162 GROWTH. [CH. VI 



or increasing or decreasing, the culture is darkened by a 

 thick cardboard cover placed over the glass. The rotation 

 should be continued and temperatures taken by a ther- 

 mometer of which the bulb is inside the glass vessel 

 covering the fungus. It is a good plan to wet the 

 cardboard cover on the outside, so that the temperature 

 during the dark period may be slightly cooler than during 

 the period of illumination. After half-an-hour or an hour 

 the dark cover 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 curve representing growth must therefore 



be drawn on a fairly large scale. 



1 Francis Darwin, Ueber das WacJisthum negativ heliotropischer 

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



