274 COMPENDIUM OF GENERAL BOTANY. 



motile organ of the Oxalis-lesii there is an upper and a lower 

 cushion of cells with an intermediate elastic vascular tissue- 

 bundle. In the dark the turgor and volume of the entire organ 

 increases ; ' in the light it decreases. Although the decrease 

 and increase in the volume of the two cushions of cells begins 

 at the same time, it does not proceed equally fast, so that in the 

 dark the swelling of the upper cushion proceeds more rapidly, 

 causing the leaf to turn downward. On exposure to light the 

 turgor-difference is again manifest, but in the inverse order ; that 

 is, the upper cushion loses its turgidity more rapidly, causing 

 the leaf to be turned upward by the more turgid lower cushion. 

 The explanation of this phenomenon becomes still more difficult 

 and complicated by the so-called " after effects." A nyctitropic 

 plant previously exposed to the changes of day and night, when 

 placed for days in either continuous dark or continuous light, still 

 continues to produce to-and-fro movements. These movements, 

 which are as yet unexplained, unite with the movements due to 

 the change of illumination (BBiiCKE, PFEFFEB, MILLABDET, SACHS). 



The common garden bean furnishes a good example of this 

 phenomenon. It need hardly be mentioned that we cannot see 

 the causal relation between the variations in turgor and the 

 variations in illumination. We can only establish the fact of 

 the existence of such relative variations. 



One-sided illumination acts in such a manner upon the longi- 

 tudinal growth of an organ that the side exposed to the light is 

 retarded in its growth, thus producing a curvature toward the 

 light. Most shoots and leaves show such positive heliotropism. 

 One-sided illumination may also have the opposite effect ; that 

 is, the side turned toward the light increases in growth and the 

 organ is turned away from the light. This phenomenon, which 

 is called negative heliotropism, occurs in the climbing shoots of 

 ivy, in many aerial roots, and in some subterranean roots. 



To give a causal-mechanical explanation of this phenomenon 

 is also impossible. We feel certain that it is due to the behavior 

 of the living cell-plasm ; the question of the cause of such be- 

 havior is quite another thing. Many investigators will of course 

 be imbued with the idea that a teleological principle also con- 

 trols these relations, since this principle forms, so to speak, the 



J In this case the expression "sleep" does not mean relaxation. 



