MOVEMENTS RESULTING FROM SHOCK 517 



appears to be identical in both cases. Mimosa is also sensitive to chemical 

 stimuli (CoRRENS, 1892). Many substances which induce movement, such as 

 hydrochloric acid vapour, may certainly injure the plant so much as to cause 

 death ; but ammonia may be made to induce movements without at the same 

 time causing injury, provided the doses be carefully regulated, and repeated 

 responses may be obtained by successive applications of the stimulant. Electric 

 currents will also induce movements (Bert, 1870), and it is possible that the 

 reactions which are consequent on high temperature and intense illumination 

 (P- 505) are to be regarded rather as related to those resulting from a blow than 

 to those which are induced by change in light intensity. Detailed investigations 

 on this point have still to be undertaken, however. 



The sensitivity above described is not confined to Mimosa ; other Legumi- 

 nosae, such as Neptunia oleracea and Desmanthus pienus, and some Oxalidaceae, 

 such as Biophytum sensitivum, are known to be very sensitive. To a more 

 limited extent all Leguminosae and Oxalidaceae, possibly all plants with pulvi- 

 nate leaves, are to be considered as sensitive to shock (Hansgirg, 1893), only in 

 these cases the movements are induced by more powerful stimuli and take place 

 only under optimal external conditions. One blow is frequently not sufficient 

 to bring about a visible response, although several blows, by summation of 

 stimuli, gradually induce a movement {Robinia, Oxalis sp.). The sensitivity 

 of these plants recalls in its character that of tendrils ; sensitivity to contact is 

 closely related to sensitivity to shock, for between Mimosa and tendrils, as the 

 two extremes of the series, there are many intermediate conditions. Into the 

 discussion of these, however, we need not enter. 



We have yet to consider the transmission of the stimulus, which has been 

 most thoroughly studied in Mimosa and Biophytum. Let us look at the phe- 

 nomenon as it manifests itself in Mimosa. When this plant is grown under 

 suitable external conditions, not only does the leaf droop when friction is applied 

 to the primary articulation, but after a brief interval the pinnae also assume the 

 folded rest position. If, on the other hand, one of the outermost leaflets be stimu- 

 lated, not only does that leaflet move but the movement spreads to the opposite 

 leaflet and onwards to the pinnae inserted lower down the petiole, all of which 

 fold together in pairs. It has already been noted that a wound operates much 

 more effectively than mere contact. If the terminal pinnae be scorched with 

 a glowing splinter or with a burning-glass, the stimulus is rapidly conveyed to 

 the base of the secondary petiole and on to the three other secondary petioles, 

 the pinnae of which also fold together from the base outwards. The secondary 

 pulvini also take up the sleep position and shortly afterwards the primary articu- 

 lation follows suit. But the process is not yet complete. After a short time 

 the primary petiole of the leaf next above or next below suddenly droops and 

 the stimulus soon affects the rest of the pulvini also. The stimulus may also 

 be transmitted from the stem itself. If a deep cut be made in the stem, care 

 being taken that it be not at the same time shaken, after a short interval move- 

 ments appear in the neighbouring leaves. The stimulus may, when the con- 

 ditions are favourable, be transmitted to a distance of half a metre, and the 

 transmission is effected with a rapidity which, though indeed feeble when com- 

 pared with the conduction of impulses in the nerves of animals, is very 

 considerable as compared with other cases of like nature in plants. Various 

 values have been recorded by different authors for the rapidity of transmission 

 of the stimulus, and this is not to be wondered at, seeing that the plant is not 

 always in the same physiological condition and that the stimulus may also vary ; 

 stiU, allowing for the personal equation, there are many differences \yhich are as 

 yet inexplicable. At least it is certain that the stimulus is transmitted at the 

 rate of several milhmetres (2-15) per second ; by way of comparison we may 

 remember that the speed of transmission in the nerves of the higher animals is 

 about a thousand times as great, while the most rapid heliotropic response 



