<) 



Tile scnsilivciicss of the leatlots to diri'd sunlight is consid- 

 or;il)l(' as can lie sccii tVoiii an examination (»f plants having an 

 eastern ex[i(isur('. Soon at'tei- tlu' sun hi-iraii to shine upon them 



Figure 4.— Same plant as in two picviuus fiynr.s aft.r prolonged exposure 

 to full sunlight. 



early in the mornintr, tlieir leaflets beji:an to turn towards the 

 east, so that before 8 o'clock (in June) the edgewise position 

 was reached. At this time of day the sun's heat is not at all 

 intense, so that one is le^l to believe that it is light rather than 

 heat that acts as the stimulus to motion. Further proof of 

 sensitiveness to light is seen in the response to strong diffuse 

 light such as prevails at midday. Leaves which are not too 

 greatly shaded turn their leaflets partially towards the light, 

 though, of course, not to the extent that they do in full sun- 

 light. If a plant is covered by an umbrella early in the morning, 

 and is examined at midday while still covered, its leaves show 

 an ui)wanl turn wliicli is evident though not pronounced. Leaves 



