CH. VIl] TRANSMITTED STIMULUS. 167 



in the holes by little plugs of cotton-wool, and should 

 be so arranged that the radicles are vertical. The beaker 

 is then placed near the window in a box with a lateral 

 opening. After a variable number of hours the roots will 

 show a strong curvature from the light. 



(214) Combined action of light and gravitation. 



Phycomyces is strongly heliotropic as well as apogeo- 

 tropic ; Elfvdng^ has shown that if the surface on which 

 the spores are sown is illuminated from below by means of 

 an oblique mirror, the hyphse grow downwards in obedi- 

 ence to the stimulus of light, but in opposition to their 

 apogeotropic tendency. 



(215) Transmitted stimmlus''. 



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

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



^ Acta Societatis Sc. Fennite, T. xii. 1880. 



'■' See Power of Movement in Plants, Ch. ix, for experiments on 

 Phalaris, &e. The observations on Setaria were published by Eothert 

 in the Berichte deut. iot. Ges., Jahrg. x. 



