CIIEMICAT. REAGENTS ON LONGITUDINAL RESPONSE 131 



as already stated, this is not the most excitable period for the 

 tissue, yet it affords us the advantageof simplified conditions, 

 inasmuch as, owing to cessation of growth, the line of record 

 before stimulation, or the base-line, now remains horizontal. 

 Stimulation, producing contraction, is represented by up 

 movements of response, and the recovery brings the curve 

 back to the base-line. 



Effect of carbonic acid gas. — I shall now proceed to 

 describe a few typical experiments, on the modification of 

 response by chemical reagents, out of a large number, which 

 were performed on the radial organs of plants of various 

 kinds, in the course of the season. And first we shall take 



the effect of carbonic acid. 



The effect of this gas, after 

 the lapse of about half an 

 hour, is one of considerable 

 depression. By this time 

 the responses are reduced 

 to about half their normal 

 value, and this depression, 

 though slow, is continuous. 

 This may be looked for as 

 the general effect, after a 

 certain length of time, of 

 the action of carbonic acid 



gas. The immediate result of the sudden introduction of an 

 abnormal factor may, however, be slightly different in different 

 cases, according to the tonic condition of the tissue. This 

 immediate effect is sometimes one of brief depression followed 

 by equally brief exaltation, to be succeeded by the true 

 depression. Or there may be a short exaltation, followed by 

 the true depression. The restoration of the normal con- 

 dition, however, is generally followed in the case of carbonic 

 acid gas by a gradually increasing exaltation of the response, 

 which may culminate in double its ordinary height, and after 

 this it again attains the normal (fig. 68). 



Effect of hydrogen gas. — We next investigate the 



vvv\ wmvaN^^i^'v 



Fig. 68. Effect of Carbonic Acid Gas 

 on Longitudinal Contractile Response 



(rt) Normal response ; (d) after exposure 

 to carbonic acid ; {c) transient exalta- 

 tion after readmission of air. 



