LEGUiMINOS^ 159 



the Southern harvesting ants from dragging them 

 down into their nests. I have seen Medicago pods 

 lying in little heaps at the entrances to such nests in 

 Italy and Algeria. The plant is slightly hairy, some- 



times glandular. 



Melilotus 



Of this genus we have three species. M. alhus has 

 white flowers, M. officinalis and M. arvensis yellow ; the 

 former has the pods irregularly, the latter transversely, 

 wrinkled. The general structure of the flower resembles 

 that of Medicago, but at one place the epidermal cells 

 of the wings are interwoven with those of the keel, so 

 that they may almost be said to have grown together. 

 They therefore move together when pressed down by an 

 insect, but on the removal of the weight resume their 

 original position. 



The leaves sleep at night, the leaflets becoming 

 vertical. In doing so the leaflets could, of course, twist 

 so that their upper surfaces should face to either side. 

 As a matter of fact they face noi^th, but as they move at 

 the same time towards the terminal leaflet, the upper 

 surface of the one faces about N.N.W., and that of the 

 other N.N.E. The terminal leaflet, on the other hand, 

 twists to either side, the upper surface sometimes facing 

 east and sometimes west, generally indeed west. It 

 also " moves in another and more remarkable manner, for 

 whilst its blade is twisting and becoming vertical, the 

 whole leaflet bends to one side, and invariably to the 

 side towards which the upper surface is directed ; so 

 that if this surface faces the west the whole leaflet bends 

 to the west, until it comes into contact with the upper 

 and vertical surface of the western lateral leaflet. Thus 

 the upper surface of the terminal and of one of the two 

 lateral leaflets is well protected."^ Darwin proved ex- 

 perimentally that leaves which were forced to remain 

 horizontal suffered more from frost. 



^ Darwin, Movementu of Plants. 



