5<D PLANT RESPONSE 



the periodic turgor-tension characteristic of the morning has 

 passed off, the plant exhibits normal mechanical responses. 

 We have again seen that when the plant Mimosa is placed 

 for a time in a dark room, and an abnormal condition of 

 turgor — as seen in the erection of its leaves — is induced, 

 the strongest blow will often produce no mechanical 

 response. 



It is thus clear that the fulfilment of certain conditions 

 is necessary, in order that a plant may exhibit its state 

 of excitation, by mechanical response. The absence of this 

 response is therefore no proof of the insensitiveness of the 

 plant. Having thus shown that sensitive plants so called 

 may under certain conditions fail to give motile indications, 

 we shall in the next chapter see whether, on the other hand, 

 ordinary plants- -commonly assumed to be insensitive — may 

 not be found to exhibit mechanical response to excitation ; 

 such mechanical response having been hitherto overlooked, 

 either in consequence of our own imperfect observation, or 

 from the fact that in radial organs excitatory reactions would 

 be likely to have a multi- radial character, which would cause 

 them to balance each other. 



Explanation of absence of lateral response in radial 

 organs. — Taking first, then, the case of a radial stem, that is 

 to say, one whose properties are the same in all directions, 

 we shall find that a single stimulus applied simultaneously 

 on all sides — in other words, a diffuse stimulus — would, even 

 if it produced responsive contraction, result in no visible 

 lateral movement like that seen in Mimosa leaf. This would 

 be due to the fact that the contractions at various diametri- 

 cally opposite points would be antagonistic, and balance each 

 other. Though lateral movement would thus not take place 

 in a radial organ, yet it is possible under favourable circum- 

 stances, as will be shown in the next chapter, to obtain longi- 

 tudinal contraction as seen in muscle. Lateral movement in 

 response to a diffuse stimulus can therefore take place only 

 when there is some difference of excitability as between two 

 opposite halves of an organ. The movement will then be 



