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 aiid 
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 agent? 
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, 1 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. Excitability 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 
1 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. 
