480 



PLANT RESPONSE 



Fig. 



192. Effect of Carbonic Acid 

 Gas on Growth 



the Unbalanced Method. The normal rate of growth was 

 •006 mm. per minute. On passing C0 2 into the plant 

 chamber the immediate effect was an acceleration, the rate 

 for the next five minutes being '009 mm. or i^ times the 

 normal. Under the continued action of carbonic acid, how- 

 ever, the rate underwent a rapid diminution, and, as is seen 



by the slope of the curve 

 becoming horizontal, growth 

 was arrested fifteen minutes 

 after the introduction of 

 carbonic acid into the plant 

 chamber. On the re-intro- 

 duction of fresh air the growth 

 was slowly renewed, and 

 gradually returned to its 

 original rate. The effect of 

 carbonic acid on growth- 

 response, then, is a prelimi- 

 nary exaltation, followed by 

 depression and arrest, which 

 arrest, if the action be not too 

 long continued, proves to be 

 only temporary. 

 Action of ether. — This experiment shows the curious 

 difference of results which occurs, according as an applica- 

 tion is external or internal. In this and in the following 

 cases, with the exception of the experiments on acids and 

 alkalis, I shall use the Balanced Method, as this brings out 

 even transient variations in a very striking manner. The 

 balanced horizontal record is seen to the left of each figure. 

 In curve a of fig. 193 is shown the effect of an external appli- 

 cation of this reagent, ether vapour being introduced into 

 the plant chamber. It will be seen that there was an imme- 

 diate retardation of growth, which lasted for more than a 

 minute. This was followed by an acceleration of growth, 

 which lasted for two minutes. There was then a depression, 

 which continued, and culminated in the arrest of growth. 



The first arrow indicates introduction 

 of C0. 2 , which induces preliminary 

 enhancement of growth, followed 

 by subsequent arrest. The second 

 arrow indicates the introduction of 

 fresh air, followed by the revival 

 of growth. Record taken by Un- 

 balanced Method. 



