CH. VIl] TRANSMITTED STIMULUS. 183 



good mirror so arranged that the bread is illuminated from 

 below. After one or two days the cover should be removed 

 and the vertically downward growth of the mould noted. 



(215) Transmitted stimulus'^. 



Sow Setaria italica in a pot in which the soil is level 

 with the rim of the pot, and let it remain in absolute 

 darkness for from 3 to 5 days, when the seedlings should 

 be 12 — 15 mm. in height. In Setaria and some allied 

 genera the seedlings are remarkable for the long hypocotyl 

 on which the cotyledon is borne ; heliotropic curvatures 

 are produced by the bending of the hypocotyl, but the 

 cotyledon alone is sensitive to lateral illumination, and 

 when it is kept dark no heliotropic bend occurs, although 

 the hypocotyl itself is lighted from one side. To darken 

 the cotyledon small caps of tin-foil must be made in the 

 following way. The foil is cut into squares of 8 x 8 mm. 

 and these are rolled, each like a cigarette paper, round the 

 base of a small pin from which the head has been cut. 

 The pipes so made are closed by pinching them at one end 

 with a forceps. The pinched part should be about 3 mm. 

 in length and should be bent as well as pinched. The 

 little hollow cylinders so constructed can be picked up with 

 a forceps and slipped over the cotyledons of some 6 — 8 

 Setaria seedlings; the pot should then be exposed to 

 lateral illumination. When time has been allowed for 



1 See Poiver of Movement in Plants, Ch. ix, for experiments on 

 PJialai-is, &c. The observations on Setaria are given by Kothert in the 

 Berichte d. deut. hot. Ges., Jahrg. x, also, in his work Ueher Heliotropismus 

 (Breslau), 1894. 



