GROWTH AND MOVEMENT 



433 



cease entirely. When the stimuli recur very frequently, the responses 

 become for a time combined, so that the organ assumes a fixed position 

 unlike the unstimulated one. This quite resembles the condition of a 

 muscle in tetanus, as can be seen by comparing the records in fig. 672. 

 After a period of tetanus, however, the reactions cease until rest from 

 excitation permits recovery. If stimulation, too brief to produce the 



Fig. 671. — Uniform electrical response in radish lo repeated stimulation. — After BosE. 



end reaction, be repeated at proper intervals, the separate effects be- 

 come combined and suffice presently to call forth the end reaction. 

 This summation of stimulation seems to be a sort of tetanic piling up of 

 the earlier excitations of the series, which finally becomes sufficient to 

 transmit its effects to the active region. 





Fig. 672. — Records of tetanic contraction in muscle (a, i) and in style of Datura (c, d) : 

 a, c, incomplete; b, d, more complete. — After BoSE. 



Reaction time. — Some time elapses between the beginning of stimu- 

 lation and the end reaction, and this is appropriately called reaction 

 time. Whereas in animals this is usually measured by a fraction of a 

 second, in plants it is much longer, occasionally a few seconds, but often 

 minutes or even hours. This tardiness is due not so much to a low 

 degree of sensitiveness, for the first reaction (perception) takes place 

 almost instantly, as to slow propagation and especially to the sluggish- 

 ness of the mechanism of growth. By contrast, turgor mechanisms usu- 

 ally respond quickly. Naturally the reaction time is made up of the 

 perception time (a small fraction of a second), the transmission time (the 

 rate varies commonly from o to 4 cm. per second), and the growth time, 

 which is far the greater part of the whole period. 



