Feb. 1902.] BOTANICAL SOCIETY OF EDINBURGH 1*79 



to induce and maintain growth is regarded as tonic, while 

 any variation in these conditions produces growth varia- 

 tions, and is described as stimulant. It has to be re- 

 membered, however, that the difference here pointed out 

 is, after all, one of degree and not of kind. From this 

 point of view plant growth is a continuous manifestation 

 of irritability, and it is this power of responding, of 

 manifesting irritability that, in the first instance, 

 enables us to distinguish betw^een a living and a dead 

 organism. 



We take a stimulus here in its widest sense, as defined 

 by Pfeffer (" Physiology," p. 11), as "a push to which the 

 organism responds according to its inherent nature and the 

 means at its disposal," while any result immediately 

 accruing or appearing only after days or weeks have 

 elapsed is a manifestation of irritability. 



When a motor stimulus is applied to resting plant 

 protoplasm, as is well known, an interval of time elapses 

 before the movement begins; and conversely, when the 

 stimulus is one to inhibit a movement already in progress, 

 a corresponding interval passes before inhibition. These 

 intervals are the familiar latent periods, and it is during 

 these periods that the protoplasm exhibits its property 

 called functional inertia (Harris, Brit. Med. Assoc, Aug. 

 1900). In the first case, it is functional inertia of 

 anabolism ; in the second, that of katabolism. One or 

 other is always in evidence after a stimulus, and in many 

 cases both, following in the order either of (1) anabolism, 

 (2) katabolism, or the reverse. Plant physiology supplies 

 copious examples of both phases. 



External influences are efficient as growth stimuli only 

 when they are ap^jlied within definite limits of intensity. 

 We have thus to recognise the existence of stimulatory 

 limits, maxima and minima. The protoplasmic molecules 

 have their own limit of swing and rate of vibration, and 

 no amount of pushing, i.e. of stimulation, will induce them 

 to swing farther or faster. At the outset we are met with 

 a " refractory period " — a non-responsiveness of living 

 matter beyond certain limits. This is the expression of 

 its functional inertia. 



We take up seriatim cases of plant irritability with 



