17 



sufficiently dwelt upon. That the pulvini themselves, not the 

 blades, receive the li^ht stimuli in response to whieh their motion 

 oeeurs in direct sunlight is demon.strable in several ways. Ab- 

 scision of the blades does not prevent the pulvini turning upward 

 in direct sunlight; nor does covering the blades Avith tinfoil or 

 black paper. While such results show that the pulvini are sus- 

 ceptible to light stimuli, others prove that the bades are not like- 

 svise susceptible. To demonstrate that pulvini alone are sensitive 

 to direct sunlight whereby they turn their blades edgewise, it was 

 found necessary to preserve the blades intact, and shade the 

 pulvini. A plant was therefore so placed that it would be partly 

 in the .shadow and partly in the direct sunlight. One of the 

 leaves in the sunlight was used for experiment, all the other 

 leaves on the plant serving as controls. The leaf in question 

 was in an expanded condition, as were all the others. A narrow 

 piece of cardboard was so placed as to cast a shadow wide enough 

 to just cover the pulvini of the lateral leaflets. The blades were 

 therefore expo.sed to full sunlight. This leaf, together with those 

 entirely in the shade, showed no sign of turning the leatlets edge- 

 wise. Thosr' entirely exposed to sunlight, on the contrary, 

 responded (juickly and soon had their leatlets pointed toward 

 the sun. 



Hence while the blades are the organs probably directly 

 benefited by such paratonic movements, they depend on their 

 pulvini for sunlight stimulation and for shift of position. 



It would appear that a contraction of that part of the motile 

 organ takes place on whieh the sun shines. This applies to 

 whatever side of the pulvinus is inten.sely illuminated. The 

 result is a bend in the pulvini of approxinuitely 90=^. There is 

 little or no twisting motion induced by intense illumination. 



Quite another problem presents itself when the behavior of 

 leaflets under diffusf fJaiflight is considered. The pulvini may 

 be unecpuilly illmuinated. but the results of such variation are 

 not consistent unless the blades are taken into consideration. 

 Two leaves, for instance, both in the expanded condition, were 

 placed each in a different light relation. One was turned with 

 its tip toward a source of diffuse or reflected light, the rays 

 lying parallel to the midrib. The other was placed with its 

 nii(lril) pci-i)endicular to the light rays and with the spongy 

 pai'cncliyiiia turned toward the light. In the first instance the 



