MOVEMENTS OF IRRITATION'. 4G1 



of the structures by illuminating from one side in a heliotropic 

 chamber (see 178) straight sporangiophores grown on cubes of 

 bread in the dark. And if now, in clinostat experiments with 

 access of light, the sporangiophores grow out obliquely to the 

 surface of the cube of bread, the side which they turn towards 

 the face of the cube, whether that face is directed towards the 

 window or away from it, is always more feebly illuminated than 

 the opposite one. Heliotropic effects must therefore follow, and 

 these, like the hydrotropic processes which are certainly of pri- 

 mary importance, would bring the sporangiophores into a direction 

 at right angles to the substratum. 



We now fix on the shaft of the clinostat soaked cubes of turf 

 (length of side about 5 cm.), and sow them with seeds of Lepidium 

 sativum or Sinapis nigra, leaving the two flanks unsown. The 

 seeds cling to the wet turf, and do not require specially fixing. 

 The clinostat, with the glass case in situ, is placed with its shaft 

 parallel to the window panes, exposed to bright diffused light, 

 and set in motion. After several days we observe that the de- 

 veloping roots, owing to their positive hydrotropism, are clinging- 

 tightly to the moist substratum, even penetrating it in part. The 

 hypocotyls at first vigorously nutate ; soon, however, they take 

 up a position at right angles to the surface of the turf. This, as 

 was shown by Dietz, is not due at all, or only in a subordinate 

 way, to hydrotropism, or contact stimulus. Here we really have 

 to do with heliotropic effects, brought about in fact in the same 

 manner as was described in the case of the sporangiophores of 

 Mucor. If, viz., we allow cubes of turf, sown with Lepidium 

 or Sinapis, to rotate slowly about an horizontal axis in the dark, 

 then the hypocotyls do not stand at right angles to the sub- 

 stratum, but assume the most various directions. Seedlings of 

 Plileum pratense are also very suitable for experiments with the 

 clinostat, but they do not grow so rapidly as, e.g., seedlings of 

 Lepidium. If we sow cubes of turf standing in water with seeds 

 of Phleum, and cover with a cardboard box, the plumule of the 

 seedlings on the horizontal upper surface grows vertically up- 

 wards. The plumule in the seedlings lying on the vertical sides 

 of the cube arches upwards, owing tq its great geotropic irrita- 

 bility. 



1 See Sachs, Arleiten des botan. Inst. in Wiirzlurg, Bd. I., p. 597, and Bd. II. 

 p. 215. 



