198 STECKBECK— ON COMPARATIVE HISTOLOGY 



In illustration of the above eight phases, the following illustrative 

 example might be cited in a stimulation act for Mimosa pudica: — When 

 a mechanical stimulus in the form of a forceps pinch is applied to, say, 

 the tenth secondary leaflet from the apex on a left-handed primary 

 leaflet, an extremely short latent period ensues, followed by rise of the 

 stimulated leaflet and almost instantly thereafter by rise of the neigh- 

 boring leaflet. Thus irrito-contraction of the pair initiates movement 

 in the leaf. A propagation of the stimulus then travels in two direc- 

 tions; one more rapidly in an upward direction causing successive closure 

 of all of the leaflets to the tip; another more slowly in downward direc- 

 tion causing similar closure. Such propagation is effected under opti- 

 mum summer conditions in eight seconds to the terminal pair of second- 

 ary leaflets, and ten seconds to basal pair. The distances through which 

 the stimulus is propagated are equal. This, therefore, indicates that 

 the rate of propagation of stimuli is slightly higher in a centrifugal than 

 in a centripetal direction. When the stimulus reaches the base of this 

 primary leaflet a flush change passes over the pulvinus, followed by a 

 shght converging of the secondary petioles. Burnett and Mayo (8, 

 p. 81), Paul Bert (2, p. 12) and successive observers noted the flush 

 changes of the secondary and primary pulvini. The tertiary pulvini 

 also show these changes just before the pairs of secondary leaflets close. 

 Propagation of the stimulus is rapidly effected down the petiole to the 

 primary pulvinus, and a drop of the entire leaf follows in two to three 

 seconds after the change in the position of the secondary petioles had 

 taken place. 



The stimulus next travels into the leaflet adjoining and almost at 

 the same time into the leaflet opposite. A short time thereafter the 

 stimulus reaches the remaining leaflet of the four. In all three leaflets 

 the secondary leaflets close, the stimulus moving toward the apex of 

 each leaflet. Thus in twenty-five to thirty seconds after the stimulus 

 had been applied all the secondary leaflets are closed. 



The neutral period which now ensues may last on an average of three 

 to five minutes. The leaflets then re-expand and the whole leaf rises 

 to its original position. The re-expansion period is ten or eleven min- 

 utes. Thus are illustrated successively a stimulation act, latent period, 

 response of leaflets, propagation of stimuli to successively removed 

 centers, neutral period and re-expansion period. 



Having noted the similarity in sensitive movement and in distri- 

 bution in the Leguminosae and the Oxalidaceae, the histological study 



