Xii PHYSIOLOGY OF GROWTH AND MOVEMENT 157 



EXPT. 150. Obtain a box which will just cover a pot of Musk. 

 Cover the pot with the box, and so arrange matters that the light from 

 a window can shine on the plant. Examine in twenty-four hours. 

 Note 



(i) The plants turn towards the light. 



Turn the box so that the light can only shine into one corner. 



(ii) On the following day the plants will have turned again to seek 

 the light. 



Turn the box so that the plant can only receive light from the room. 



(iii) At the end of another day the plants turn once more to catch the 

 diffused light. 



(iv) Plants like Musk are light-seekers. They always arrange them- 

 selves so as to receive the maximum amount of light. 



Constituents of White Light which Produce 

 Heliotropism. If a plant is grown so that the red and 

 yellow rays of the spectrum (p. 123) can fall on it, there is either 

 only a little curvature or none at all. But under the influence 

 of the blue and violet rays nearly as much curvature takes place 

 as in white light. 



EXPT. 151. Obtain two boxes similar to the one used in Experiment 

 150. Make grooves in each, so that the open sides of the boxes can 

 have slips of glass inserted. Germinate three pots of Cress and mark 

 them A, B, C. Place pot A on a window sill exposed to white light. 

 Cover B with a box and slide a piece of red glass into position, so that 

 red rays can only fall on the plants. Cover C with the other box and 

 slide a piece of blue glass into position, so that blue rays can only pass 

 to the plants. Note 



(i) The plants in pot A turn towards the window in the same way as 

 the Musk did. 



(ii) The plants in pot B grow erect ; they do not curve in any 

 direction. 



(iii) The plants in pot C curve towards the source of light just as do 

 the plants in pot A. 



Experimental Results. It must consequently be con- 

 cluded : 



(1) That plants growing in a window, and more strongly 

 illuminated on one side than the other, bend towards the source 

 of light. 



(2) That plants which receive only, red rays grow erect, and 

 do not curve towards the source of light. Or, they grow as 

 they would in the open air. The rays from the red end of the 

 spectrum are not instrumental in producing curvature. 



