GROWTH AND MOVKMKNT 



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 to 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. 



fl0\ 



Fig. 672. 



Records of tetanic contraction in muscle (a, b) and in style of Datura (<, 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. 



C. B. & C. BOTANY 



28 



