chyma of the two lateral leatlets face each other. A second is a 

 diurnal one in which ditiused light or moderate sunlight is 

 brought to bear upon the leaves. The leatliets as.sume an ex- 

 panded position so as to bring their palisade surfaces against 

 the light and approximately at right angles to it. The leatlets 

 would lie nearly in the same plane. A third position is that 

 assumed in th.- intense sunaght of midday. The leaflets turn 

 vertically or edgsewise, so as to bring their surfaces at an acute 

 angle with the light rays, if net parallel with them. 



The purpose of the "sleep" position is generally stated to 

 be the checking of radiation of heat whereby plants would be 

 enabled to withstand a lower temperature. Darwin has shown 

 that this actually works in practice ; but such a theory is worth- 

 less when plants of tropical lowlands are considered, since 

 danger of injuriously low temperatures never exists.. Stahl 

 suggests a possible purpose in the more etftcient shedding of dew, 

 so that transpiration could proceed with less interference. 



The manifest purpose of the extended position of the leaflets 

 is to bring them into favorable light relation. 



The edgewise position prevents a dangerous exposure to too 

 intense sunlight, for it has been shown that chlorophyll is in- 

 jured by such exposure. Credit is sometimes given to the belief 

 that lessening of transpiration is accomplished by huddling of 

 the leaves, since by overlapping, their exposed surface is reduced. 

 Transpiration would be diminished by the decrease in tempera- 

 ture resulting from reduced illumination, or by bringing stomata- 

 bearing surfaces into contact with other leaf surfaces. In 

 Erythrina, the under or stomata-bearing sides of the leaflets are 

 not brought together, but face outwards, resulting in greater 

 exposure probably than when they lie parallel to the ground. 

 In some instances there is no attempt made to overlap the leaf- 

 lets, as sometimes occurs in Phaseolus. Instead of bending its 

 leaflets up or down, a horizontal leaf will rotate them ninety 

 degrees, leaving them still extended with their midribs in a 

 horizontal position. 



Erythrina herbacea will probably be found not to differ 

 from many tropical plants, as soon as their leaves are investi- 

 gated. It differs from Phaseolus in the time required for a 

 response to variation in light intensity and in the behavior of its 

 leaflets in direct sunlight. Part of the experimentation on rapid- 

 ity of response to changes in light intensity was made on a plant 

 of low stature, growing in a dark thicket to which practically 



