IG 



parenchyma directly toward the light. In the baek-to-back, 

 edgewise arrangement neither side received direct illumination, 

 but nevertheless a normal condition was established in that the 

 palisade side obtained more light than the spongy side. One 

 could conclude from that, that insufficient though proper distri- 

 bution of light on the two sides of leatiets was more desirable 

 than sufficient light intensity on the wrong side. 



In the third instance, a downward turn of 90° of the terminal 

 leaflet would adjust it perpendicularly to the light, and a 

 twist of 90° on the part of the lateral leaflets would place them 

 also in a proper light relation. These movements occurred 

 abundantly in the field as could be seen by examining plants 

 having one-sided illumination. The laboratory experiments re- 

 peated them. 



In the fourth instance, the expected happened as in the pre- 

 vious experiment. The terminal leaflet turned up to bring the 

 palisade surface toward the light, while the laterals twisted. 



The pulvini behaved much like ball and socket joints. The 

 pulvinus of the terminal leaflet can twist 90° in either direction, 

 or 180° in all. Its amplitude as a hinge joint is at least 300°. 

 The pulvini of the lateral leaflets can twist 90" in either direc- 

 tion, or 180° in all, and can bend at least 90° in any direction, 

 giving them an extreme amplitude of 180°. 



Twisting and bending may occur simultaneously, thus pro- 

 ducing a motion permissible in a ball and socket joint. 



The base of the midrib of the leaf is provided with a large 

 pulvinus, but it is neither responsive nor motile to anything 

 like the extent that is the case with the leaflet pulvini. The 

 midrib itself is stiff and straight to the place of attachment of 

 the first pair of leaflets. At this point a difference in direction 

 is usually taken by the remaining portion of the midrib, and in 

 certain instances this difference in direction is undoubtedly 

 accountable by the attempt to bring the terminal leaflet into 

 a more favorable light relation, for instance, out of the shadow 

 of the lateral leaflets, when they happen to lie in the path of 

 the light. Slow changes take place in the midrib pulvinus when 

 the main stem becomes so altered in direction that certain leaves 

 find it difficult to get proper access to the light. 



The pulvini of the leaflets are at once the organs for receiving 

 and responding to a light stimulus. The latter point has been 



