i82 TKANSMITTBD EFFECTS OF LIGHT. Chap. IX 



the case of Phalaris, whether the skin covered with a rathei 

 tliick crust of dry Indian ink did not mechanically prevent 

 their curvature. 



Btta vulgaris. — A few analogous experiments were tried on 

 this plant, which is not very well adapted for the purpose, as the 

 basal part of the hypocotyl, after it has grown to above half an 

 inch in height, does not bend much on exposure to a lateral 

 light. Four hypocotyls were surrounded close beneath their 

 petioles with strips of thin tin-foil, -2 inch in breadth, and they 

 remained upright all day before a paraffln lamp ; two others 

 were surrounded with strips '15 inch in breadth, and one of 

 these remained upright, the other becoming bowed ; the band- 

 ages in two other cases were only • 1 inch in breadth, and both 

 of these hypocotyls became bowed, though one only slightly, 

 towards the light. The free seedlings in the same pots were 

 all fairly well curved towards the light; and during the follow- 

 ing night became nearly upright. The pots were now turned 

 round and placed before a window, so that the opposite sides 

 of the seedlings were exposed to the light, towards which all 

 the unprotected hypocotyls became bent in the course of 7 h. 

 Seven out of the 8 seedlings with bandages of tin-foil remained 

 upright, but one which bad a bandage only ■ 1 inch in breadth, 

 became curved to the light. On another occasion, the upper 

 halves of 7 hypocotyls were surrounded with painted gold- 

 beaters' skin ; of these 4 remained upright, and 3 became a little 

 curved to the light: at the same time 4 other seedlings sur- 

 rounded with unpainted skin, as well as the free ones in the 

 same pots, all became bowed towards the lamp, before which 

 they had been exposed during 22 hours. 



Badicles of Sinapis alba. — The radicles of some plants are 

 indifferent, as far as curvature is concerned, to the action of 

 light ; whilst others bend towards and others from it.* Whether 

 these movements are of any sei-vice to the plant is very doubtful, 

 at least in the case of subterranean roots ; they probably result 

 from the radicles being sensitive to contact, moisture, and gravi- 

 tation, and as a consequence to other irritants which are never 

 naturally encountered. The radicles of Sinapis alba, when 

 immersed in water and exposed to a lateral light, bend from it, 

 or are apheliotropic. They become bent for a length of about 

 4 mm. from their tips. To ascertain whether this movement 



* Sachs, 'PhyslologieT^g^tale,' 1868, p. 44. 



