[Illustration: FIG. 9.--Flower of _Catasetum_; _c_, column; _h_, horns; _l_, labellum.]
Here again we have adaptive behaviour of exquisite nicety, and we have the transmission of an impulse very rapidly along the cells of the irritable horns, followed by the sudden rupture of a membrane. Beautiful, however, as is the adaptation, effective as it is to a definite biological end, the organic behaviour does not afford any indication of the guidance of consciousness. Among plants we have many interesting and admirable examples of organic behaviour; but nowhere so much as a hint of that profiting by individual experience which is the criterion of the effective presence of conscious guidance and control.
[Illustration: FIG. 10.--_Catasetum_; C, diagram of column; _a_, anther; _an_, horn; _d_, adhesive disc; _f_, filament of anther; _g_, ovarium; _ped_, pedicel; D and E, pollinium; _p_, pollen-mass. (From Darwin’s “Orchids.”)]
It is sometimes said that the tentacles of the Sun-dew leaf indicate a primitive kind of reflex action in plants, and that they afford evidence of discrimination. “It is,” says Romanes, “the stimulus supplied by continuous _pressure_ that is so delicately perceived, while the stimulus supplied by _impact_ is disregarded.”[11] And, comparing this with what is observed in the Venus’s Fly Trap, he says: “In these two plants the power of discriminating between these two kinds of stimuli has been developed to an equally astonishing extent, but in opposite directions.”[12] It is well, however, to avoid terms which carry with them so distinctively a conscious implication as “discrimination” and “perception” do for most of us. Just as the photographer’s film reacts differently according to the quality of light-rays, violet or red, which reach it, so do many organic substances react differently to stimuli of different quality, irrespective of their intensity. The “discrimination” of plants and of some of the lower animals is of this kind, and it is better to speak of it simply as differential reaction. There can then be no chance of its being confused with conscious choice.
Nor should the movements of the Sun-dew tentacles or of those of the Sea-anemone be termed in strictness reflex action. As originally employed by Marshall Hall, and, since that time, by common consent, _reflex action_ involves a differentiated nervous system. There is, first, an afferent impulse from the point of stimulation passing inwards to a nerve-centre; secondly, certain little-understood changes within this centre; and thirdly, an efferent impulse from the centre to some organ or group of cells which are thus affected. In plants there is no indication of anything analogous to this specialized mode of response. The impulse passes directly from the point of stimulation to the part affected without the intervention of anything like a nerve-centre. In the sensitive Oxalis the impulse passes directly to the point of insertion of the leaflet or leaf-stalk; in Catasetum, from the horn to the retaining membrane; in the Sun-dew, from the affected tentacle to those in its neighbourhood. Even in the Sea-anemone, though there is a loosely diffused nervous system, the passage of the impulse from one part of the circlet of tentacles to other parts, seems to follow a direct rather than a reflex course, and there do not appear to be any specialized centres by which the impulses are received and then redistributed.