OTHER STIMULI 131 



Unpublished researches by Pollock have shown that if the aerial organs are 

 caused to bend gently to and fro, their growth is often at first accelerated, owing 

 either to changes of the tissue-strains or to plastic stretching. Subsequently 

 a certain retardation of growth ensues, and finally, in spite of the continued 

 disturbances, approximately the original rate of growth is resumed. Horvath, 

 Schmidt, and also Meltzer 1 state that violent shaking retards the development of 

 bacteria in nutrient liquids, and ultimately kills them, while Meltzer has shown that 

 some species are more sensitive than others. It is not, however, quite certain that 

 the result is entirely a mechanical effect, for other factors alter when a culture-fluid 

 is continuously shaken. Plasmodia seem to be very sensitive to mechanical 

 shocks and vibrations, whereas the algae which live beneath waterfalls must be 

 highly resistant. Bacteria, yeast 2 , and fungi 3 often develop a little more rapidly 

 when the culture-fluid is gently stirred, probably because the supply of food and 

 oxygen is favourably affected. 



The molecular movements in the protoplast, to which all physiological 

 reactions are ultimately due, may not only be affected by movements in 

 mass caused by mechanical shocks, but also may be affected by wave 

 vibrations in the ether, such as those of radiant heat, light, and electricity, 

 or by directly transferred molecular vibrations, as when heat is conducted to 

 a plant by an enveloping medium. Just as a particular tone will cause a 

 string to vibrate and iodide of nitrogen placed on it to explode, so it is also 

 possible that particular sound-waves may induce resonant vibrations in the 

 protoplasm, and hence produce molecular disturbances involving physio- 

 logical reactions. Reinke 4 in fact found that after 24 hours bacteria 

 had developed hardly at all in a culture-fluid in which a gilded brass tube 

 had been kept in continued longitudinal vibration, and hence concluded 

 that sound-waves hinder development. The experiment is not, however, 

 conclusive, for in small vessels the reflected waves would produce 

 irregular vibrations, and probably considerable internal friction. The 

 true effect of the sound-waves in liquids should be merely one of regular 

 compression and relaxation, and this, if violent, might easily produce a 

 marked effect. 



If the mechanical agency causes an injury, the usual wound-reaction 

 ensues, but even without this, changes of tone and correlative influences 



1 Horvath, Pfliiger's Archiv, 1878, Bd. XVII, p. 125. Cf. also Nageli, Theorie d. Gahrnng, 

 1878, p. 88; B. Schmidt, Centralbl. f. Bact., 1892, Bd. XI, p. 691 ; Meltzer, Zeitschr. f. Biol., 1894, 

 Bd. xxx, p. 464. A summary is given by FHigge, Mikroorganismen, 3. Aufl., 1896, I, p. 135. 



3 Hansen, Meddelelser fra Carlsberg Laboratoriet, 1882, Bd. I, French abstract, p. 94. 

 On the influence of shaking on fermentation cf. Buchner and Rapp, Zeitschr. f. Biol., 1899, N. F., 

 Bd. xix, p. 108. 



3 Ray, Rev. G^n. d. Bot., 1897, T. IX, p. 254. The elimination of the directive action of 

 gravity may be one co-operating factor. 



* Reinke, Pfliiger's Archiv f. Physiol., 1880, Bd. xxm, p. 434. The action of enzymes has been 

 supposed to be due to the transference of molecular vibrations. 



K 2 



