202 BIOLOGICAL LECTURES. 



leaf was mechanically stimulated and fell through 58 in 155 

 seconds, again stimulated it fell through 37 in 114 seconds, 

 again stimulated it fell through 16 in 85 seconds, and on 

 fourth stimulus it fell through 9 in 69 seconds. It thus swept 

 through an arc of 120 in little more than 7 minutes, but as I 

 have since proved, by shortening the time intervals between 

 stimuli, the same movement can be got in less than half 

 the -period. The average rate of expansion is from 12-15 

 minutes in the first three species now under consideration, but 

 under certain conditions may be only 7-9. Ice and ice-water, 

 hot water and dry-heat stimuli, alcohol, ether, etc., are all 

 irritants. 6 f> ether when placed not merely at the base of a 

 leaflet but even in the middle of it, excites to movement. 



I have already referred to the fact that Cassia nictitans, 

 though closely resembling Mimosa pudica in its leaf move- 

 'ments, shows a greatly reduced capacity for propagation of 

 stimuli from one leaflet or pair of leaflets to another. The 

 four species now under consideration are still less sensitive in 

 this respect, for it is possible to make a terminal or lateral 

 leaflet fall through 38-67 without participation of the other 

 two leaflets in the change. 



Desmodium rotnndifolium is the least sensitive of the genus, 

 for in its sluggish action and limited amplitude of movement 

 (amounting to io-25) it more nearly resembles some of the 

 Lespedezas. Now AmpJiicarpcea, Desmodium canescens, and 

 Dcsmodinm panicnlatum are all upright growers, and are 

 therefore exposed in their leaflets to the full effects of night 

 cold and heat radiation from the tissues, and I believe that 

 this may largely explain why in evolutionary development they 

 have become much superior to Desmodium rotundifolium, whose 

 long sucker-like shoots run along the ground, and give off 

 leaflets that nestle amongst surrounding herbage. 



AmpJiicarpcea, Desmodium and Lespcdeza all seem to resemble 

 the species of O.ralis, and to differ from such as Mimosa 

 pudica or Cassia nictitans in that the primary stalk of the leaf 

 does not appear to move, or only moves so slightly that it 

 has hitherto escaped my observation. But this difference is 

 secondary and not fundamental, I believe, for in two such 



