1 90 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, but 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 



