76 RESEARCHES ON IRRITABILITY OF PLANTS 



often be obtained in this way after a preliminary period of 

 variation. 



The periodic variation seen in the above cases some- 

 times finds still more complex expression. This is the case 

 where waning and waxing occur in series instead of simple 

 alternation. That is to say, response may undergo a 

 continuous diminution in a sequence of three or more, to 

 be followed by a corresponding sequence in which the 

 amplitudes wax larger and larger, such serial alternations 

 being repeated. 



Staircase Response 



We have seen the responses that characterise highly 

 excitable specimens, in which there is an exhibition of 

 growing fatigue. Taking a specimen in the contrasted 

 condition of more or less sub-tonicity, we obtain an equally 

 characteristic effect, which is the antithesis as it were of 

 that which we have been considering. In this, successive 

 responses undergo a gradual enhancement, or what is known 

 in muscle-response — with which it is exactly parallel — as a 

 staircase increase (figs. 37, 38). After attaining a maximum 

 excitabiUty, under successive stimulations, there generally 

 ensues a fatigue-decline. 



Before entering on a detailed description of this parti- 

 cular response it would be well to discuss certain pheno- 

 mena characteristic of a relatively a-tonic condition of the 

 tissue. In a specimen in the normal condition there is a 

 certain amount of tonicity, accompanied by a moderate 

 degree of contraction. When deprived of the invigorating 

 influences of favourable external stimuli the plant becomes 

 sub-tonic, such relative a-tonicity being characterised by 

 relaxation or the absence of normal tonic contraction. 

 Under the action of successive stimuli the tonic condition 

 of the specimen will be improved. The loss of tone, with 

 its consequent relaxation, will gradually give place to a 

 better tone with increasing tonic contraction. Or the 



