PROPAGATION OF MECHANICAL AND CHEMICAL STIMULI 95 



when an incision is made in the vascular bundles. Similarly the transference of an 

 excitation from one pair of leaflets to another in the intact plant only takes place 

 when it is sufficiently turgid, and may hence safely be assumed to be due to the 

 hydrostatic pulsation aroused by the sudden escape of water from the directly- 

 stimulated cells, possibly aided by the sudden bending of the part of the vascular 

 bundles lying in the, pulvinus. Since this pulsation is usually comparatively feeble, 

 it is only natural that the abscission or burning of a leaflet should produce an effect 

 which, being more intense, is propagated to a greater distance than that due to 

 touching a single leaflet. 



According to Haberlandt, the conduction of stimuli takes place in the tannin- 

 tubes l of the phloem, which transfer positive or negative pressure waves to the pul- 

 vini, and these mechanically excite the motile cells. Macdougal, however, denies that 

 the stimuli are transmitted by hydrostatic pulsation in this manner. Haberlandt's 

 conclusion is mainly based upon the fact that the drop of liquid which escapes from 

 an incised vascular bundle is, for the most part, derived from these tannin-sacs. 

 Transmission is, however, also possible in their absence, for Dutrochet 2 found a con- 

 duction of stimuli was still possible when incisions were made through all the tissues 

 excepting the wood. Haberlandt has also overlooked the fact that in a dead portion 

 of the stem the conditions for the transference of a pressure wave through the sap- 

 containing tannin-sacs are not fulfilled 3 . In addition Borzi 4 has found that the con- 

 duction of stimuli in Aeschynomene indica and Neptunia oleracea takes place in tissues 

 which do not possess any continuous system of tannin-sacs. 



Macdougal found that no stimulation was produced when as large a cut surface 

 as possible of the shoot was submerged in a solution of potassium nitrate, so that 

 a sudden fall of turgor was produced in the exposed tissues, including the tannin-sacs. 

 Negative results were also obtained when the pressure with which water was driven 

 into a cut surface of the stem was suddenly raised by three to eight atmospheres. 

 Macdougal found that the rise of pressure was rapidly transmitted in the xylem vessels, 

 and also in the tannin-sacs, to the furthermost shoots and leaves, so that it is evident 

 that not every movement of water or change of pressure is able to transmit a stimulus 

 to the pulvinus. 



[These results of Macdougal's do not necessarily show that the transference of 

 the stimulus is due to a special stimulatory substance, and indeed do not afford con- 

 clusive proof that the transference is not due to a hydrostatic pulsation. 



The cells of Char a and Nitella, for instance, respond to seismonic stimulation 



1 For details concerning these tubes and their contents, cf. Haberlandt, 1890, 1. c., Physiol. 

 Anat, 2. Aufl., 1896, p. 482 ; Baccarini, Bot. Centralbl., 1893, Bd. LIV, p. 171 ; Borzi, L'apparato 

 di moto delle sensitive, 1899. (A reference is given in the Bot. Centralbl., 1899, Bd. LXXX, p. 351.) 

 Since these tubes occur in other plants, and are primarily together with their contents of metabolic 

 importance, they can only secondarily have developed a power of conducting stimuli in certain 

 plants. 



2 Dutrochet, 1824, 1. c., p. 69. Confirmatory results have been obtained by Haberlandt, 1890, 

 1. c. ; Macdougal, 1. c. 



3 The living portion of the tube shuts itself off from the injured portion, according to Haberlandt, 

 and without this no restoration of turgor would be possible in the tube. 



* Borzi, I.e., p. 4. 



