434 PHYSIOLOGY 



Presentation time. — In order to produce any reaction a stimulus of 

 given intensity must act for a definite time, called the presentation time. 

 For the primary reaction this is extremely brief — practically instan- 

 taneous. But end reactions, especially those due to growth, require 

 some minutes or even an hour or more. Thus, roots must be kept hori- 

 zontal for 15-30 minutes or even longer (depending upon the plant and 

 its condition), in order that gravity may cause a curvature. This means, 

 apparently, that the excitation must reach a given pitch through con- 

 tinuous or summated stimulation, before it can be propagated to the 

 active region and affect the growth mechanism. Once that pitch is 

 attained, the end reaction will follow; and if the initial stimulus cease 

 to act, it will follow as an after effect. If the intensity of the stimulus be 

 increased, presentation time is correspondingly shortened (within limits, 

 the ratio is inverse). 



Excitability. — To obtain a reaction it is not enough that a stimulus 

 act upon a plant. The protoplasm must be in a certain condition, or 

 excitation cannot follow. This is clearly recognized when it is said that 

 a " dead " plant no longer responds to stimulation as before. It was 

 once said: " The dead organism is ' dead ' merely because it has lost its 

 irritability; " but this is true only by an extension of the term irritability 

 beyond its usual sense. Closer study reveals the fact that many agents 

 that do not produce death temporarily abolish or reduce or even exalt 

 excitability. When protoplasm is in a condition of excitability, it is 

 also in a condition to carry on well its usual activities; irritability there- 

 fore is associated with other normal physiological qualities covered by 

 the term tone. One experiences the feeling of well-being and vigor ; it 

 comes when all the functions of the body are proceeding properly. 

 So under favorable conditions the plant's functions are all effective and 

 this tonic condition may be assumed as the norm,^ the result of the com- 

 bined responses to many simultaneous external and internal stimuli. 

 Retardation or acceleration of particular functions may then be brought 

 about by the intensification or weakening of particular stimuli of this 

 complex, or by the application of unusual ones. 



Loss of irritability. — ExcitabiUty may be diminished or abolished 

 temporarily by a dose of anesthetics, like chloroform and ether, certain 

 other functions being also interfered with. The precise mode of action 

 is not known. After a time the effect passes away and tonic irritability 



' Note that this is not a fixed or well-defined condition; it is merely the usual, the ordi- 

 nary; and it is assumed purely for convenience. 



