5i8 TRANSFORMATION OF ENERGY 



observed as yet, viz. 0-3 mm. per second (Brodiaea congesta ; ROTHERT, 1894, 

 137), is 10 to 50 times slower than the rate of transmission in Mimosa. 

 Apparently the transmission of the stimulus in tendrils comes next to that 

 of Mimosa, so far as rapidity is concerned. 



One must not, however, compare the transmission of stimuli j in the 

 animal nerve with transmission in Mimosa, seeing that in the former con- 

 duction is effected by living protoplasm which is not the case in the latter. As 

 a matter of fact, the stimulus in Mimosa may travel by way of tissues which have 

 been killed by narcotics (PFEFFER, 1873 b) or by heating (HABERLANDT, 1890), 

 and hence the conception of a transmission by living cells, and especially by 

 intercellular protoplasmic strands, is excluded from consideration. DUTROCHET 

 (1837) assumed that the stimulus travelled by way of the vascular bundle, and 

 PFEFFER has endeavoured to confirm this view. The chief argument used 

 was derived from an experiment of DUTROCHET' s in which vigorous stimuli 

 were propagated through a portion of a stem which had been deprived of its 

 cortex, and in which the pathway available for the transmission of the stimulus 

 was exclusively the vascular elements. PFEFFER endeavoured to show that the 

 movement of water in the vascular tissue might serve as the medium of trans- 

 ference of the stimulus. Some of the water extruded from the pulvinus on 

 stimulation might, he thought, enter the vascular bundle, and this sudden move- 

 ment of water might be propagated along the vessels and so release a stimulus in 

 other pulvini. It is immaterial how this movement of water arises provided only 

 it be rapid, for movements of water connected with transpiration are well 

 known to produce no effect at all. If an incision be made in the stem, the stimu- 

 lus, as we have seen, is transmitted to neighbouring pulvini, but only when the 

 vascular bundles are touched and when the extravasation of liquid from them 

 demonstrates directly the movement of water taking place in them. 



HABERLANDT'S (1890) elaborate researches have completely upset this 

 theory. This author showed that the sap referred to in PFEFFER' s experiment 

 did not by any means come out of the vessels, but from tubular cells in the 

 phloem, corresponding to the tannin canals of other Leguminosae, but differing 

 from them in the possession of numerous fine pores in their transverse walls. 

 These pores allow of easy transference from place to place of the whole of the 

 cell-contents, and every injury inflicted on such a cell permits an abundant 

 extravasation of sap, just as in the case of sieve- tubes, and which may be recog- 

 nized by its characteristic constituents. Movement also takes place when the 

 stem is cut only when an alteration in pressure follows in these tubular cells, and 

 such a change in pressure may be transmitted just as well in the dead as in the 

 living tube. HABERLANDT has further shown that in DUTROCHET' s decortica- 

 tion experiment the whole of the tissue, right into the wood, was not removed, 

 but that the entire phloem along with the conducting hyphae was retained. 

 These tubular cells HABERLANDT regards as the specific transmitters of stimuli 

 in Mimosa. The one criticism, however, which militates against HABERLANDT'S 

 theory, is derived only from the result of the experiment where the cortex has 

 been completely removed. This experiment showed that after the removal of 

 the whole of the conducting tissue the stimulus could be transmitted by the xylem 

 only. Although HABERLANDT has very ingeniously fitted this fact into the tail 

 end of his theory by a subsidiary hypothesis, we are inclined to regard it some- 

 what in the light of the heel of Achilles ! Further experiments specially directed 

 to this point must show how transmission of the stimulus is effected when the 

 xylem is interrupted but when the conductive tissue remains intact. 



If this experiment should show that the xylem is indispensable for the 

 transmission of the stimulus then we must fall back on the DUTROCHET- 

 PFEFFER theory. One further point must, according to our idea, be cleared up 

 in this theory, viz., how the liquid which exudes from the cells of the sensitive 

 half of the articulation succeeds in entering the vessels. PFEFFER himself was 



