130 PLANT RESPONSE 



effects of chemical reagents are subject to modification, as is 

 fully explained later (pp. 322 and 477), by the tonic con- 

 dition of the specimen. 



The method of procedure consists in first obtaining a 

 series of responses to uniform stimuli under normal con- 

 ditions, and then a similar series after the application of the 

 reagent. The uniform individual stimuli in both series arc 

 applied at intervals which allow of complete recovery. The 

 intensity of stimulus, and the time-intervals, are kept constant 

 throughout the experiment. The chemical agent may take 

 the form of gas or vapour, or it may consist of a liquid. In 

 the former case, by the turning of the three-way stop-cock 

 described on p. 72, water-vapour is passed through the 

 plant chamber, and the responses thus obtained are taken as 

 the normal. By a quick manipulation of the stop-cock, the 

 gaseous or vaporous reagent is next introduced, and the 

 responses now obtained exhibit the physiological modifica- 

 tion induced by it. The effect produced by some agents is 

 permanent, by others transitory. This difference may be 

 demonstrated by turning the three-way cock once more, so as 

 to allow water-vapour torepiace the specific gaseous medium. 

 The responses now obtained exhibit the after-effect. 



The difficulty in this investigation lies in the selection of 

 specimens which exhibit complete recovery, together with 

 uniformity of successive responses. For this purpose we 

 may take any radial organ, one of the most excitable, and 

 therefore suitable, of these being the filament of the corona 

 of Passifloi^a. In this case high magnification is not necessary, 

 and very moderate stimulus is sufficient. Failing Passiflora, 

 I have frequently used with success the staminal filaments of 

 the Uriclis Lily, and Broivnca ariza, and the styles of Datura 

 and Euchans Lily. It should be borne in mind that the 

 excitability of the tissue is to a certain extent influenced by 

 seasonal conditions, being different under different circum- 

 stances of time and weather. 



It is best to choose specimens from flowers which are 

 already open, and in which growth has just ceased. Although, 



