514 TRANSFORMATION OF ENERGY 



appearance of the plants and smaller ones will be frightened by the movement 

 itself. In the great majority of cases, however, where leaves react in this way 

 to a shock stimulus, the response, even after a vigorous excitation, appears very 

 slowly. The object of the movements in stamens and stigmas is much more 

 apparent, for in these cases they are obviously almost always related to the 

 mode of pollination of the flower. 



Let us now study the movement in Mimosa a little more closely. As already 

 stated curvatures take place in all three sets of articulations. In order to study 

 the changes which precede these movements we will naturally confine ourselves 

 to the largest pulvinus at the base of the primary petiole. Curvature in this pul- 

 vinus occurs not only when the whole plant is shaken but also when the articula- 

 tion itself is subjected to a slight shock. It may be easily shown that it is only 

 the under half of the pulvinus that is sensitive, for even a gentle friction of that 

 side with a piece of stick at once induces a response, while the upper side may 

 be much more vigorously stimulated without any result ensuing. Since at last 

 a response is obtained by vigorous stimulation of the upper side, the reason for 

 this lies in the fact that the stimulus has been transmitted to the lower half 

 of the cushion. Experimental evidence on this point was first brought forward 

 by LINDSAY in 1790, who showed that when the upper half of the pulvinus 

 was removed the leaf remained capable of carrying out these movements, but 

 that rigor set in when the lower half was cut off. (For the older literature 

 see PFEFFER, 1873 a). 



The more exact conditions of curvature are to be obtained by investigating 

 the resistance to flexion, and by determining the alterations in volume taking place 

 in the two halves of the pulvinus. The resistance to flexion increases in the night 

 position but decreases markedly after a shock stimulus. BRUCKE (1848) found 

 the difference in the angles (a-a) in the two positions (compare p. 506) after 

 stimulation to be two or three times as great as in the unstimulated condition, 

 and so was able to establish a fundamental difference between the similar ' sleep ' 

 and ' stimulated ' positions of Mimosa. It is to PFEFFER (1873 a) that we 

 owe accurate investigations as to the changes in volume of the articulation. He 

 proved by measurements, taken with the aid of the microscope, that the upper 

 half of the articulation showed a quite insignificant increase during the curva- 

 ture, but that the under half markedly decreased. Since the movement was 

 carried out just as well when the epidermis had been carefully removed, and 

 since the axial vascular bundle need not, on account of its rigidity, be taken into 

 account, the reduction in volume observed can be accounted for only by a con- 

 traction in the parenchyma on the under side. This contraction is accompanied 

 by a relaxation in the two halves of the pulvinus, which may be directly 

 estimated if the effort to droop on the part of the leaf be prevented by some 

 opposing force. In a more recent research PFEFFER has estimated this pressure 

 at between 2 J and 5 atmospheres. The expansive powers of the under half of 

 the pulvinus is reduced to this extent, while apparently no change of any kind 

 occurs in the upper half. 



Whence this decrease of expansive force on the under side of the pulvinus 

 arises cannot be settled straight away. In order to answer this question clearly 

 experiments on amputated leaves are essential. Such leaves exhibit rigor at once 

 on amputation, but, if they be kept in a saturated atmosphere, they regain their 

 capacity of response to stimulus after a time, the primary articulation exhibiting 

 inward curvature after contact, which may have a radius of 5 mm., while 

 normal articulations exhibit a curve with a radius of only 3-4 mm. After 

 stimulation of the amputated leaf it is possible to see a certain extravasation 

 of liquid from the cut surface, which, if evaporation be prevented, is again 

 absorbed as the curvature becomes gradually relaxed. This liquid is excreted 

 from the parenchyma of the sensitive half of the pulvinus, and it is only after 

 vigorous stimulation that the upper half helps in producing the excretion, and 



