CHAPTER XI. 



IRRITABILITY; INFLUENCE OF LIGHT; 

 CHEMIOTACTIC PHENOMENA. 



Irritability. 



hitrodiiction. — Irritability, or the response of a living 

 organism to external influences, is characterized by a move- 

 ment of an organism as a whole, or in part, either towards 

 or away from the source of influence. Capacity for move- 

 ment is one of the properties of protoplasm, and is as general 

 among the lower plants as the lower animals. The higher 

 plants are rooted by their habit to the soil, and so are bound 

 by their physical conditions, but even they make a partial 

 response to external influences. For example, when young 

 cress seedlings are made to grow at the dark side of a room 

 their stems assume a horizontal position in their endeavour 

 to reach the light. Movements either of attraction or of 

 repulsion may be caused by light, gravity, water, injury, and 

 various chemical substances. The movement is usually, but 

 not invariably, one that is distinctly beneficial to the plant. 

 As an exception may be mentioned the movement of certain 

 bacteria towards corrosive sublimate which is toxic to them. 



Much has yet to be learnt about the response of the sulphur 

 bacteria to external influences, but there are considerable data 

 on the effect of light and of various chemicals in inducing 

 movements. Some of the coloured bacteria of this group are 

 probably the m.ost sensitive of all organisms to light, and the 

 response both of coloured and of uncoloured sulphur bacteria 

 to certain chemical compounds is very marked. 



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