198 BIOLOGICAL LECTURES. 



weeks old respectively, it was found that expansion in a leaf of 

 one of the first took place within 4^-5^ minutes, in one of the 

 second (selecting the fifth or sixth leaf from the cotyledons) 

 the expansion period was 8Jy-ioV minutes, in one of the third 

 (selecting the eleventh or twelfth leaf) the expansion period was 

 13-16 minutes. To fully verify the observations, I compared 

 plants of known age in several establishments, and found the 

 results in all cases to agree, the longest period occupied in 

 expansion — 23 minutes — having been witnessed in a plant 

 kindly placed at my disposal by Dr. Schively and which was 

 four or five years old, but had formed fresh shoots for the 

 season. 



The relative rapidity of contraction and expansion shown by 

 leaves at different levels on the stem is to a large extent retained 

 during the life of each, though with age the movements 

 become more sluggish. 



As regards rate of propagation of stimuli Dutrochet calculated 

 it to be 2-3 m. m. per second in the stem, and 8-15 m. m. in 

 the petiole. Bert in comparing his own results with those of 

 Dutrochet considered that this estimate was much too high and 

 gave 2-5 m. m. per second as the rate. But undoubtedly the 

 estimates of both observers are greatly below the true rate for 

 certain parts of the leaf, as the following will prove. With 

 forceps I delicately pinched the tip of a terminal leaflet borne 

 on a primary leaflet that had 17 pairs of secondary ones. The 

 end leaflets closed within a second, the next lower pair after 5 

 seconds, and all had closed within 9| seconds. Now, even if we 

 grant that the wave of excitation started immediately on applica- 

 tion of the excitatory stimulus, the entire distance having been 

 65 m. m. an average rate of 7 m. m. per second would be the 

 outcome. But many experiments which cannot here be detailed 

 convinced me not only that excitations can be started over any 

 part of the leaflets, but that there is a rapid propagation along 

 the leaf stalk. The starting of cold stimuli by ice clearly 

 verified this. Whether delicately held in the leaf axil against 

 the pulvinus, or placed alongside the pulvinus or against its 

 lower surface that is specially irritable, a bit of ice made the 

 leaf fall within i^ — ij] seconds. Now the length of each 



