CYTOTAXIS 365 



is also an instance of chemotropic cytotaxis, and if sensitive Spirillum forms 

 are used these collect a little distance away from the cell. Chemotropic 

 cytotaxis is probably also involved in the attraction of the pollen-tube to 

 the embryo-sac, as well as in the penetration of a host by the hyphae of 

 a parasitic fungus, and in the formation of Lichens. 



In some cases osmotropic, thigmotropic, and even also hydrotropic 

 stimuli may be used for purposes of physiological interaction, whereas 

 thermal, galvanic and photic stimuli are of little or no value in this respect *. 

 Thus few plants are luminous, and the electric currents and differences of 

 temperature due to vital activity are so trifling as to be unable to exert 

 any appreciable tropic stimulation. Reflected rays, or local heating due to 

 external radiation, may, of course, exercise some effect, but these are not 

 within the control of the plant. Thigmotropic reactions, on the other hand, 

 are responsible for the attraction and fusion of the sperm and ovum of 

 Fucus, as well as for the coiling of tendrils round each other. 



The stimuli may either act across short distances, or only when the cells 

 are in contact in the former case attraction being ensured, while in the latter 

 case accidental contact is made permanent. Small objects may be brought 

 together by surface-tension forces, and also repelled from one another 

 without their possessing any special motile organs. In such cases we have 

 a purely physical movement produced in the same way as when an oil-drop 

 comes into contact with a soap-solution on one side. The movement is 

 only physiological in the sense that metabolism causes the production of 

 the substances responsible for the modifications of surface-tension. According 

 to Roux 2 , it is by a physiochemical action of this kind that the separated 

 fragments of a frog's egg creep together again. The plasmodial aggregation 

 of the Myxamoebae of Acrasieae may be brought about in the same way ; 

 but, since the Myxamoebae have a power of independent movement, it 

 seems more probable that we have here another instance of physiological 

 chemotaxis. The fact that the aggregation ceases under certain conditions 

 shows nothing, for it might be due to a cessation of the secretory activity 

 on which the changes of surface-tension and the chemotaxis might alike be 

 dependent. 



1 A regular arrangement may also arise from purely mechanical causes. 



2 Roux, Archiv f. Entwickelungsmechanik, 1894, p. 43; Rhumbler, Biolog. Centralbl., 1898, 

 Bd. xviii, p. 22; Ergebnisse d. Anat. u. Entwick. von Merkel und Bonnet, 1898, Bd. vm, p. 587. 



