5o6 TRANSFORMATION OF ENERGY 



pulvinus, (2) when it decreases in the concave side, (3) when both these conditions 

 occur, or (4) when an alteration in turgidity of the same kind but of unequal 

 amount takes place in both halves of the pulvinus. We might imagine that it 

 would not be difficult to determine which of these four possibilities is the cor- 

 rect one with the aid of the plasmolytic method ; but Hilburg (1881) found it 

 impossible to demonstrate any plasmolytic difference between darkened and 

 illuminated articulations. This is all the more astonishing when we remember 

 that in articulations which are geotropically stimulated, differences in the 

 turgidity of the cells on opposite sides may readily be demonstrated by the 

 plasmolytic method. The reason for the failure in the present instance is as 

 yet by no means apparent, nevertheless we must assume that differences in 

 turgidity are the causes of nyctitropic movement, and efforts must be made in 

 some roundabout way to obtain definite information on the subject, as, for 

 example, by determining the resistance to flexion and by experiments with 

 articulations which have been operated upon. 



As long ago as 1848, BrIjcke showed that the resistance to flexion in 

 the articulation was increased by darkening. In order to determine this resist- 

 ance he simply used the statical moment of the leaf and carried out his experi- 

 ment on the primary articulation of Mimosa in the following manner. He 

 inclined the plant carefully until the petiole of the leaf under consideration 

 occupied a horizontal position, that is, until the weight of the leaf exerted its 

 maximum effect on the articulation. After measuring the angle (a) between the 

 petiole and the stem, he turned the plant through an angle of 180° until the 

 petiole was again horizontal and once more measured the angle (a). The 

 difference between these two angles (a-a) affords an estimate of the resistance 

 to flexion of the articulation, since as the resistance increases the value of 

 the remainder decreases. In two experiments with Mmosa, Brucke found this 

 difference to be quite as great in the evening as in the morning or afternoon, but 

 in two other experiments the difference was markedly less (namely 12° instead 

 of 21°, and 15° instead of 27°). Pfeffer (1875) found that in Phaseolus the 

 difference was 18° to 20° in daylight, 9° to 10° in the dark, and he was able to 

 establish the fact that in many other plants an increase in the resistance took 

 place in the evening. The increase in the resistance to flexion expresses 

 nothing more, however, than that the tension of the parenchyma of the articu- 

 lation against the central vascular bundle has increased ; how this increase in 

 tension is distributed in the articulation, whether all longitudinal areas parti- 

 cipate or whether the osmotic swelling takes place only at definite situations, 

 we do not at present know. It is even possible that turgor is reduced on one 

 side only when it is increased on the opposite side. 



Since no conclusion can be drawn as to the mechanical cause of the bending 

 from the increase in the resistance to flexion, it will be necessary to experi- 

 ment with half articulations after removal of the other halves. With this end 

 in view Pfeffer removed the upper half of the articulation in the case of one 

 primary leaf of Phaseolus, and the under half in another leaf, and arranged the 

 leaves in an appropriate dynamometer so that the pressure activity of the ex- 

 panding articulation could be recorded. The results of these two experiments 

 were very remarkable ; both halves of the articulation reacted in exactly the 

 same way : when darkened expansion took place, when illuminated contraction, 

 in each half. The movements which the leaf executed, when provided with one 

 half-articulation only, were exactly opposite in direction according to whether 

 the upper or the under half of the articulation was retained. The leaf provided 

 with the upper half of an articulation became depressed upon darkening, and 

 elevated itself when illuminated ; the leaf provided with the under half of the arti- 

 culation elevated itself in the dark and became depressed in light. Since, in the 

 dark, both halves of the articulation show increased turgidity the conditions 



