72 Wilson and Greenman. — Movements of tJic Leaves of 



the light it can, while the one from red glass is putting itself 

 rapidly into its sleep position. The leaves under the arrows 

 have already tucked themselves up in the peculiar manner 

 previously indicated in the sleep of this plant under red glass, 

 shown in PI. XIII, Fig. 8, and in PI. XI) Fig. 4. 



The red glass must cut off certain rays of the spectrum 

 which serve to determine the ordinary sleep positions. No 

 difference in the night positions could be found under the 

 blue or white glass. 



The following conclusions seem obvious from the work 

 done and the mass of material under observation : 



(1) That there are great numbers of plants which put their 

 leaves in a special or hot sun position. 



(2) That these hot sun positions have come to exist in order 

 to protect the plants possessing them from a too rapid trans- 

 piration. 



(3) That these hot sun positions are not dependent on light 

 alone, but that the heat rays play a very important part in 

 determining them ; and also that the water supply of the 

 plant, both in the air as well as in the soil, exercises a direct 

 influence. 



(4) That for some reason, not yet well understood, the 

 leaves of Mclilotus alba take a different position at night 

 under red light from the one ordinarily assumed in the so- 

 called sleep of this plant. 



DESCRIPTION OF PLATES. 

 Illustrating paper by Dr. Wilson and Mr. Greenman. 



Plate IX. 



Fig. 1. Ordinary sleep or night position of leaves of Melilotus alba. 



Fig. 2. Day position of the leaves. 



Fig. 3. Leaves slightly elevated. First state of the hot sun position. 



Fig. 4. Hot sun position at twelve o'clock on an extremely hot and 

 dry day. The leaves are pointed directly at the sun. They make the 

 least possible shadow in this position. The transpiration caused by both 

 heat and light is reduced to a minimum. 



