322 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



The negatively phototactic movement of the plasmodia of Myxomycetes 

 is possibly directly due to the retarding action of the strong illumination 

 upon the amoeboid activity of the exposed side, but it is uncertain whether 

 similar phototropic movements and aggregations may be produced within 

 cells covered by cell-walls, apart from those shown by the chloroplastids of 

 Mesocarpus and other plants. 



In many cases a sufficient rise in the intensity of the illumination 

 causes the positive phototaxis to become negative, so that Weber's law can 

 only apply within certain limits. Strasburger * observed only an attraction 

 to the light in the case of the zoospores of Botrydium granulatum, and of 

 the Flagellate Infusorian Chilomonas curvata. This was possibly due to 

 the fact that the organisms were temporarily adapted to high intensities 

 of light, since Stahl 2 found that the zoospores of Botrydium granulatum 

 readily performed negatively phototactic movements. Moderately strong 

 sunlight is sufficient to produce this in most zoospores, but in others 

 comparatively feeble light suffices, as, for instance, in the case of most 

 Diatoms, while no positive phototactic action at all has been detected in 

 Myxomycetes. 



Zoospores hence usually collect at a certain distance from the source 

 of illumination, but instead of coming to rest, continually cross and recross 

 the imaginary line of equilibrium, where, we may assume, they would take 

 on a diaphototropic position if incapable of locomotion. Owing to the 

 unequal irritability of different individuals, the position of equilibrium varies 

 even in the same species. In addition, periodic autogenic changes of tone 

 may occur, which, under constant conditions, may cause the zoospores to 

 swim at intervals from one side to the other of a drop of water in the path 

 of the light 3 . In some cases this reversal is slowly produced, but in the 

 zoospores of Ulothrix zonata it may take place so rapidly, that the 

 zoospore, immediately it has reached one side, swims back to the other. 

 Another instance of autogenic reversal is afforded by the fact that 

 the Desmid Closterium moniliferum turns first the young end and then 

 the older one towards the light at intervals of six to thirty-five minutes 4 . 



Changes of tone also occur during development 5 , and they may be in- 

 duced to a greater or less extent by alterations in the cultural and external 

 conditions. Thus it requires a stronger illumination to change positive into 

 negative phototaxis when zoospores are used which have developed in strong 

 light than when they have developed under feeble illumination 6 . Similarly, 



1 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 26. 

 3 Stahl, Einige Bemerkungen u. d. richtenden Einfluss d. Lichts auf Schwarmsporen, 1879. 

 Reprint from Verh. d. phys.-med. Ges. zu Wiirzburg, N. F., Bd. XIV. 



8 Strasburger, 1. c., pp. 17, 38. * Stahl, Bot. Ztg., 1880, p. 396. 



5 Cf. Strasburger, 1. c., p. 38 ; Oltmanns, Flora, 1892, p. 187. 

 c Strasburger, 1. c., p. 39 ; Oltmanns, 1. c., p. 191. 



