THE CELL 15 



cause the adjacent fluid to move over the cell. Such mobile 

 processes when permanent have been called cilia. 



These movements are modified by the various STIMULI 

 which modify the activity of the chemical changes in the 

 protoplasm (p. 5). Thus, cooling diminishes, and finally stops 

 them. Gentle heat increases them, but when a certain 

 temperature is reached, they are stopped. Drying and various 

 drugs, such as chloroform, quinine, etc.,- also arrest the 

 movements. 



Changes in the surroundings may cause either contraction 

 or expansion, may repel or attract. When an attracting or a 

 repelling influence, a positive or negative stimulus, acts at 

 one side of the cell unilateral stimulation it may lead to 

 movement of the cell away from it or towards it. Movements 

 are produced by various chemical substances (ehemiotaxis), 

 or by light (phototaxis), or by electricity (galvanotaxis). If 

 the action is towards the stimulus, it is said to be positive, 

 if away from it, negative. 



Chemiotaxis is the attraction or repulsion produced by one- 

 sided application of chemical stimuli. This is well seen in 

 the plasmodial masses of a^thalium septicum which grow on 

 tan. Oxygen and water both attract it towards them, and 

 exercise a positive ehemiotaxis. It is also seen in the stream- 

 ing of the white cells of the blood to disintegrating tissues, 

 or to various micro-organisms which have to be destroyed to 

 prevent their poisoning the organism, and in the attraction 

 exercised by the ovum upon the male element in reproduction. 



Barotaxis is the effect of unilateral pressure or mechanical 

 stimulation. Many protozoa appear quite unable to leave 

 the solid substance e.g. the microscope slide with which 

 they are in contact, the unilateral pressure seeming to cause 

 a positive attraction in that direction. 



Phototaxis. Light, which plays so important a part in 

 directing the movements of the higher plants, also acts 

 positively or negatively on many unicellular organisms. Thus 

 the swarm spores of certain algse 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 plays an important part in directing 

 the movements of certain bacteria. 



