RESPONSE TO CERTAIN STIMULI 41 



oxygen in some form is necessary, and to most the free 

 oxygen of the air is sufficient. Some forms, however, 

 cannot use this, and so they must obtain the oxygen 

 they require by the analysis of compounds containing it. 



Free oxygen seems to have a stupefying effect on 

 anaerobic bacteria, and they lie dormant until all the 

 free oxygen has been excluded ; on the other hand, 

 aerobic bacteria, if the oxygen supply be deficient, 

 usually associate themselves with diatoms in order to 

 profit by the oxygen thrown off by the diatoms. 



All living organisms respond to electrical stimuli, or 

 exhibit galvanotropism. An electric current, if too 

 violent, causes the death of the animal or plant sub- 

 jected to it, but if it be of mild intensity it may cause 

 the organisms to cease all motion, or they may try to 

 swim away from or to the positive pole. What the 

 exact effect on the living organism is, is not known for 

 certain, but one thing seems to be pretty sure, that a 

 current of fairly high potential is injurious to animal 

 life, whereas it seems to have a beneficial effect on plants 

 when discharged in the air in their vicinity. 



We have seen that plants as well as animals respond 

 to stimuli. The chief difference between the plants and 

 the animals is that in the plant the sensitiveness is more 

 or less diffuse ; in the animal the perception of the 

 stimulus is localised, and there are centres to which these 

 stimuli are transmitted. The animal analyses the 

 stimulus there, and the reaction is caused by a stimulus 

 generated at this centre. In the simplest case the cell 

 that receives the stimulus also reacts, but in most 

 multi-cellular forms the receiving and reacting elements 

 are distinct. In the higher types of animal life this is 



