heliotropism, implies that a plant, when unequally illuminated on the two sides, bends from the light, instead of, as in the last sub-class of cases, towards it; but apheliotropism is comparatively rare, at least in a well-marked degree.  There is a third and large sub-class of cases, namely, those of “transversal-Heliotropismus” of Frank, which we will here call diaheliotropism.  Parts of plants, under this influence, place themselves more or less transversely to the direction whence the light proceeds, and are thus fully illuminated.  There is a fourth sub-class, as far as the final cause of the movement is concerned; for the leaves of some plants when exposed to an intense and injurious amount of light direct themselves, by rising or sinking or twisting, so as to be less intensely illuminated.  Such movements have sometimes been called diurnal sleep.  If thought advisable, they might be called paraheliotropic, and this term would correspond with our other terms.

It will be shown in the present chapter that all the movements included in these four sub-classes, consist of modified circumnutation.  We do not pretend to say that if a part of a plant, whilst still growing, did not circumnutate—­though such a supposition is most improbable—­it could not bend towards the light; but, as a matter of fact, heliotropism seems always to consist of modified circumnutation.  Any kind of movement in relation to light will obviously be much facilitated by each part circumnutating or bending successively in all directions, so that an already existing movement has only to be increased in some one direction, and to be lessened or stopped in the other directions, in order that it should become heliotropic, apheliotropic, etc., as the case may be.  In the next chapter some observations on the sensitiveness of plants to light, their [page 420] rate of bending towards it, and the accuracy with which they point towards its source, etc., will be given.  Afterwards it will be shown—­and this seems to us a point of much interest—­that sensitiveness to light is sometimes confined to a small part of the plant; and that this part when stimulated by light, transmits an influence to distant parts, exciting them to bend.

Heliotropism.—­When a plant which is strongly heliotropic (and species differ much in this respect) is exposed to a bright lateral light, it bends quickly towards it, and the course pursued by the stem is quite or nearly straight.  But if the light is much dimmed, or occasionally interrupted, or admitted in only a slightly oblique direction, the course pursued is more or less zigzag; and as we have seen and shall again see, such zigzag movement results from the elongation or drawing out of the ellipses, loops, etc., which the plant would have described, if it had been illuminated from above.  On several occasions we were much struck with this fact, whilst observing the circumnutation of highly sensitive seedlings, which were unintentionally illuminated rather obliquely, or only at successive intervals of time.