508 TRANSFORMATION OF ENERGY 



ments as possible, we must, at the same time, consider the possible effect of geo- 

 tropism on sleep movements. In fact, when we remember that a leaf when 

 executing sleep movements comes into entirely different relations with gravity, 

 the question arises whether gravity does not operate so as to bring it back again 

 into the old position. This effect of gravity, however, has not as yet been demon- 

 strated, indeed the effect is quite otherwise. In 1875, PFEFFER found that when 

 a bean was placed in an inverted position, the sleep movements took place in 

 the reverse direction so far as the plant was concerned ; that is to say, they 

 retained their relation to the direction of gravity. FISCHER (1890) snowed 

 that nyctitropic movements in the bean ceased when the plant was rotated 

 on a klinostat. From these observations we may conclude that we have to deal 

 in this case not with nyctitropic, but with geotropic movements, and that the 

 plant in consequence of being darkened, reacts geotropically otherwise than 

 before. Similar alterations in the geotropic rest position we found took place when 

 rhizomes and roots were illuminated ; but plants do not all behave in the same 

 manner. Whilst Lupinus albus agrees in all essential features with Phaseolus, 

 FISCHER found that in the case of Amicia, Desmodium, Acacia, Mimosa, &c., 

 even after twelve days' rotation on the klinostat, nyctitropic movements retained 

 completely their original direction. From this we must conclude that there are 

 two types of plants which, following FISCHER, may be distinguished us auto- 

 nyctitropic and geonyctitropic. We confine ourselves merely to the statement 

 of this fact, but we may at least note that FISCHER'S experiments are not so 

 easy of explanation as one might at first sight think. NOLL (1892) has drawn 

 attention to the possibility that dorsiventral organs are geotropically stimulated 

 when rotated on a klinostat, and SCHWENDENER (1892) has shown that Phaseolus, 

 at least in the first days during which the plant was rotated, preserved its sensi- 

 tivity to changes in illumination. Further research in this subject will be 

 necessary before we can give a clear account of the phenomena. 



A very large number of, though by no means all, plant organs which carry 

 out nyctitropic movements present difficulties in the explanation of the mechanics 

 of the process of curvature for one special reason ; the nyctitropic movements 

 continue after the cause, namely alteration in illumination, ceases (periodic move- 

 ments). This phenomenon may be remarkably well seen in Mimosa or Acacia 

 lophantha, when these plants are kept in the dark at a constant temperature. 

 For days they open their leaflets in the morning and close them in the 

 evening at approximately the same time as those plants which are exposed to 

 periodic alterations in illumination. These movements, which are of the nature 

 of after-effects, gradually cease, since the leaf becomes pathological when kept 

 in continued darkness, and changes are set up in it which lead, in the first place, 

 to abolition of the power of movement, that is to say, to darkness-rigor (SACHS, 

 1863), and in the long run even to death. The absence of light induces this con- 

 dition of rigor only indirectly, for we can render the leaves of Mimosa incapable 

 of movement when completely illuminated also, if we at the same time deprive 

 them of access to carbon-dioxide (VocHTiNG, 1891). On the other hand, leaves 

 which have been cultivated in the dark live and exhibit movements for a much 

 longer time in absence of light than those which have grown to maturity in light. 

 Obviously the rigor is due to injuries suffered by the leaf in consequence of an 

 interference with the chlorophyll function. 



As we have mentioned above, these after-effects are also met with in leaves 

 which carry out nyctitropic curvatures by growth, such as the leaves oiNicotiana ; 

 they are also met with in many flowers. On the other hand, they occur in leaves 

 which, after being in the dark for a short time, cease to move, although 

 they still retain their capacity for movement, as, for example, Tulipa, Robinia 

 pseudacacia, or which carry out movements in the dark which are quite irregular 

 and which bear no relationship to variations in light. These latter movements, 

 the so-called autonomous movements, we will treat of in Lecture XLI. 



