Uses of heliotropism—­Insectivorous and climbing plants not heliotropic—­ Same organ heliotropic at one age and not at another—­Extraordinary sensitiveness of some plants to light—­The effects of light do not correspond with its intensity—­Effects of previous illumination—­Time required for the action of light—­After-effects of light—­Apogeotropism acts as soon as light fails—­Accuracy with which plants bend to the light—­ This dependent on the illumination of one whole side of the part—­Localised sensitiveness to light and its transmitted effects—­Cotyledons of Phalaris, manner of bending—­Results of the exclusion of light from their tips—­ Effects transmitted beneath the surface of the ground—­Lateral illumination of the tip determines the direction of the curvature of the base—­ Cotyledons of Avena, curvature of basal part due to the illumination of upper part—­Similar results with the hypocotyls of Brassica and Beta—­ Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips—­ Concluding remarks and summary of chapter—­Means by which circumnutation has been converted into heliotropism or apheliotropism.

No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated.  They are thus enabled to decompose carbonic acid.  But the sheath-like cotyledons of some Gramineae, for instance, those of Phalaris, are not green and contain very little starch; from which fact we may infer that they decompose little or no carbonic acid.  Nevertheless, they are extremely heliotropic; and this probably serves them in another way, namely, as a guide from the buried seeds through fissures in the ground or through overlying masses of vegetation, into the light and air.  This view [page 450] is strengthened by the fact that with Phalaris and Avena the first true leaf, which is bright green and no doubt decomposes carbonic acid, exhibits hardly a trace of heliotropism.  The heliotropic movements of many other seedlings probably aid them in like manner in emerging from the ground; for apogeotropism by itself would blindly guide them upwards, against any overlying obstacle.

Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light.  Drosera rotundifolia is one of the few plants the leaves of which exhibit no trace of heliotropism.  Nor could we see any in Dionaea, though the plants were not so carefully observed.  Sir J. Hooker exposed the pitchers of Sarracenia for some time to a lateral light, but they did not bend towards it.* We can understand the reason why these insectivorous plants should not be heliotropic, as they do not live chiefly by decomposing carbonic acid; and it is much more important to them that their leaves should occupy the best position for capturing insects, than that they should be fully exposed to the light.