IRRITO-CONTRACTILITY IN PLANTS. 195 



But as I shall point out later, it would be a mistake to suppose 

 that the actual rate of propagation of stimulus from apex to 

 base of the leaf is so slow as this. If the leaf operated on was 

 the fifteenth from the growing apex of the stem and the tenth 

 leaf be now chosen and similarly operated on, the time interval 

 between contraction of succeeding pairs of leaflets will be 

 1 1 2 seconds; with the seventh leaf from the apex the interval 

 will be i^-if seconds; and finally a delicate light-green leaf, 

 such as the second or third from the apex, shows closure of 

 successive pairs with an interval between of i-i|- seconds. 

 The latent period shown after general stimulus in such a set 

 of leaves is equally variable, and under optimum surroundings 

 is only \- f of a second in young leaves such as the third from 

 the apex, | i second in the seventh or eighth, i i^ in the 

 tenth or eleventh, and \\- if in the fifteenth. 



As mentioned above, the angle through which the leaflets 

 fall on excitation, is from 40 -45. If a general shock 

 be given to the entire leaf, the leaflets fall simultaneously 

 through an angle of equal amplitude. But summation action 

 can now be brought to bear, for on second stimulus the leaflets 

 will fall through an angle of 24-28 and in a correspondingly 

 short time. On third stimulus the leaflets will fall through 

 1 2 - 1 4 in a still shorter interval, while a fourth stimulus will 

 cause a fall through 4- 5 in a shorter interval than the last. 

 By this time the leaflets will be folded downward back to back 

 as in night-sleep, or a fifth stimulus effecting a slight additional 

 movement may be needed to complete the process. 



A small bit of ice weighing 1 of a grain, if delicately placed 

 on the tip of a terminal leaflet so as not to touch or directly 

 stimulate the pulvinus cells, starts motion in the one opposite 

 within 7 seconds. A steady impulse is then propagated down 

 the leaf -stalk from pair to pair, the time interval, as in 

 mechanically stimulated leaves, depending on the age of the 

 leaf. If, as has repeatedly been noticed, a pair of leaflets is 

 encountered which is somewhat benumbed from the effect of 

 previously applied chemical or other agents, these will remain 

 motionless, or nearly so, but the same interval of time will elapse 

 before the next pair beneath will contract as would have been 



