CHAPTER IX 



ON THE UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 

 IN RESPONSE 



Uniform response in plants — Staircase effect — Fatigue due to molecular strain 

 — Fatigue in plant-responses — Periodic fatigue — Fatigue under continuous 

 stimulation — Explanation of anomalous erection of \ez.i of M/'wosa under con- 

 tinuous stimulation— Conductivity and excitability of tissue diminished through 

 incomplete protoplasmic recovery— Relatively greater fatigue in a motile than 

 conducting organ — Disappearance of the motile excitability earlier than con- 

 ductivity — Refractory period — Absence of responsive effect when stimulus 

 falls within refractory period. 



The mechanical response of plants is fundamentally due, as 

 we have seen, to those molecular changes which are the 

 result of stimulus. These changes bring about contractions 

 of the excited cells, in consequence of which water is ex- 

 pelled, and we obtain longitudinal response in radial organs, 

 or lateral movement in dorsi-ventral organs, the latter being 

 simply a special case of differential longitudinal contraction. 

 On the cessation of stimulus the expelled water is reabsorbed, 

 and the organ resumes its original position. In the case, for 

 example, of the leaves of Mimosa^ this position of equilibrium 

 is, approximately speaking, at an angle of 45° above the 

 horizon, and this, for convenience, may be called the erect 

 position. After a period of rest, then, molecular equilibrium 

 being re-established, the protoplasm recovers its original 

 properties, of which excitability is one, and response takes 

 place on stimulation as before. This resumption by the leaf 

 of its original position may thus be taken as a rough indica- 

 tion of the restoration of its original protoplasmic properties. 

 But this is only true in a general way, for there may be 

 cases, as we shall see, in which the apparent return of the leaf 



