MECHANICAL RESPONSE IN ORDINARY LEAVES 63 



records of such movements in these different cases on the 

 same revolving drum. And with the purpose of making the 

 angular movements, in the -various instances of long and 

 short leaves, comparable, I rendered the virtual length the 

 same in all cases, by attaching longer or shorter indicators, 

 as was necessary, and the movement of the tip of the indi- 

 cator or its projection was then recorded on the drum. The 

 longest leaf experimented on in this series was Mimosa, 

 having a length of 5 cm. In this case no additional indicator 

 was attached ; and in other cases the indicator-lengths which 

 were added made each leaf have a virtual length of 5 cm. 



Fatigue-reversals in ordinary leaves, as in Mimosa. — 

 In comparing the response-records of various specimens, it 

 is necessary to understand one peculiarity which is observed 

 in certain responses under long-continued stimulation. In 

 order to do this, I must anticipate a certain characteristic 

 of Mimosa response, which will be more fully described in 

 Chapter IX. It is found that the leaf of Mimosa gives imme- 

 diate response to stimulus, by a fall or contraction of the 

 lower half. But when the stimulus is long continued, the 

 leaf rises gradually to its original position, or even, some- 

 times, beyond this. This position is, however, only apparently 

 normal, being really a posture of fatigue. For whereas, 

 while the leaf is fresh, it responds to stimulus by depression, 

 it is now, though it occupies the same position, entirely in- 

 susceptible of excitatory depression. In exhibiting the effect 

 of long-continued stimulation on ordinary leaves, I shall be 

 able to show that they resemble Mimosa, not only in the 

 extent of their responses, but also, in certain cases, even with 

 regard to this specific characteristic. 



In the records given (fig. 33) the first curve is of Mimosa 

 leaf. It should be remembered that the experiments in all 

 these cases were carried out with the leaves held perpendicu- 

 larly downwards, the excitatory movement being produced 

 by strong and continuous tetanic shocks. The excitatory 

 effect recorded, therefore, is due to the differential contraction 

 of the two halves of the motile organ, lifting the leaf against 



