LOCOMOTORY DIRECTIVE MOVEMENTS 549 



they have for their object the placing of the organism in the most favourable 

 relationship to light and to heat. When the intensity of the heat or the light is 

 graduated, the organisms respond, becoming positively photo- and thermo-tactic 

 at infra-optimal intensities and negatively so at supra-optimal intensities. 



There are but few data available as to thermotactic phenomena, so that we 

 may dismiss the subject in a sentence. Thermotaxis has been shown to occur 

 in certain Infusoria, Amoebae ( Verworn, 1901, p. 473), and specially in Myxomy- 

 cetes. In the case of the last-named organisms Stahl (1884) has demonstrated 

 positive thermotaxis by placing one side of a plasmodium of Fuligo in water at 

 a temperature of 30° and the other in water of 7°. He believed that at higher 

 temperatures positive thermotaxis would change into negative, and this idea has 

 been confirmed by Wortmann's experiments (1885). The optimum tempera- 

 ture for Fuligo lies about 36°, above which negative thermotaxis ensues. Whether 

 or not a difference in the temperatures of the two sides of as much as 20° is 

 necessary has not been determined, but it is probable that it need not be quite 

 as much as that. 



Phototaxis, and especially the negative form of it, has also been shown to 

 occur in plasmodia, but the phototactic influence of light has been much more 

 thoroughly studied in relation to swarmspores, whose aggregation on the bright- 

 est sides of vessels placed in diffuse light has long been known. Phototaxis is 

 best seen in the swarmspores of Algae, but it occurs also in the colourless swarm- 

 spores of Chytridium, Polyphagus, &c. ; it is not exhibited, however, by fern 

 antherozoids. Strasburger (1878) has also shown that in many cases the 

 phototactic reaction is dependent on the intensity of the light. If a vessel con- 

 taining swarmspores be placed at a certain distance from a window, it will be 

 seen that as a rule they arrange themselves so that their long axes are parallel 

 with the path of the incident rays, and with their anterior ends facing the light. 

 Further, the spores proceed to make for the most illuminated spot, moving in 

 straight lines. If the vessel be brought gradually closer and closer to the window 

 the light becomes at length so intense as to cause the swarmspores to retreat 

 from it. Obviously a certain optimum intensity exists between these two inten- 

 sities, the attainment of which explains the phototactic movement. Oltmanns' 

 (1892) experiments on this subject are very instructive. He placed motile 

 colonies of Volvox in light of very varied intensity and noticed that they always 

 strove to place themselves where they would be subjected to light of a definite 

 intensity. Not all the colonies, however, behave precisely in the same 

 manner ; the light requirements of each colony, or — as one might say — its 

 disposition with regard to light varies with its developmental condition. The 

 female colonies, in Oltmanns' experiment, placed themselves under much feebler 

 illumination than the asexual specimens, where they effected movements which 

 were extremely peculiar but as yet inexplicable. External influences play also 

 an important part in this light disposition. When the illumination is con- 

 tinuous and bright, and when the temperature is at the same time raised, the 

 light requirements are greater, and the colonies migrate to where the light is 

 more intense (Strasburger, Oltmanns). 



The crowding together of swarmspores at one point inside a vessel exposed 

 to light of gradually changing intensity is to be ascribed to positive and nega- 

 tive phototactic movements, and also to indifference to that intensity to which 

 the region where the crowding occurs is exposed. Although in the case of 

 many swarmspores the region of indifference is by no means sharply defined, in 

 other cases it would appear that it is extremely restricted ; for Strasburger 

 observed positive phototactic swarmspores becoming negatively phototactic 

 quite rapidly as the light was increased without remaining for any length of 

 time in the indifferent condition. 



As in the case of heliotropism, so in phototaxis, the question frequently 



