CHAPTER III. THE PLANT 
STIMULUS AND RESPONSE 
GENERAL RELATIONS 
144, The nature of stimuli. Whatever produces a change in the func- 
tions of a plant isa stimulus. The latter may be a force or a material ; it may 
be imponderable or ponderable; effect, not character, determines a stimulus. 
Consequently, reaction or response decides what constitutes a stimulus. 
The presence of the latter can be recognized only through an appreciable 
or visible response, since it is impossible to discriminate between an impact 
which produces no reaction and one which produces a merely latent one. 
From this it is evident that quantity is decisive in determining whether the 
impact becomes a stimulus. Plants grow constantly under the influence 
of many stimuli, all varying from time to time in amount. Small changes 
in these are so frequent that, in many cases at least, the plant no longer 
appreciably reacts to them. Such-changes, though usually measurable, are 
not stimuli. Futhermore, it must be clearly recogriized that plants which 
are in constant response to stimuli are stimulated anew by an efficient in- 
crease or decrease in the amount of any one of these. As is well known, 
however, stich increase or decrease is a stimulus only within certain limits, 
and the degree of change necessary to produce a response depends upon 
the amount of the factor normally present. The entire absence of a force 
usually present, moreover, often constitutes a stimulus, as is evident in the 
case of light. The nature of the plant itself has a profound bearing upon 
the factors that act as stimuli. Many species are extremely labile, and 
react strongly to relatively slight stimuli; others are correspondingly stable, 
and respond only to stimuli of much greater force. Some light is thrown 
upon the nature of this difference by the behavior of ecads. A form which 
has grown under comparatively uniform conditions for a long time seems 
to respond less readily, and is therefore less labile than one which is sub- 
ject to constant fluctuation. In many cases this is not true, however, and 
the degree of stability, i. e., of response, can only be connected in a general 
way with taxonomic position. 
145. The kinds of stimuli. The factors of a habitat are external to 
the plant, and consequently are termed external stimuli. Properly speak- 
ing, all stimuli are external, but sirice the response is often delayed or can 
