igO BIOLOGICAL LECTURES. 



leaflet, a latent period of 3^^ seconds elapses, followed by a 

 period of slow but gradually accelerating contraction during 

 the next 4 seconds. From the seventh to the twentieth 

 second the motion is rapid, but thereafter slows down gradually 

 to the thirtieth second, and then becomes increasingly slow till 

 the forty-fifth second when the contraction ceases. After 

 15-18 minutes expansion begins, and a very slow rise can be 

 traced till the leaf regains its expanded state in 45-50 minutes. 

 The angle through which such a leaf falls is on the average 

 37°, but varies with the daily periodicity of cell tension, as well 

 as the temperature and moisture of the air and soil. 



It should be stated that young, active — not necessarily 

 growing — leaves are to be preferred for experiment, but even 

 the oldest and fully matured leaves are sensitive to a definite 

 degree, though less so than those younger. Some interesting 

 statistics will be adduced later in this connection. 



When the three leaflets of a leaf are simultaneously excited, 

 all contract, and the movement shown is as follows: The 

 terminal leaflet simply drops and folds its halves together 

 more or less toward their upper faces, ])ut the two side leaflets 

 move forward and downward so that each describes a segment of 

 an ellipse. If care be taken, however, to stimulate the terminal 

 one of the three, it alone will contract, or, at most, the side 

 leaflets will move to a small extent. This, among other things, 

 proves that the capability of conducting a stimulus through its 

 tissues is comparatively feeble in this species. 



In no case have I found that a single shock is sufficient to 

 produce a contraction approaching in amount to the nyctitropic 

 or the most pronounced parathermotropic states. The angle 

 fallen through varies from 25° to 48°. A summation of 

 stimuli, however, produces very different results. If an excited 

 leaflet be left till the downward movement has ceased, and 

 a second stimulus be applied, it will fall still further, but 

 through a less angle than before, and its motion will cease 

 sooner ; if a third stimulus be then applied there will be an 

 additional fall less in extent than the second, and greatly 

 less than the first, the time of motion being correspondingly 

 shortened. A fourth stimulus will give a slight but appreciable 



