CH. VIIl] CIRCUMNUTATION. 209 



Section B. Autonomous Movements : Periodicity. 



(258) Gircumnutation. 



A really good method of observing circumnutation 

 (where the movement is small) has yet to be devised. One 

 of the methods described in Darwin's Power of Movement 

 m Plants (p. 7) is, in spite of certain faults, perhaps the 

 best for our present purpose. 



Any rapidly growing plant will serve for observation, 

 for instance, a seedling cabbage or sunflower. The most 

 essential precaution is that the plant shall not be 

 subjected to lateral illumination, to insure which the 

 experiment ought to be conducted in a room lighted from 

 above. If this is not possible the plant must be in a 

 cylinder blackened inside, and must be illuminated by an 

 oblique mirror hung above the mouth of the cylinder so 

 as to throw the light vertically downwards. The plant 

 should if possible rest on a steady stone floor. 



To the upper end of the hypocotyl a delicate glass 

 filament, 20 mm. in length, is fixed vertically by shellac 

 varnish, which should be thick enough to dry rapidly. 

 Before it is fixed the following preparations are necessary. 

 A minute equilateral triangle of paper (2 or 3 mm. 

 to the side) is pierced in the centre and is slipped 

 over the glass filament, pushed down to the base and 

 there fixed with shellac, so that it is at right angles to 

 the filament. At the other end of the filament a minute 

 bead of black sealing-wax is fixed. A sheet of glass 

 (2 ft. X 2 ft.) fixed horizontally about 2 ft. above the apex 



D. A. 14 



