The Power of Movement in Plants

scope with a micrometer eye-piece, s0 arranged that each division equalled ;3,th of an inch. After an interval of 30 m. the apex was observed, and it was seen to cross a little obliquely two divisions of the micrometer in 9 m. 15 s.; and after a few minutes it crossed the same space in 8 m. 50s. The seedling was again observed after an intervalof three-quarters of an hour, and now the apex crossed rather obliquely two divisions in 10 m, We may therefore conclude that it was travelling at about the rate of Jjth of an inch in 45 minutes. We may also conclude from these and the previous observations, that the seedlings of Phalaris in breaking through the surface of the soil circum- nutate as much as the surrounding pressure will permit. This fact accounts (as in the case before given of the asparagus) for a circular, narrow, open space or crack being distinctly visible round several seedlings which had risen through very fine argillaceous sand, kept uniformly damp. Zea mays (Graminez).—A glass filament was fixed obliquely ~ to the summit of a cotyledon, Fig. 51. rising -2 of an inch above the ground; but by the third morn- ing it had grown to exactly thrice this height, so that the distance of the bead from the mark below was greatly in- creased, consequently the trac- ing (Fig. 51) was much more magnified on the first than on the second day. The upper part of the cotyledon changed Fn ae its course by at least as muck Zeu mays: circumnutation of cotyle- a8 a rectangle six times on each pg eat Ae eae ken oe of the two days. The plant Movement of bead casi an Was illuminated by an obscure average about 25 times. light from vertically above.

This was a necessary precau- tion, as on the previous day we had traced the movements of cotyledons placed in a deep box, the inner side of which was feebly illuminated on one side from a distant north-east window, and at each observation by a wax taper held for a minute or two on the same side; and the result was that the cotyledons travelled all day long to this side, though making in their course some couspicuous flexures, from which fact alone we might have

concluded that they were circumnutating; but we thought it advisable to make the tracing above given. Radicles—Glass filaments were fixed to two short radicles, placed so as to stand almost upright, and whilst bending down- wards through geotropism their courses were strongly zigzag ; from this latter circumstance cirecumnutation might have been inferred, had not their tips become slightly withered after the first 24h., though they were watered and the air kept very damp. ,Nine radicles were next arranged in the manner formerly described, so that in growing downwards they left tracks on smoked glass-plates, inclined at various angles between 45° and 80° beneath the horizon. Almost every one of these tracks offered evidence in their greater or less breadth in dif- ferent parts, or in little bridges of soot being left, that the apex had come alternately into Fig. 52.

more and less close contact with the glass. In the accompanying figure (Fig. 52) we have an accurate copy of one such track. In two instances alone (and in these the plates were highly inclined) there was some evidence of slight lateral movement. We presume therefore that the friction of the apex on the smoked surface, little as this could have been, sufficed } to check the movement from side to side of these Zz . Avena sativa (Graminez).—A cotyledon, 13 smoked _glass- inch in height, was placed in front‘of a north- Plate by tip

‘ of radicle in east window, and the movement of the apex growing down- was traced on a horizontal glass during two wards. days. It moved towards the light in a slightly zigzag line from 9 to 11.30 a.m. on October 15th; it then moved a little backwards and zigzagged much until 5.p.m., after which hour, and during the night, it continued to move towards the window. On the following morning the same movement was continued in a nearly straight line until 12.40 p.m., when the sky remained until 2.385 extraordinarily dark from thunder-clouds. During this interval of 1h. 55m., whilst the light was obscure, it was interesting to observe how circumnutation overcame heliotropism, for the apex, instead of continuing to move towards the window in a slightly zigzag line, reversed its course four times, making two small narrow ellipses. A diagram of this case will be given in the chapter on Heliotropism.

A filament was next fixed to a cotyledon only { of an inch in height, which was illuminated exclusively from above, and as it was kept in a warm greenhouse, it grew rapidly ; and now there could be no doubt about its circumnutation, for it described a figure of 8 as well as two small ellipses in 53 hours. Fig. 53. came up by chance in a flower- pot near its parent. The frond, as yet only slightly lobed, was only ‘16 of an inch in length and / very thin glass filament, which projected for a length of ‘36 of an inch, was fixed to the end of the frond. The movement was

Nephrodium molle: cireumnutation go, highly magnified that the of very young frond, traced in A darkness on horizontal glass, figure (Fig. 58) cannot be fully from 94M. to 9. P.M. Oct. 30th. trusted; but the frond was Movement of bead magnified 48 constantly moving in a complex famnes. . Manner, and the bead greatly changed its course eighteen times in the 12 hours of observation. Within half an hour it often returned in a line almost parallel to its former course. The greatest amount of movement occurred

between 4 and 6p.m. The circumnuta- Fig. 54. tion of this plant is interesting, because the species in the genus Lygodium are well known to circumnutate conspicuously s, and to twine round any neighbouring 3 object. Sclaginella Kraussii (?): Selaginella Kraussit (2) (hycopodiacez). circumnutation of —A very young plant, only ‘4 of an inch young plant, kept in in height, had sprung up in a pot in the darkness, traced from 444 house. An extremely fine glass fila- 8.45 a.m. to 10 Pat. JOG. S888 ue Oct. Blot. ment was fixed to the end of the frond-

like stem, and the movement of the bead traced on a horizontal glass. It changed its course several times, as shown in Fig. 54, whilst observed during 13h.15m., and returned at night to a point not far distant from that whence it had started in the morning. There can be no doubt that this little plant cireumnutated. GENERAL CONSIDERATIONS ON THE Movements AND GROWTH UF SEEDLING PLants. Generality of the circumnutating movement—Radicles, their circum- nutation of service—Manner in which they penetrate the ground— Manner in which hypocotyls and other organs break through the ground by being arched—Singular manner of germination in Megar- thiza, &e.—Abortion of cotyleduns— Circumnutation of hypocotyls and epicotyls whilst still buried and arched—Their power of straightening themselves—Bursting of the seed-coxrts—Inherited effect of the arching process in hypogean hypocotyls—Cireumnuti- tion of hypocotyls and epicotyls when erect—Ciicumnutation of cotyledons—Pulvini or joints of cotyledons, duration of their activity, rudimentary in Oxalis corniculata, their development— Sensitiveness of e.tylcdons to light and consequent disturbance of their periodic mcvements—Sensitivevess of cotyledons to coutact.

Tue circumnutating movements of the several parts or organs of a considerable number of seedling plants have been described in the last chapter. A list is here appended of the Families, Cohorts, Sub-classes, &c.. to which they belong, arranged and numbered ac- cording to the classification adopted by Hooker.* Any one who will consider this list will see that the young, plants selected for observation, fairly represent the whole vegetable series excepting the lowest cryptogams, and the movements of some of the latter when mature will hereafter be described. As all the seedlings which were observed, including Conifers, Cycads and Ferns, which belong to the most ancient

* As given in the ‘General System of Botany,’ by Le Maout and Decaisne, 1873. types amongst plants, were continually circumnu- tating, we may infer that this kind of movement is common to every seedling species. 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 svil 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, &., 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 pene- trates 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 ard of the inorganic matter dissolved in it. The mechanical aid afforded by the root-hairs in pene- trating 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 70 ACTION OF THE RADICLE. Caar. U growing part continues to do so, probably for as long as growth continues. This movement of the radicle has been described in Brassica, AZsculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of its occurrence was inferred by Sachs,* from radicles placed vertically upwards being acted on by geotro- pism (which we likewise found to be the case), for if they had remained absolutely perpendicular, the attrac- tion 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 fila- ments 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. In- stituis in Wiirzburg, Heft iii. 1873, p. 460. This memoir, be- sides its intrinsic and great in- terest, deserves to be studied as a model of careful investigation, aud we shall have oceasion to refer to it repeatedly. Dr. Frank had previously remarked (‘ Bei trige zur Pflanzenphysiologie, 1868, p. $1) on the fact of radicles placed vertically upwards being acted on by geotropism, and ha explained it by the supposition that their grcwth was not equal on all sides.

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 #4 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 fila- ments were placed so as to stand up almost vertically, they curved downwards through the action of geotro- pism, circumnutating at the same time, and their courses were consequently zigzag. Sometimes, how- ever, 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 them- selves 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 circum- nutating 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

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. Never- theless, 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 soft- ened 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 down- ward 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 such open passage; and we have observed that roots commoniy run down the old burrows of worms.*

When a radicle is placed in a horizontal or inclined position, the terminal growing part, as is well known, bends down towards the centre of the earth; and Sachs f has shown that whilst thus bending, the growth of the lower surface is greatly retarded, whilst that * Sece,also, Prof. Hensen’sstate- | rows made by worms, ments (‘ Zeitxchrift fiir Wissen, t ‘Arbeiten des bot. Inst. Zool.,’ B. xxviii. p. 354, 1877) to Wurzburg,’ vol. i. 1873, p. 461. the same effect. He goes so far See also p. 397 for the length of as to believe that roots are able the growing part, and p. 45] ou to penetrate the ground toa great the force of geotropism. fepth only by means of the bur-

of the upper surface continues at the normal rate, or may be even somewhat increased. He has further shown by attaching a thread, running over a pulley, to a horizontal radicle of large size, namely, that of the common bean, that it was able to pull up a weight of only one gramme, or 15°4 grains. We may therefore conclude that geotropism does not give a radicle force sufficient to penetrate the ground, but merely tells it (if such an expression may be used) which course to pursue. Before we knew of Sachs’ more precise observations we covered a flat surface of damp sand with the thinnest tin-foil which we could procure (‘02 to ‘03 mm., or ‘00012 to ‘00079 of an inch in thickness), and placed a radicle close above, in such a position that it grew almost perpendicularly down- wards. When the apex came into contact with the polished level surface it turned at right angles and glided over it without leaving any impression; yet the tin-foil was so flexible, that a little stick of soft wood, pointed to the same degree as the end of the radicle and gently loaded with a weight of only a quarter of an ounce (120 grains) plainly indented the tin-foil.

Radicles are able to penetrate the ground by the force due to their longitudinal and transverse growth ; the seeds themselves being held down by the weight of the superincumbent soil. In the case of the bean the apex, protected by the root-cap, is sharp, and the growing part, from 8 to 10 mm. in length, is much more rigid, as Sachs has proved, than the part immediately above, which has ceased to increase in length. We endeavoured to ascertain the downward pressure of the growing part, by placing germinating beans between two small metal plates, the upper one of which was loaded with a known weight; and the

radicle was then allowed to grow into a narrow hole in wood, 2 or 8 tenths of an inch in depth, and closed at the bottom. The wood was so cut that the short space of radicle between the mouth of the hole and the bean could not bend laterally on three sides; but it was impossible to protect the fourth side, close to the bean. Consequently, as long as the radicle con- tinued to increase in length and remained straight, the weighted bean would be lifted up after the tip had reached the bottom of the shallow hole. Beans thus arranged, surrounded by damp sand, lifted up a quarter of a pound in 24 h. after the tip of the radicle had entered the hole. With a greater weight the radicles themselves always became bent on the one unguarded side; but this probably would not have occurred if they had been closely surrounded on all sides by compact earth. There was, however, a possible, but not probable, source of error in these trials, for it was not ascertained whether the beans themselves go on swelling for several days after they have germinated, and after having been treated in the manner in which ours had been; namely, being first left for 24 h. in water, then allowed to germinate in

the radicle of a bean grew. Thick- ness of stick at narrow end ‘08 inch, at broad end “16; depth of hole +1 inch. very damp air, afterwards placed over the hole and almost surrounded by damp sand in a closed box. We succeeded better in ascertaining the force exerted transversely by these radicles. Two were so placed as to penetrate small holes made in little sticks, one of which was cut into the shape here exactly copied (Fig. 55). The short end of the stick beyond

end. As the wood was highly elastic, the split or fissure closed immediately after being made. After six days the stick and bean were dug out of the damp sand, and the radicle was found to be much enlarged above and beneath the hole. The fissure, which was at first quite closed, was now open to a width of 4 mm.; as soon as the radicle was extracted, it imn.e- diately closed to a width of 2 mm. ‘The stick was then suspended horizontally by a fine wire passing through the hole lately filled by the radicle, and a little saucer was sus- pended beneath to receive the weights; and it required 8 lbs. 8 ozs. to open the fissure to the width of 4 mm.—that is, the width before the root was ex- tracted. But the part of the radicle (only ‘1 of an inch in length) which was embedded in the hole, probably exerted a greater transverse strain even _, than 8 Ibs. 8 ozs., for it had split the solid wood for a length of rather more than a quarter of an inch (exactly -275 inch), and this fissure is shown in Fig. 55, Wooden pincers, kept closed by A second stick was tried in the hole (-14 inch in diameter same manner with almost ex- 224 ‘6 inch in depth) bored through the narrow closed actly the same result. part, through which a radicle We then followed a better fF 2 bean was allowed to grow. Temp. 50°-60° F.

plan. Holes were bored near the narrow end of two wooden clips or pincers (Fig. 56), kept closed by brass spiral springs. Two radicles in damp sand were allowed to grow through these holes. The pincers rested on glass-plates to lessen the friction trom the sand. The holes were a little larger (viz. 14 inch) and considerably deeper (viz. *6 inch) than in the trials with the sticks; so that a greater length of a rather thicker radicle exerted a transverse strain. After 13 days they were taken up. The distance of two dots (see the figure) on the longer ends of the pincers was now carefully measured; the radicles were then extracted from the holes, and the pincers of course closed. They were then suspended horizontally in the same manner as were the bits of sticks, and a weight of 1500 grams (or 3 lbs. 4 ozs.) was necessary with one of the pincers to open them to the same extent as had been effected by the transverse growth of the radicle. As soon as this radicle had slightly opened the pincers, it had grown into a flattened form and had escaped a little beyond the hole; its diameter in one direction being 4°2 mm., and at right angles 35 mm. If this escape and flattening could have been prevented, the radicle would probably have exerted a greater strain than the 3 lbs. 4 02s. With the other pincers the radicle escaped still further out of the hole; and the weight required to open them to the same extent as had been effected by the radicle, was only 600 grams.

With these facts before us, there seems little diffi- culty in understanding how a radicle penetrates the ground. ‘The apex is pointed and is protected by the root-cap; the terminal growing part is rigid, and increases in length with a force equal, as far as our observations can be trusted, to the pressure of at least a quarter of a pound, probably with a much greater force when prevented from bending to any side by the surrounding earth. Whilst thus increasing in length it increases in thickness, pushing away the damp

earth on all sides, with a force of above 8 pounds in one case, of 3 pounds in another case. It was impos- sible to decide whether the actual apex exerts, relatively to its diameter, the same transverse strain as the parts a little higher up; but there seems no reason to doubt that this would be the case. The growing part there- fore does not act like a nail when hammered into a board, but more like a wedge of wood, which whilst slowly driven into a crevice continually expands at the same time by the absorption of water; and a wedge thus acting will split even a mass of rock.

Manner in which Hypocotyls, Epicotyls, &e., rise wp and break through the ground.—After the radicle has penetrated the ground and fixed the seed, the hypo- cotyls of all the dicotyledonous seedlings observed by us, which lift their cotyledons above the surtace, break through the ground in the form of an arch. When the cotyledons are hypogean, that is, remain buried in the soil, the hypocotyl is hardly developed, and the epicotyl or plumule rises in like manner as an arch through the ground. In all, or at least in most of such cases, the downwardly bent apex remains for a time enclosed within the seed-coats. With Corylis avel- lena the cotyledons are hypogean, and the epicotyl is arched; but in the particular case described in the last chapter its apex had been injured, and it grew laterally through the soil like a root; and in consequence of this it had emitted two secondary shoots, which likewise broke through the ground as arches.

Cyclamen does not produce any distinct stem, and only a single cotyledon appears at first; * its petiole * Thisistheconelusionarrived considered by other botanists as at by Dr. H. Gressner (‘Bot. the first true leaf is really the Zeitung, 1874, p. 837), who — second cotyledon, which is greatly maintains that what las been delayed in its development. Persicum : seedling, figure en- larged: c, blade of cotyledon, not yet expanded, with arched petiole beginning to straighten itself; A, hypocotyl developed into acorm 5 r,second- ary radicles.

Abronia also has only a single fully developed cotyledon, but in this case it is the hypocotyl which first emerges and is arched. <Alronia umbellata, however, presents this peculiarity, that the enfolded blade of the one developed cotyledon (with the enclosed endosperm) whilst still beneath the surface has its apex upturned and parallel to the descending leg of the arched hypocotyl; but it is dragged out of the ground by the con- tinued growth of the hypocotyl, with the apex pointing downward.

Acanthus mollis: seedling, with the hypogean cotyledon on the near side removed and the radicles cut off: a, blade of first leaf begin- ning to expand, with petiole still paitially arched; 6, second and opposite Jeaf, as yet very imper- fectly developed; c, hypogean cotyledon on the opposite side. and a true leaf first breaks through the ground with its petiole forming an arch. In the genus Acanthus the cotyledons are likewise hypogean. In A. mollis, a single leaf first breaks through the ground with its petiole arched, and with the opposite leaf much less developed, short, straight, of a yellowish colour, and with the petiole at first not half as thick as that of the other. The undeveloped leaf is protected by stand- ing beneath its arched fel- low; and it is an instruc.

tive fact that it is not arched, as it has not to force for itself a passage through the ground. In the accom- panying sketch (Fig. 58) the petiole of the first leaf has already partially straightened itself, and the blade is beginning to unfold. The small second leaf ulti- mately grows to an equal size with the first, but this process is effected at very different rates in different individuals: in one instance the second leaf did not appear fully above the ground until six weeks after the first leaf. As the leaves in the whole family of the Acanthacez stand either opposite one another or in whorls, and as these are of equal size, the great in- equality between the first two leaves is a singular fact. We can see how this inequality of development and the arching of the petiole could have been gradually acquired, if they were beneficial to the seedlings by favouring their emergence ; for with A. candelabrum, spinosus, and latzfolius there was great variability in the inequality between the two first leaves and in the arching of their petioles. In one seedling of A. can- delabrum the first leaf was arched and nine times as long as the second, which latter consisted of a mere little, yellowish-white, straight, hairy style. In other seedlings the difference in length between the two leaves was as 3 to 2, or as 4 to 3, or as only ‘76 to -62 inch. In these latter cases the first and taller leaf was not properly arched. Lastly, in another seedling there was not the least difference in size between the two first leaves, and both of them had their petioles straight; their lamine were enfolded and pressed against each other, forming a lance or wedge, by which means they had broken through the ground. Therefore in different individuals of this same species of Acanthus the first pair of leaves breaks through the ground by two widely different methods; and if