470 PHYSIOLOGY OF GROWTH. 



narrow slit into a dark room. After the beam has traversed a 

 biconvex lens placed at a distance from the slit not quite equal to 

 twice its focal length, it falls on the flint glass prism, which has 

 an angle of 60. Since glass vigorously absorbs the highly 

 refrangible rays, it is very advisable to provide the heliostat with 

 a silver mirror, and to use a quartz lens and a quartz prism. 



We may employ for investigation, e.g., vetch seedlings grown in 

 the dark in very small earthenware vessels. They are placed 

 within the spectrum in such a way that ihe flanks of the stems 

 are directed towards the incident rays of light. Seedlings of 

 Sinapis and Triticum also serve well for the investigations. In 

 the course of a few hours the result of the experiment stands out 

 clearly. Neglecting the so-called lateral flexion, it is seen es- 

 pecially that the rays at the limit between violet and ultra-violet 

 display the greatest heliotropic energy. Towards very sensitive 

 objects the heliotropic energy of the rays falls off from violet to- 

 green ; yellow is inactive ; a second smaller maximum lies in the 

 ultra-red. 



If we cultivate Mucor Mucedo on bread, in darkness, as de- 

 scribed in 181, and then illuminate the culture unilaterally, the 

 unicellular sporophores curve towards the light. They are thus 

 positively heliotropic, which, as I specially set forth in my Lehrbuch 

 der Pflanzenphysiologie, p. 308, is of significance in connection with 

 the theory of heliotropism. 



When heliotropic nutations are exhibited, the side of the strnc^ 

 ture which becomes convex always grows more strongly, in 

 consequence of the light stimulus, than the side which becomes 

 concave. Moreover, growing structures alone are capable of 

 heliotropic nutations, as is shown by a very simple experiment. 

 We grow bean seedlings in the dark. When the epicotyl has 

 reached a length of some centimetres, we make fine ink-lines on 

 the stem at intervals of about 5 mm., and illuminate unilaterally. 

 The heliotropic curvature only appears in the upper, still growing 

 region of the epicotyl. 



If we illuminate etiolated vetch seedlings for about twelve hours 

 unilaterally, and then cut off the epicotyls and place them in a 15 

 per cent, solution of common salt, the heliotropic nutation is not 

 reversed again by the plasmolysis, since it has already been com- 

 pletely fixed by growth. If, however, we only continue the one- 

 sided illumination long enough to make the epicotyl bend slightly, 

 then the curvature is somewhat diminished by plasmolysing, since 



