182 NEGATIVE HELIOTROPISM. [CH. VII 



(213) Negative heliotropism. 



Allow mustard seed to germinate in sawdust, which 

 should be thrown lightly into the flower-pot, not pressed 

 down, and should not be too wet. When the radicles are 

 15 20 mm. in length the seedlings are pulled out of the 

 sawdust and placed with their roots in water. A disc 

 of thin cork plate is pierced with holes of 5 6 mm. 

 in diameter and is supported by 3 bent wires in the mouth 

 of a glass beaker about 1 2 mm. above the surface of the 

 water. The mustard seedlings are supported 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) Struggle between the effects of light and gravitation. 



Phycomyces is strongly heliotropic as well as apogeo- 

 tropic ; Elfving 1 has shown that if the surface on which 

 the spores are sown is illuminated from below by means of 

 an oblique mirror, the hyphae grow downwards in obedi- 

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

 apogeotropic tendency. The bread on which the Phy- 

 comyces grows should be fixed to the inside of the cover of 

 a stoppered jar. A little water should be poured into the 

 jar to keep the air damp. The jar is then darkened, 

 except below, by black material tied round it. It must 

 be suspended vertically and placed near a window, with a 



1 Acta Societatis Sc. Fennice, T. xn. 1880. 



