The Power of Movement in Plants. Charles Darwin
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Conifers, Cycads and Ferns, which belong to the most ancient
* As given in the 'General System of Botany,' by Le Maout and Decaisne, 1873. [page 68]
types amongst plants, were continually circumnutating, we may infer that this kind of movement is common to every seedling species.
SUB-KINGDOM I.—Phaenogamous Plants.
Class I.—DICOTYLEDONS.
Sub-class I.—Angiosperms. Family. Cohort. 14. Cruciferae. II. PARIETALES. 26. Caryophylleae. IV. CARYOPHYLLALES. 36. Malvaceae. VI MALVALES. 41. Oxalideae. VII. GERANIALES. 49. Tropaeoleae. DITTO 52. Aurantiaceae. DITTO 70. Hippocastaneae. X. SAPINDALES. 75. Leguminosae. XI. ROSALES. 106. Cucurbitaceae. XII. PASSIFLORALES. 109. Cacteae. XIV. FICOIDALES. 122. Compositae. XVII. ASTRALES. 135. Primulaceae. XX. PRIMULALES. 145. Asclepiadeae. XXII. GENTIANALES. 151. Convolvulaceae. XXIII. POLEMONIALES. 154. Boragineae. DITTO 156. Nolaneae. DITTO 157. Solaneae. XXIV. SOLANALES. 181. Chenopodieae. XXVII. CHENOPODIALES. 202. Euphorbiaceae. XXXII. EUPHORBIALES. 211. Cupuliferae. XXXVI. QUERNALES. 212. Corylaceae. DITTO
Sub-class II.—Gymnosperms. 223. Coniferae. 224. Cycadeae.
Class II.—MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae. XI. LILIALES. 41. Asparageae. DITTO 55. Gramineae. XV. GLUMALES.
SUB-KINGDOM II.—Cryptogamic Plants.
1. Filices. I. FILICALES. 6. Lycopodiaceae. DITTO [page 69]
Radicles.—In all the germinating seeds observed by us, the first change is the protrusion of the radicle, which immediately bends downwards and endeavours to penetrate the ground. In order to effect this, it is almost necessary that the seed should be pressed down so as to offer some resistance, unless indeed the soil is extremely loose; for otherwise the seed is lifted up, instead of the radicle penetrating the surface. But seeds often get covered by earth thrown up by burrowing quadrupeds or scratching birds, by the castings of earth-worms, by heaps of excrement, the decaying branches of trees, etc., and will thus be pressed down; and they must often fall into cracks when the ground is dry, or into holes. Even with seeds lying on the bare surface, the first developed root-hairs, by becoming attached to stones or other objects on the surface, are able to hold down the upper part of the radicle, whilst the tip penetrates the ground. Sachs has shown* how well and closely root-hairs adapt themselves by growth to the most irregular particles in the soil, and become firmly attached to them. This attachment seems to be effected by the softening or liquefaction of the outer surface of the wall of the hair and its subsequent consolidation, as will be on some future occasion more fully described. This intimate union plays an important part, according to Sachs, in the absorption of water and of the inorganic matter dissolved in it. The mechanical aid afforded by the root-hairs in penetrating the ground is probably only a secondary service.
The tip of the radicle, as soon as it protrudes from the seed-coats, begins to circumnutate, and the whole
* 'Physiologie Végétale,' 1868, pp. 199, 205. [page 70]
growing part continues to do so, probably for as long as growth continues. This movement of the radicle has been described in Brassica, Aesculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of its occurrence was inferred by Sachs,* from radicles placed vertically upwards being acted on by geotropism (which we likewise found to be the case), for if they had remained absolutely perpendicular, the attraction of gravity could not have caused them to bend to any one side. Circumnutation was observed in the above specified cases, either by means of extremely fine filaments of glass affixed to the radicles in the manner previously described, or by their being allowed to grow downwards over inclined smoked glass-plates, on which they left their tracks. In the latter cases the serpentine course (see Figs. 19, 21, 27, 41) showed unequivocally that the apex had continually moved from side to side. This lateral movement was small in extent, being in the case of Phaseolus at most about 1 mm. from a medial line to both sides. But there was also movement in a vertical plane at right angles to the inclined glass-plates. This was shown by the tracks often being alternately a little broader and narrower, due to the radicles having alternately pressed with greater and less force on the plates. Occasionally little bridges of soot were left across the tracks, showing that the apex had at these spots been lifted up. This latter fact was especially apt to occur * 'Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in Würzburg,' Heft iii. 1873, p. 460. This memoir, besides its intrinsic and great interest, deserves to be studied as a model of careful investigation, and we shall have occasion to refer to it repeatedly. Dr. Frank had previously remarked ('Beiträge zur Pflanzenphysiologie, 1868, p. 81) on the fact of radicles placed vertically upwards being acted on by geotropism, and he explained it by the supposition that their growth was not equal on all sides.
[page 71] when the radicle instead of travelling straight down the glass made a semicircular bend; but Fig. 52 shows that this may occur when the track is rectilinear. The apex by thus rising, was in one instance able to surmount a bristle cemented across an inclined glass-plate; but slips of wood only ¼0 of an inch in thickness always caused the radicles to bend rectangularly to one side, so that the apex did not rise to this small height in opposition to geotropism.
In those cases in which radicles with attached filaments were placed so as to stand up almost vertically, they curved downwards through the action of geotropism, circumnutating at the same time, and their courses were consequently zigzag. Sometimes, however, they made great circular sweeps, the lines being likewise zigzag.
Radicles closely surrounded by earth, even when this is thoroughly soaked
and softened, may perhaps be quite prevented from circumnutating. Yet we
should remember that the circumnutating sheath-like cotyledons of Phalaris,
the hypocotyls of Solanum, and the epicotyls of Asparagus formed round
themselves little circular cracks or furrows in a superficial layer of damp
argillaceous sand. They were also able, as well as the hypocotyls of
Brassica, to form straight furrows in damp sand, whilst circumnutating and
bending towards a lateral light. In a future chapter it will be shown that
the rocking or circumnutating movement of the flower-heads of Trifolium
subterraneum aids them in burying themselves. It is therefore probable that
the circumnutation of the tip of the radicle aids it slightly in
penetrating the ground; and it may be observed in several of the previously
given diagrams, that the movement is more strongly pronounced in radicles
when they first
[page 72]
protrude from the seed than at a rather later period; but whether this is
an accidental or an adaptive coincidence we do not pretend to decide.
Nevertheless, when young radicles of Phaseolus multiflorus were fixed
vertically close over damp sand, in the expectation that as soon as they
reached it they would form circular furrows, this did not occur—a fact
which may be accounted for, as we believe, by the furrow being filled up as
soon as formed by the rapid increase of thickness in the apex of the
radicle. Whether or not a radicle, when surrounded by softened earth, is
aided in forming a passage for itself by circumnutating, this movement can
hardly fail to be of high importance, by guiding the radicle along a line
of least resistance, as will be seen in the next chapter when we treat of
the sensibility of the tip to contact. If, however, a radicle in its
downward growth breaks obliquely into any crevice, or a hole left by a
decayed root, or one made by the larva of an insect, and more especially by
worms, the circumnutating movement of the tip will materially aid it in
following