472 PHYSIOLOGY OF GROWTH. 



necessary to set this conjecture on a firm basis ; but the results 

 of certain experiments of Oltmanns 2 are certainly in favour 

 of it. If young seedlings of Lepidium grown in a flower-pot 

 are placed in a box blackened on the inside, and provided with 

 a slit about 3 cm. wide, and, after being pushed close up to the 

 slit, are exposed to direct sunlight (we take care to rotate the box 

 so as to have it always in approximately the same position with 

 respect to the sun), then the hypocotyls remain straight, whereas 

 in less intense light they exhibit positive heliotropism. We may 

 further arrange an experiment in which the sun's rays are directed 

 into a dark chamber, by means of a heliostat, through a thick 

 layer of concentrated alum solution, and then passed through a 

 large biconvex lens, behind which is a flower-pot with Lepidium 

 seedlings. The seedlings are planted in a row, and this is pre- 

 sented to the rays diverging from the focus of the lens at an 

 angle of about 45. In this way one seedling stands nearly at the 

 focus of the lens, the rest more and more remote from it without 

 shading each other. With sufficiently strong illumination we find 

 that the stems of the seedlings which stand nearest to the focus of 

 the lens undergo negatively heliotropic curvature. The third or 

 fourth seedling of the row remains quite straight. The fifth and 

 sixth and seventh experience positive heliotropic nutations. 



If we lay across the middle of a vessel containing algal 

 swarmers a small strip of wood, so that it is directed at right 

 angles to the window, and consequently nearly parallel to the 

 incident rays of light, the swarmers do not collect at the front of 

 the dish, but on either side of the strip of wood in its penumbra. 

 Famintzin and especially Oltmanns (Flora, 1892, p. 203) con- 

 clude from this that the phototactic movement of the swarmers is 

 determined not by the direction of the light rays, but by the 

 intensity of the light or the rate at which it falls off. Oltmanns 

 assumes that this is also the case in heliotropic nutations, and 

 some investigations instituted by myself perhaps favour this view. 

 The important question here touched upon requires, however, 

 further very searching investigation, and in the following experi- 

 ments, as in those above alluded to, it appears to me that very 

 special attention must be paid to the effect of reflected light, in 

 order to avoid errors in the conclusions. For my experiments I 

 employed a box blackened on the inside, and having in one wall a 

 slit of slight width but considerable length. Within the box, 

 behind this slit, was placed a wedge-shaped glass bottle containing 



