IRRITABILITY. 379 



Pfeffer. From his measurements it is evident that each 

 variation, whether it be a rise or a fall of temperature, is 

 followed by a temporary acceleration of the mean rate of 

 growth of the perianth-leaf as a whole ; after a time the rate 

 of growth adapts itself to the tonic influence of the tempera- 

 ture, whether it be higher or lower, to which the flower has 

 been exposed. 



All the explanation which can be given of these facts, 

 without venturing on speculation, is this ; that the perianth- 

 leaves of these plants are so constituted that they respond to 

 the stimulating effect of a rise of temperature within certain 

 limits by an accelerated growth of the upper surface, by what 

 we may term induced epinasty, and to all falls of temperature, 

 as well as to all rises above the upper limit, by an accelerated 

 growth of the lower surface, by induced hyponasty. Unlike 

 the- great majority of plant- organs, these perianth-leaves are 

 endowed with a specific irritability to variations of tempera- 

 ture, as is clearly proved by the fact that any variation of 

 temperature is followed by an acceleration of their mean rate 

 of growth. That this acceleration should express itself in the 

 form of heterauxesis we may probably attribute to the dorsi- 

 ventrality (p. 366) of the organs, but we are quite unable to 

 account for the fact that the acceleration due to a rise of 

 temperature should particularly and constantly affect the 

 upper surface, and that due to a fall of temperature should 

 similarly affect the lower surface. 



LIGHT. 



In entering upon the consideration of the influence of 

 light upon the rate of growth, we will begin by enquiring into 

 the tonic influence of light, that is, into its relation to the 

 irritability and motility of the protoplasm of growing organs, 

 and we will then go on to study the stimulating action of 

 variations in the intensity of light, and, finally, the combined 

 tonic effect of light and of temperature. 



