- 

 ment : The cut end of a Mimosa stem is placed in water. 

 A large amount of water is now found to be absorbed, and 

 an abnormal turgidity is produced in the tissue, in conse- 

 quence of which the leaves are erected almost vertically. 

 If stimulus be now applied, there is no responsive movement 

 owing to the difficulty of the expulsion of water from the 

 gorged tissue. But the specimen is found to exhibit its 

 state of excitation by electrical response, thus proving that 

 not its sensitiveness, but its power of manifesting it mechani- 

 cally, has been arrested. 



We must bear in mind that, in these cases of differential 

 response, the efficiency of the motile apparatus depends 

 upon a delicacy of poise as between the two halves of the 

 organ, which is capable of being easily upset, under the 

 action of stimulus. This poise is determined by the antago- 

 nistic tissue-tensions of the two halves, and this again must 

 be modified by the distribution of water, or the relative 

 turgor-variations, in the two halves. Any deviation from 

 the normal distribution of turgidity might, therefore, be ex- 

 pected to affect the exhibition of the motile effect. Thus, 

 early in the morning, owing to excess of turgor-tension, 

 the leaflets of Biophytuiii show hardly any response, and 

 their motility disappears altogether, when the turgor is 

 raised still higher, on wet days. But later in the day, when 



E 



