26 VETERINARY PHYSIOLOGY 



they are in contact, the unilateral pressure seeming to cause 

 a positive attraction in that direction. It is well seen in 

 climbing plants. 



Phototaxis. — Light, Avhich plays so important a part in 

 directing the movements of the higher plants, also acts posi- 

 tively or negatively on many unicellular organisms. Thus, 

 the swarm spores of certain algas are positively attracted 

 by moderate illumination, streaming to the source of 

 light, while they are negatively stimulated by strong 

 light, and stream away from it. Light also pla3's an 

 important part in directing the movements of certain 

 bacteria. 



Thermotaxis. — The unilateral influence of temperature is 

 well seen in the plasmodium of cethalium septicmn which 

 streams from cold water towards water at a temperature of 

 about 30° C. 



Galvanotaxis. — A current of electricity has a marked effect 

 in directing the movements of many cells. Certain infusoria, 

 when brought between the poles of a galvanic battery, stream 

 towards the negative pole, while other organisms move to the 

 positive. 



The etfects of this unilateral stimulation are of g-reat 

 importance in physiology and pathology, since they explain 

 the streaming of leucocvtes to the intestine durinof dig-estion 

 and to parts of the body infected by micro-organisms and 

 other poisons. They also explain the apparently volitional 

 acts of unicellular organisms. Many of these organisms 

 appear definitely to select certain foods, but in reality tliey 

 are simply impelled towards them by this unilateral 

 stimulation. 



B. Nucleus. 



(1) Structure. — The nucleus, seen with a moderate magni- 

 fying power, appears in most cells as a well-defined circular 

 or oval body situated towards the centre of the cell (figs. 2 (c) 



