TRANSMISSION IN MIMOSA 643 



The stimulus-transmitting elements of Mimosa pudica are not 

 situated, as Dutrochet, Sachs and Pfeffer, among others, have supposed, 

 in the woody cylinder, or in the hadrome portions of the primary 

 bundles, but on the contrary occur in the leptome-strands, where they 

 take the form of elongated tubular cells arranged in longitudinal series. 

 In the main petioles and stems, these cells attain a length of "6-1 '2 mm., 

 and an average width of - 018 mm. Their membranes are thin, 

 soft and colourless, and give a deep violet reaction with chlor-zinc- 

 iodine. The longitudinal walls are well provided with pits. Each of 

 the transverse septa, which may or may not be oblique, bears not 

 necessarily at its centre a single wide circular pit, the closing-mem- 

 brane of which is perforated by minute pores containing protoplasmic 

 threads (Fig. 266). The cell-contents comprise a thin peripheral layer 

 of cytoplasm, a large rounded or somewhat elongated nucleus, and cell- 

 sap. The composition of this cell-sap is of interest ; it contains, in 

 addition to a gummy or mucilaginous substance, a large amount of 

 aglucoside or glucoside-like compound in solution, which is responsible 

 for the intense reddish-violet reaction with ferric chloride and the 

 reddish coloration with ferrous sulphate and also suspended granules 

 of a resinous character. 



These stimulus-transmitting cells form a continuous system of tubes, 

 which accompany the leptome-tissue throughout the stems, main 

 petioles (Fig. 267), sub-petioles and pinnules: consequently they are to 

 be found in the pulvini of petioles and pinnules, where they are 

 separated from the motor-tissue in the sensitive half of the pulvini by 

 abundantly pitted collenchymatous elements. In the adaxial marginal 

 bundle of each sub-petiole, the transmitting elements form short-celled 

 " nodal points " between the two members of each pair of pinnules ; at 

 these points stimuli can travel transversely between opposite pinnules, 

 as well as along the sub-petiole. In the root-system, the transmitting 

 cells are confined to the central cylinder in the main root, and are 

 entirely absent from the lateral roots. 



The effects of incision (see above, p. 641) show that stimuli are 

 actually propagated in this system of highly turgescent tubes, and 

 that the mode of transmission is a hydrodynamic one. If, namely, 

 one or more of the tubular cells are laid open by an incision in 

 the stem or petiole, their cell-sap instantly escapes in the form of a 

 drop of transparent liquid ; immediately afterwards the nearest pulvinus 

 carries out its characteristic movement. The micro-chemical reactions 

 of this drop of liquid more especially the violet coloration with ferric 

 chloride prove that it is not derived from the woody cylinder, which 

 indeed need not be wounded in order to produce the aforesaid conse- 

 quences. The liquid that escapes is, in fact, not water at all, but 



