TRANSMITTED EFFECT OF LIGHT 



367 



recording lever. A long glass fibre is supported by 

 lateral pivots on jewel bearings. The seedling is attached 

 to the short arm of the lever by means of a cocoon 

 thread. The reeordiag plate oscillates to and fro once in 

 a minnte ; the successive dots give therefore the time 

 relations of the responsive movement. The positive cur- 

 vature towards light is recorded as an up-curve, the 

 negative curvature being represented by a down-curve. 



Arrangement for local stimulation by light. — The device 

 of placing tin foil caps on the tip employed by some 

 observers labours under the disadvantage, that it causes 

 mechanical irritation of the sensitive tip. The appliance 

 seen in figure 13") is free from this objection and offers 



S h 



1 



Fig. 1.35. — Arrangement for local application of light to the tip and the 

 growing region. 0, O', aperture.^ on a metallic screen. Light is focussed 

 by a lens on the tip, and on the growing region at o, o.* Figure to the 

 right shows front view of the shutter resting on a pivol and worked by string, T. 



many advantages. A metallic screen has two holes 

 and 0^; these apertures are illuminated by a parallel 

 beam of light from an arc lamp. A lens focusses the 

 light from 0, on the hypocotyl, and that from 0\ on the 

 tip of the cotyledon. A rectangular pivoted shutter S, 

 lies between the apertures and 0'. In the intermediate 

 position of the shutter, light acts on both the tip and the 

 growing region. The shutter is tilted up by a pull on 

 the thread T, thus cutting off light from the growing 

 region ; release of the thread cuts off light from the tip. 

 Thus by proper manipulation of the shutter, the tip or the 

 growing hypocotyl, or both of them, may be subjected to 



