Preliminary Observations on the flovements 



of the Leaves of Melilotus alba, L. 



and other Plants. 



By W. P. Wilson, D.Sc, 



ASSISTED BY 



Jesse M. Greenman. 



(WITH PL,ATES IX, X, XI, XII AND XIII.) 



THE leaves of many plants may, under different condi- 

 tions, take three distinct positions, each one of which 

 may be assumed to give some advantage to the plant. 

 These three positions may be designated as (i) a normal day- 

 light position ; (2) a hot sun position ; and (3) a night position. 

 Nearly all the genera of Leguminosce furnish examples which 

 may take any one of these positions when the surroundings 

 are favorable. Many other widely separated families of plants 

 furnish scattered illustrations of the same movements and 

 changes in the position of leaves. Oxalis, Pyrus (Americana 

 Be), Sambucus, Rhododendron, Croton, Myriophyllum, and Mar- 

 silia give examples sufficiently separated to show that family 

 relationships have nothing to do with it. In some of these 

 the hot sun and night positions are the same. This is illus- 

 trated in the genera Oxalis, Myriophyllum and Marsilia. 



In the sleep of plants, or night position, the leaves are inva- 

 riably so disposed or folded together as to lessen the leaf sur- 

 face displayed in the daytime. Darwin came to the conclu- 

 sion, supporting it with interesting experiments, that these 

 night positions lessened the surface exposed to radiation and 

 thus protected the plant from cold. 



I wish to emphasize the fact that the hot sun positions, 

 over 200 of which I have already examined, occurring so fre- 



