CH. VI] LIGHT. 149 



is removed and the fungus allowed to grow in light 

 for an hour, during which its growth should be noted once 

 or twice. The rate of growth in the dark should differ 

 from that in the light by something like 20 p.c. 



(189) Light. 



The effect of light on the growth of roots should 

 also be demonstrated on the apheliotropic roots of Sinapis 

 alba, on account of the interest of the fact in relation 

 to the theories of heliotropic curvature ^ The seedling 

 mustards are supported by plugs of cotton-wool in holes 

 in a cork plate so that the roots dip in water. 



The details of the experiment are practically the same 

 as in exp. 188, but the periods of light and dark should be 

 longer, say 2 or 3 hours. 



(190) Periodicity. 



A Narcissus kept for 24 hrs. in the dark room will 

 record its periodic changes in growth rate. This will 

 probably not be evident on merely inspecting the tracing 

 on the drum ; a graphic representation must therefore be 

 made on a fairly large scale. 



' Francis Darwin, Ueier das Wachsthum negativ heliotropischer Wur- 

 zeln. Sachs' Arbeiten, ii. p. 521. 



