68 Wi/son and Grecnman. — Movements of the Leaves of 



night position is assumed in a different manner by nearly 

 every genus of Leguminosce. Nor do the leaves always sleep 

 in the same manner in a given species. In the plant we are 

 examining, the very young leaves sometimes elevate them- 

 selves on the general petiole, instead of sinking down, and 

 then bring the apices of all three leaflets up together to the 

 zenith until they touch each other. If the young leaf hap- 

 pens to be at, or near the growing bud, the leaflets will often 

 rise up and encircle it at night. Darwin l has called attention 

 to the fact that the sleep of the young leaves of certain plants 

 resembles the sleep of the adult leaves in other genera. This 

 may indicate relationships of descent not otherwise easily 

 seen. 



From the night position, the leaves as daylight approaches 

 gradually change into that of the day, in which the plane of 

 the leaves are generally so placed as to receive the rays of 

 the sun at right angles to their surfaces. Most of the leaves 

 are quite as low below the horizon as in their sleep positions, 

 but their faces are turned broadly to the light. 



On a cool day, with atmosphere nearly saturated with mois- 

 ture, the leaves may retain this relative position to the sun's 

 rays (PI. IX, Fig. 2). If, however, the air and soil are dry and 

 the sun hot, the leaves quickly take another position ; the gen- 

 eral petiole becomes slightly elevated and the leaflets rise up 

 above the plane of the horizon. By eight o'clock in the morn- 

 ing we may have the position indicated in PI. IX, Fig. 3. At 

 nine o'clock the leaves will have still further changed. The 

 leaflets will now be parallel with each other and also with the 

 sun's rays. PI. XII, Fig. 1 was photographed at nine o'clock 

 in the morning on a hot, dry day. The leaflets have all taken 

 a position parallel with the incident ray, and therefore cast 

 the least possible shadow. In order to get a profile view of 

 the angles made by the leaves with the general direction 

 of the stem the camera was set south thirty degrees west of 

 the plant. The leaves continue to rotate with the sun, keep- 

 ing themselves parallel with its rays. At twelve o'clock the 

 leaves will point vertically up to the zenith, as exhibited in 



' Movements of Plants, Eng. ed., 18S0, p, 345. 



