78 PHYSIOLOGY CHAP. 



stimulus is i'oimd capable not merely of exciting protoplasmic 

 movements, but even of determining their direction up to a certain 

 point. Verworn observed that amoebae always move in a direction 

 opposite to the thermal stimulus, i.e. they exhibit negative 

 thermotaxis. Mendelssohn studied on a ciliated infusorium, 

 Paramoecium, the thermotactic influence of different grades of 

 temperature. When one end of a vessel full of liquid, and 

 swarming with Pararnoecia, is heated to 24-28 C., the creatures 

 move to the cooler end of the vessel ; when, on the contrary, one 

 end of the vessel is cooled below the said degrees, the infusoria 

 move towards the warmer end. Thus there may be positive or 

 negative thermotaxis according to the degree of temperature. 



In this case, as in cherno- and barotaxis, the movements are 

 determined by the difference of temperature at the two poles of 

 the Paramoecium, differences which can be estimated at about 

 0-01 C. 



X. Light rays, like heat rays, act as a direct stimulus on 

 comparatively i'ew elementary organisms. In the higher animals 

 they only affect the nervous elements of the retina, and great 

 intensity is required to stimulate the cutaneous endings of the 

 thermal nerves as well. The skin of invertebrates is also excitable 

 to light. 



Many observations have been made in order to determine the 

 nature of the action of light upon Protista, and to ascertain 

 whether excitability to light is a general property of protoplasm, 

 or first appears during the phylogenic evolution of living beings. 

 The results with amoebae were purely negative. Other Rhizopoda, 

 however, were seen to contract on sudden illumination. 



E. Oehl (1886-91) saw that the leucocytes of the blood both of 

 man and frog, when exposed to bright sunshine under the 

 microscope, reacted by active migratory and amoeboid movements, 

 which were not present previous to the photic stimulation. 



The work of Strasburger and others shows that intensity of 

 light exerts a great influence on bacteria and diatoms, so that up 

 to a certain point of intensity they exhibit positive phototaxis, 

 and approach the source of light ; with greater intensity they 

 move farther off, and exhibit negative phototaxis; at a mid-point 

 they show themselves wholly indifferent. The wave-length of the 

 light rays is also of great importance. Engelmann has shown 

 that the Bacterium photometricum (observed in the micro- 

 spectroscope) swarms into the region of the ultra-red rays, and to 

 a less extent into that of the orange and yellow rays, i.e. towards 

 Frauenhofer's D-line (Fig. 21). 



The term heliotaxis has long been employed to denote the 

 common property of plants to turn on their axis in the direction 

 of the sun's rays. The phenomenon is particularly conspicuous 

 in plants grown inside the house. Both stems and petioles curve 



