BIOLOGICAL LECTURES. 





The photograph was taken from the south. It will be seen 

 that the three leaflets, elevated on their sensitive petioles, are 

 all pointing directly at the sun in the south-east. The leaflets 

 being parallel to the heat and light rays, escape much of the 

 consequent transpiration of water which would occur were they 

 thrown down to the sun's rays as in Plate No. 4. 



Another plant, Plate No. 6, photographed under similar 

 conditions of dryness and hot sun at twelve o'clock in the 



morning, shows the leaves 

 pointing directly to the 

 zenith, thus again parallel 

 to the sun's rays and for 

 similar reasons. 



Plate No. 7 shows still 

 another plant taken on the 

 same hot and dry day, 

 with the leaves pointing 

 directly toward the sun. 

 This photograph was taken 

 at 2 p. M. in the afternoon 

 from a position 15 north 

 of west, at right angles to 

 a profile view of the leaves. 

 It will be seen from 

 these illustrations that 

 when the conditions are 

 ripe for rapid loss of moist- 

 ure, that these sensitive 

 leaves continually keep 

 themselves parallel with the sun's rays during the day. As 

 the leaves in the morning are first influenced by heat and 

 light, they elevate themselves and point toward the eastern 

 sun. They continue to rotate during the day, so that at noon 

 they are erect, slightly inclined to the south, and at night are 

 directed to the west. 



Plate No. 8 shows a cluster of these plants photographed 

 from the north at six o'clock in the afternoon after a compara- 

 tively warm, dry day. The leaves are mainly pointing to the 



PLATE No. 



