We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends towards the source of moisture. When the tip is excited by light (though [page 573] in the case of radicles this was ascertained in only a single instance) the adjoining part bends from the light; but when excited by gravitation the same part bends towards the centre of gravity. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, no doubt in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.
Abies communis, effect of killing or injuring the leading shoot, 187 — pectinata, effect of killing or injuring the leading shoot, 187 —, affected by Aecidium elatinum, 188
Abronia umbellata, its single, developed cotyledon, 78 —, rudimentary cotyledon, 95 —, rupture of the seed coats, 105
Abutilon Darwinii, sleep of leaves and not of cotyledons, 314 —, nocturnal movement of leaves, 323
Acacia Farnesiana, state of plant when awake and asleep, 381, 382 —, appearance at night, 395 —, nyctitropic movements of pinnae, 402 —, the axes of the ellipses, 404 — lophantha, character of first leaf, 415 — retinoides, circumnutation of young phyllode, 236
Acanthosicyos horrida, nocturnal movement of cotyledon 304
Acanthus candelabrum, inequality in the two first leaves, 79 —, petioles not arched, 553 — latifolius, variability in first leaves 79 — mollis, seedling, manner of breaking through the ground, 78, 79 —, circumnutation of young leaf, 249, 269 — spinosus, 79 —, movement of leaves, 249