5i6 TRANSFORMATION OF ENERGY 



to stimuli, repeated at J to ^-hourly intervals. No investigations appear to have 

 been made as to whether or not the extent of the movement remains unaltered 

 according to the length of time the stimulation is continued. Before the normal 

 position of the leaf is reached, however, about 5 minutes after the first stimu- 

 lation, the leaf may again react to a new stimulus, but then the amplitude of the 

 movement is less. It may be supposed that in this case the capacity for reaction 

 is still limited, because the original expansive force in the under half of the 

 pulvinus has not yet been recovered ; on the other hand, the sensitivity may 

 still be normal. Conversely the sensitivity is obviously inhibited by con- 

 tinuous, feeble vibrations, while the capacity for response might be restored. 

 We may conclude from this that the leaf returns to its normal position 

 during the shaking and once more regains its resistance to flexion ; although 

 the leaf was not sensitive to shaking continued for 2-3 hours, it became once 

 more fully sensitive in 5-15 minutes after the cessation of the stimuli (Pfeffer, 

 1873 a). From this point of view there is a great difference between the present 

 case and hapto- and nycti-tropism, where the organs do not become accustomed 

 to stimulation or do so very slowly. 



There are many allied problems in this subject which yet await solution. 

 We know only that the sensitivity is in many respects dependent on external 

 conditions. High temperature, bright light and excessive moisture render the 

 plants extraordinarily sensitive, and under such conditions feeble stimuli operate 

 as well as strong ones, the responsive movement reaching its maximum ; but 

 when the sensitivity of Mimosa is decreased, owing to low temperature, 

 drought or the application of chloroform, ether, &c., feeble shocks induce a much 

 more limited depression of the leaf than strong shocks. Although we have no 

 detailed information on the dependence of the extent of the reaction on the 

 amount of the stimulus, we are better acquainted with the nature of the stimuli 

 to which Mimosa especially will respond. We have already found that feeble 

 contact with the sensitive part of the articulation induces a response ; it has yet 

 to be determined, however, whether the immediate conditions correspond to 

 those established for tendrils. 



The fact that the reaction is induced just as well by a liquid, e.g., rain- 

 drops, as by solid bodies, shows in the plainest possible way the great difference 

 in character that exists between sensitivity as exhibited by Mimosa and by 

 tendrils. Both react to a shock only, for statical pressure is followed by no 

 response in Mimosa. Pfeffer, for example, obtained no movement when he 

 gradually increased the weight bearing on the sensitive part of the pulvinus up 

 to 30 g. Mimosa reacts, however, to every shock, if it be sufficiently intense, 

 and a solitary stimulus is generally enough to bring about a maximum reaction. 

 In the case of tendrils, however, as we have seen, such shocks are operative only 

 when they affect neighbouring parts with unequal intensity ; in their case also, 

 several simultaneous or consecutive shocks are always necessary whose indi- 

 vidual effects are capable of summation. In Mimosa, on the other hand, 

 every sudden deformation of the cells of the sensitive half of the pulvinus is per- 

 ceived and is responded to by a movement. Special hairs, which are, however, 

 not confined in their distribution to the sensitive pulvinus only, serve as 

 additional organs of perception ; when these hairs, formed of thick- walled cells, 

 become bent as a result of pressure or a blow, the deformation of these cells 

 must, owing to their varied individual connexion with the sensitive paren- 

 chyma, be greater than that due to an equally strong pressure applied to the 

 outer surface of the pulvinus (Haberlandt, 1901). 



The leaves of Mimosa respond not only to a blow but also to injuries, and 

 movements take place after an incision or after the action of a burning-glass, 

 though dissimilar in intensity to those which are consequent on a blow. The 

 stimulus effected by a wound is much more rapidly transmitted than that induced 

 by a blow, but the execution of the movement from a mechanical point of view 



