MOVEMENT IN PLANTS. 2 



it will get the light evenly all round ; put C in a chamber 

 so that it gets light only from one side, through a vertical 

 slit. After a few days note the t directions of growth of 

 roots and shoots in the three cases. 



290. Influence of Light Intensity on Nature of 

 Response. That the same organ may differ in its helio- 

 tropic response according to the intensity of the light may 

 be shown as follows. Get ready three pots of darkness- 

 grown Mustard seedlings, each covered by a cardboard 

 chamber with a vertical slit in one side. Place A at a dis- 

 tance of several yards from a window ; place B close to a 

 window but in diffuse light ; expose G to direct sunlight, 

 turning the cover from time to time so that direct sunlight 

 shall enter the slit. After several days compare the three 

 sets of seedlings. Both A and B curve towards the light, 

 while G (exposed to strong sunlight) shows little or no 

 curvature. If very strong light from a lantern is focussed 

 (passing through a layer of alum solution to minimise the 

 heating effect) on a seedling, the shoot will show negative 

 heliotropic curvature. If Cress or Mustard seedlings are 

 planted in a row in a pot or box, which is placed at an 

 angle of 45 to the beam from the lantern, so that one 

 seedling stands nearest the focus of the lens and the rest 

 are more and more remote from it, the seedling nearest the 

 focus undergoes negative curvature, the next ones remain 

 straight, and the farthest ones show positive curvature, 



291, Diaheliotropism is readily demonstrated* Dig 

 up a White Dead Nettle plant, set it erect in a pot, and 

 tie the stem firmly to a vertical stick so that it cannot 

 curve. Set the plant near a window, placed so that one 

 row of leaves faces the window, i.e. if the window faces 

 north, the four rows of leaves will face N., S., E., W. 

 After a day or two note the positions assumed by the 

 leaves in the four rows. In the N. and S. rows the petiole 

 simply curves downwards in the former case, upwards in 

 the latter. In the E. and W. rows, however, the petiole 

 undergoes twisting, in order to bring the leaf blade into 

 the right position. 



P.B. 15 



