CH. Vl] LIGHT. 161 



described : the numbers give the temperatures (C.) to 

 which the plant was exposed. 



(187) Light. 



Fit up a Narcissus as above described in the dark 

 room, and after 2 or 3 hrs. take down the shutter so that 

 the plant is illuminated ; no sunshine must be admitted, 

 otherwise the temperature of the room will be affected. 

 The period of illumination should last 3 hours, when the 

 shutters should be once more closed. In this experiment 

 it is important to take readings of the wet- and dry-bulb 

 thermometers occasionally, both during the dark and the 

 light period. 



(188) Liglit 



Vines^ has shown that light has a retarding effect on 

 the growth of Phycomyces. A ripe sporangium is allowed 

 to burst in a watch-glass of water and a few drops are 

 placed, by means of a needle, on a thick slice of bread, 

 which should be previously steamed to roughly sterilise it. 

 The bread is placed in a saucer containing a little water 

 and covered with a flat-sided glass cover. It is now placed 

 on an apparatus by which it is kept revolving once in 30 

 minutes on a vertical axis, so as to avoid heliotropic 

 curvatures-. If the hyphse are growing vigorously read- 

 ings may be taken every 15 minutes for an hour, and 

 afterwards at intervals of 30 minutes. When sufficient 

 readings have been taken to indicate the course of the 

 growth-rate, i.e. to ascertain whether the rate is steady, 



1 Sachs' Arbeiten, ii. p. 133. 



2 We use a drum turning on a strong steel axis and driven by an 

 endless band connected with a pulley driven by clockwork. 



D. A. 11 



