GENERAL RELATIONS lOt 



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 mechanical 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: (i) 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 



