GENERAL RELATIONS 101 
not be clearly traced, it may be permissible to speak of internal stimuli, i. e., 
those which appear to originate within the plant. These, however, are 
extremely obscure, and it is hardly possible to deal with them until much 
more is known of the action of external stimuli. Of the latter, certain 
forces, gravity and polarity, act in a way not at all understood, and as 
they are essentially alike for all plants and all habitats, they can here be 
ignored. Stimuli are imponderable when, like light and heat, they are 
measured with reference to intensity, and ponderable, when, as in the case 
of water-content, humidity, and salt-content, they can be expressed in mass 
or weight. It is undesirable to insist upon this distinction, however, since 
the real character of a stimulus is determined by its effect, and the latter 
is not necessarily dependent upon whether the stimulus is one of force or one 
of material. There is, however, a fundamental difference between factors 
with respect to their relation to the plant. Direct factors alone are stimuli, 
since indirect factors must always act through them. For example, the 
wind, its mechanicai influence excepted, can affect the plant only in so far 
as it is converted into the stimulus of increased or decreased humidity. Con- 
sequently, the normal stimuli of the plants of a formation are: (1) water- 
content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. 
Soil, pressure, physiography, and biotic factors influence plants only 
through these, and are not stimuli, though exceptions must be made of 
biotic factors in the case of sensitive, insectivorous, and gall-producing 
plants. 
146. The nature of response. Since plants have no special organs for 
the perception of stimuli, nor sensory tracts for their transmission, an ex- 
ternal stimulus acting upon a plant organ is ordinarily converted into a re- 
sponse at once. The latter as a rule becomes evident immediately; in some 
cases it is latent or imperceptible, or some time elapses before the chain of re- 
sponses finds visible expression. A marked decrease in humidity calls forth an 
immediate increase of transpiration, but the ultimate response is seen in the 
closing of the stomata. A response to decreased light intensity, on the 
other hand, is much less rapid and obvious. This difference is largely due 
to the fact that the functional response is more marked, or at least more 
perceptible in one case than in the other. 
Response is the reaction of the plant to a stimulus; it begins with the 
impact of an efficient factor, and ends only with the consequent final read- 
justment. The immediate reaction is always functional. The nature and 
intensity of the stimulus determine whether this functional response is 
followed by a corresponding change in structure. The consideration of this 
theme consequently gains in clearness if a functional and a structural 
