IRRITABILITY; POLARITY AND CORRELATION 345 



mal nerves, but his ideas on these questions have not yet had a 

 large following in the Western World. Response in plants is gen- 

 erally much slower than in animals, indicating that the channels 

 of transmission are not so efficient. Each cell in the plant trans- 

 mission chain is much shorter than the nerve cells with their 

 neurons, with the result that the stimulus cannot travel far in 

 one cell before it must cross the boundary of the adjacent one. 

 The stimulus travels much like the impulse one sets up when 

 tumbling down a row of blocks. As each block falls it impinges 

 upon the next one, which then transmits the impulse given it to 

 the next, and so on until the end block is overturned. There is 

 thus a series of stimuli and responses as the original stimulus 

 travels down the transmission line. 



The Response. — By the " response " is generally meant the 

 first visible end response. The change within the cells during 

 the transmission is a response to the original stimulus, but this 

 reaction (which is not the end one) is not easily seen and is not 

 the response meant. When light acts upon a stem many changes 

 may take place within the cells before the stem actually starts to 

 bend towards the light, but only this final bending is included in 

 the visible response. 



The same stimulus may bring about different reactions or re- 

 sponses, depending upon the machinery. By pressing a button 

 one may ring a bell, turn on a light, start a motor, etc. The stimulus 

 of pressing the button is the same in all cases, but the response is 

 different because the apparatus is very different. Similarly in 

 plants, the stimulus of electricity may cause a stamen to shorten, 

 the leaf of a sensitive plant to drop, or a tendril to curl. Gravity 

 may cause stems to go up and roots to go down. In all these exam- 

 ples the stimulus may be the same, but it operates through a 

 different mechanism and results in different responses. 



Reaction Time.— A stimulus presented for a very short time 

 may bring about a change in the cells in the immediate neighbor- 

 hood without ever causing any ultimate visible reaction or re- 

 sponse. To bring about the response, the stimulus must be pre- 

 sented for a minimum time, which is called the presentation time. 

 After the presentation of the stimulus, it requires some time for the 

 stimulus to be transmitted. The transmission time is relatively 

 short (a few seconds or minutes), but before the response can be 

 seen the cells in the reacting region must have time to react. The 



