15 



upper lateral leaHet responded at i»iiee in assuming a perpen- 

 dieular-to-li^ht position. The differenee in behavior of the two 

 liiteral leaflets may be at least partially explained by noting that 

 the lower was partly if not entirely shaded by the upper. There 

 is no doubt but that the terminal leaflet would, if it were on the 

 plant, and if it were given time enough, attain a perpendicular 

 relation ; but when compared with the hinge-like bending of the 

 laterals, a twist is a motion requiring considerable more time. 



In the second instance, the terminal leaflet would require a 

 turn or twist of 180° to bring the palisade surface toward the 

 light. The laterals could reach that position only by a move- 

 ment of equal amplitude. 



Only the terminal leaflet succeeded in reaching an approxi- 

 mately perpendicular position, and this was accomplished by an 

 upward and then backward bending. The response of the lateral 

 leaflets was always quick and uniform. They turned into the 

 edgewise position with their spongy tissue surfaces together, and 

 their tips toward the light. Thus they passed info a position 



Figure 7. — Leaf before and after expo.sure to diffuse light for :5o ininutes 

 Tlie liglit rays were from the right and above, and uppro.vinuuely at 

 right angles to the leaf surface. The spongy parenchyma side of" the 

 leaflets was toward the light. 



similar to that assumed in direct sunlight. It is interesting to 

 note that this light relation was preferred to one with the spongy 



