MOVEMENTS OF IRRITATION. 



467 



cylinder is covered with moist blotting-paper. On the bottom of 

 the cylinder we fasten a sheet of cork soaked with water, and 

 cover it with wet cotton wool. The seedlings are pinned to the 

 cork as far as possible from the centre. Care must be taken that 

 they are kept sufficiently moist. Seedlings of Pisum with roots 

 about 2 cm. long are very suitable for investigation. To study 

 the behaviour of secondary roots we employ seeds of Vicia Faba. 

 Experiments made with the axis of rotation horizontal are con- 

 tinued for some time, and we find that the secondary roots under 

 the influence of the centrifugal force all curve outwards. 1 



1 See Knight, Philosophical Trans., 1806, T. 1, p. 99 ; and Sachs, Arbeiten d. 

 botan. Inst. in Wiirzbury, Bd. 1, p. 607. 



178. Heliotropic Nutations. 



Many plant structures, especially young stems, when exposed 

 to one-sided illumination, bend towards the rays of light ; they 

 are positively heliotropic. Some structures, e.g. the roots of many 

 Cruciferae, are negatively heliotropic ; under one-sided illumina- 

 tion, they curve away from the light. A small glass vessel, over 

 the mouth of which is firmly tied a piece of small-meshed netting, 

 is completely filled with spring water. 

 Some seedlings of Sinapis alba, 

 germinating in sawdust, are fixed 

 in the meshes of the net by means 

 of cotton wool. The vessel is then 

 covered for a day with a bell-glass, 

 and this with a cardboard box. As 

 germination proceeds in the dark the 

 roots grow straight down into the 

 water, while the hypocotyls grow 

 vertically upwards. In order now to 

 submit our plants to the influence 

 of one-sided illumination, we place 

 the vessel with the seedlings under 

 a beaker, covered, with the exception 

 of a small slit, with dull black 

 paper, or under a cardboard box 

 provided with a slit, and covered on 

 the inside with dull black paper. 

 After a few hours it can already be clearly seen that the hypo- 



FIG. 157. Seedling of Sinapis 

 alba. The hypocotyl exhibits a 

 positive heliotropic curvature, the 

 root a negative one. 



