The Power of Movement in Plants

The position which leaves and cotyledons occupy during the day, namely, more or less transversely to the direction of the light, is due, according to Frank, to what we call diaheliotropism. As all leaves and cotyledons are continually circumnutating, there can hardly be a doubt that diaheliotropism results from modified circumnutation. TF'rom the fact of leaves and cotyledons frequently rising a little in the evening, it appears as if diaheliotropism had to conquer during the middle of the day a widely prevalent tendency to apogeotropism.

Lastly, the leaflets and cotyledons of some plants are known to be injured by too much light; and when the sun shines brightly on them, they move upwards or downwards, or twist laterally, so that they direct their edges towards the light, and thus they escape being injured. These paraheliotropic movements cer- tainly consisted in one case of modified circumnuta- tion; and so it probably is in all cases, for the leaves . of all the species described circumnutate in a con- spicuous manner. This movement has hitherto been observed only with leaflets provided with pulvini, in which the increased turgescence on opposite sides is not followed by growth; and we can understand why this should be so, as the movement is required only for a temporary purpose. It would manifestly be dige

advantageous for the leaf to be fixed by growth in its inclined position. For it has to assume its former horizontal position, as soon as possible after the sun has ceased shining too brightly on it. The extreme sensitiveness of certain seedlings to light, as shown in our ninth chapter, is highly remark- able. The cotyledons of Phalaris became curved towards a distant lamp, which emitted so little light, that a pencil held vertically close to the plants, did not cast any shadow which the eye could perceive on a white card. These cotyledons, therefore, were affected by a difference in he amount of light on their two sides, which the eye could not distinguish. The degree of their curvature within a given time towards a lateral light did not correspond at all strictly with the amount of light which they received; the light not being at any time in excess. They continued for nearly half an hour to bend towards a lateral light, after it had been extinguished. They bend with remarkable precision towards it, and this depends on the illumination of one whole side, or on the obscura- tion of the whole opposite side. The difference in the amount of light which plants at any time receive in comparison with what they have shortly before re- ceived, seems in all cases to be the chief exciting cause of those movements which are influenced by light. Thus seedlings brought out of darkness bend towards a dim lateral light, sooner than others which had pre- viously been exposed to daylight. We have scen several analogous cases with the nyctitropic move- ments of leaves. A striking instance was observed in the case of the periodic movements of the cotyledons of a Cassia; in the morning a pot was placed in an obscure part of a room, and all the cotyledons rose up closed: another pot had stood in the sunlight, and

the cotyledons of course remained expanded ; both pots were now placed close together in the middle of the room, and the cotyledons which had been exposed to the sun, immediately began to close, while the others opened; so that the cotyledons in the two pots moved in exactly opposite directions whilst exposed to the same degree of light. We found that if seedlings, kept in a dark place, were laterally illuminated by a small wax taper for only two or three minutes at intervals of about three- quarters of an hour, they all became bowed to the point where the taper had been held. We felt much surprised at this fact, and until we had read Wiesner’s observations, we attributed it to the after-effects of the light; but he has shown that the same degree of curvature in a plant may be induced in the course of an hour by several interrupted illumina- tions lasting altogether for 20 m., as by a continuous ilumination of 60 m. We believe that this case, as well as our own, may be explained by the ex- citement from light being due not so much to its actual amount, as to the difference in amount from that previously received; and in our case there were repeated alternations from complete darkness to light. In this, and in several of the above specified respects, light seems to act on the tissues of plants, almost in the same manner as it does on the nervous system of animals.

There is a much more striking analogy of the same kind, in the sensitiveness to light being localised in the tips of the cotyledons of Phalaris and Avena, and in the upper part of the hypocotyls of Brassica and Beta; and in the transmission of some influence from these upper to the lower parts, causing the latter. to bend towards the light. This influence is also trans: mitted beneath the soil to a depth where no light enters. It follows from this localisation, that the lower parts of the cotyledons of Phalaris, &., which normally become more bent towards a lateral light than the upper parts, may be brightly illuminated during many hours, and will not bend in the least, if all light be excluded from the tip. It is an interest- ing experiment to place caps over the tips of the cotyledons of Phalaris, and to allow a very little light to enter through minute orifices on one side of the caps, for the lower part of the cotyledons will then bend to this side, and not to the side which has been brightly illuminated during the whole time. In the case of the radicles of Sinapis alba, sensitiveness to light also resides in the tip, which, when laterally illuminated, causes the adjoining part of the root to bend apheliotropically.

Gravitation excites plants to bend away from the centre of the earth, or towards it, or to place them- selves in a transverse position with respect to it. Although it is impossible to modify in any direct manner the attraction of gravity, yet its influence could be moderated indirectly, in the several ways described in the tenth chapter; and under such circumstances the same kind of evidence as that given in the chapter on Heliotropism, showed in the plainest manner that apogeotropic and geotropic, and probably diageotropic movements, are all modified forms of eircumnutation.

Different parts of the same plant and different species are affected by gravitation in widely different degrees and manners. Some plants and organs exhibit hardly a trace of its action. Young seedlings which, as we know, circumnutate rapidly, are eminently sensi- tive; and we have seen the hypocotyl of Beta bending upwards through 109° in 3h. 8m. The after-effects of apogeotropism last for above half an hour; and horizontally-lai.l hypocotyls are sometimes thus car- ried temporarily beyond an upright position. The benefits derived from geotropism, apogeotropism, and diageotropism, are generally so manifest that they need not be specified. With the flower-peduncles of Oxalis, epinasty causes them to bend down, so that the ripening pods may be protected by the calyx from the rain. Afterwards they are carried upwards by apogeotropism in combination with hyponasty, and are thus enabled to scatter their seeds over a wider space. The capsules and flower-heads of some plants are bowed downwards through geotropism, and they then bury themselves in the earth for the protection and slow maturation of the seeds. This burying process is much facilitated by the rocking movement due to circumnutation.

In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits an influence to the adjoining upper part, causing it to bend towards the centre of the earth. That there is transmission of this kind was proved in an interesting manner when horizontally extended radicles of the bean were exposed to the attraction of gravity for 1 or 14 h., and their tips were then amputated. Within this time no trace of curva- ture was exhibited, and the radicles were now placed pointing vertically downwards; but an influence had already been transmitted from the tip to the adjoining part, for it soon became bent to one side, in the same manner as would have occurred had the radicle remained horizontal and been still acted on by geo- tropism. Kadicles thus treated continued to grow out horizontally for two or three days, until a new tip was

reformed ; and this was then acted on by geotropism, and the radicle became curved perpendicularly down. wards. It has now been shown that the following important classes of movement all arise from modified circum- nutation, which is omnipresent whilst growth lasts, and after growth has ceased, whenever pulvini are present. These classes of movement consist of those due to epinasty and hyponasty,—those proper to climbing plants, commonly called revolving nutation, —the nyctitropic or sleep movements of leaves and cotyledons,—and the two immense classes of move- ment excited by light and gravitation. When we speak of modified circumnutation we mean that light, or the alternations of light and darkness, gravitation, slight pressure or other irritants, and certain innate or constitutional states of the plant, do not directly cause the movement; they merely lead to a tempo- rary increase or diminution of those spontaneous changes in the turgescence of the cells which are already in progress. In what manner, light, gravita- tion, &c., act on the cells is not known; and we will here only remark that, if any stimulus affected the cells in such a manner as to cause some slight tendency in the affected part to bend in a beneficial manner, this tendency might easily be increased through the preservation of the more sensitive indi- yiduals, But if such bending were injurious, the tendency would be eliminated unless it was over- poweringly strong; for we know how commonly all characters in all organisms vary. Nor can we see any reason to doubt, that after the complete elimination of a tendency to bend in some one direction under a certain stimulus, the power to bend in a directly

opposite direction might gradually be acquired through natural selection.* Although so many movements have arisen through modified circumnutation, there are others which appear to have had a quite independent origin; but they do not form such large and important classes. When a leaf of a Mimosa is touched it suddenly assumes the same position as when asleep, but Briicke has shown that this movement results from a different state of turgescence in the cells from that which occurs during sleep ; and as sleep-movements are cer- tainly due to modified circumnutation, those from a touch can hardly be thus due. The back of a leaf of Drosera rotundifolia was cemented to the summit of a stick driven into the ground, so that it could not move in the least, and a tentacle was observed during many hours under the microscope; but it exhibited no circumnutating movement, yet after being mo- mentarily touched with a bit of raw meat, its basal part began to curve in 23 seconds. This curving movement therefore could not have resulted from modified circumnutation. But when a small object, such as a fragment of a bristle, was placed on one side of the tip of a radicle, which we know is continually circumnutating, the induced curvature was so similar to the movement caused by geotropism, that we can hardly doubt that it is due to modified circumnu- tation. A flower of a Mahonia was cemented to a stick, and the stamens exhibited no signs of circum- nutation under the microscope, yet when they were lightly touched they suddenly moved towards the pistil. Lastly, the curling of the extremity of a tendril when

* See the remarks in Frank’s 91, &c.), on natural selection in ‘Die wagerechte Richtung ven connection with geotropism, helio Pflanzenthcilen’ ‘1870, pp. 90, tropism, &e. touched seems to be independent of its revolving on circumnutating movement. This is best shown by tha part which is the most sensitive to contact, circum- nutating much less than the lower parts, or apparently not at all.* Although in these cases we have no reason to believe that the movement depends on modified cir- cumnutation, as with the several classes of movement described in this volume, yet the difference between the two sets of cases may not be so great as it at first appears. In the one set, an irritant causes an increase or diminution in the turgescence of the cells, which are already in a state of change; whilst in the other set, the irritant first starts a similar change in their state of turgescence. Why a touch, slight pressure or any other irritant, such as electricity, heat, or the absorption of animal matter, should modify the turgescence of the affected cells in such a manner as to cause movement, we do not know. But a touch acts in

, this manner so often, and on such widely distinct plants, that the tendency seems to be a very general one; and if beneficial, it might be increased to any extent. In other cases, a touch produces a very different effect, as with Nitella, in which the protoplasm may be seen to recede from the walls of the cell; in Lactuca, in which a milky fluid exudes; and in the tendrils of certain Vitacez, Cucurbitacee, and Bignoniaces, in which slight pressure causes a cellular outgrowth.

Finally, it is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed uncon- sciously by the lower animals.j With plants an * For the evidence on this pp. 173, 174. head, see the ‘Movements and + Sachs remarks to nearly the Habits of Climbing Plants,’ 1875, same effict: “ Dass sich die le astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensi- tive toa slight momentary touch. The habit of moving at certain periods is inherited both by plants and gnimals; and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmis- sion of an influence from the excited part to another which consequently moves. Yet plants do not of course possess nerves or a central nervous system; and we may infer that with animuls such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts.

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 oppo- site 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

bende Pflanzensubstanz derart lich, wie die verschiedenen Sinnes- innerlich differenzirt, dass ein- mnerven deg Thiere’ (‘ Arbvilen zelne Theile mit specifischen des Bot. Inst. in Wiirzburg, Bd, Energien ausgeriistot siud, dhn- ii. 1879, p, 282). 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 advan- tage 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 accord- ance 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 Acidium elatinum, Abutilon Darwinti, sleep of leaves and not of cotyledons, 314 Acacia Farnesiana, state of plant when awake and asleep, 381, 382 Acanthus candelabrum, inequality in the two first leaves, 79 =-—— mollis, seedling, manncr of breaking through the ground, 4écidium elatinum, effect on the lateral branches of the silver fir,

Albizzia lophantha, nyctitropie move- ments of leaflets, 383 Ampelopsis trieuspidata, hyponastio movement of hooked tips, 272- Amphicarpea monoica, circumnuta- tion and nyctitropic movemente of leaves, 265 Apios graveolens, heliotropic move- ments of hypucotyl, 422-424 —, circumnutation of vertically dependent young gynophores, 519 Arching of various organs, impor- tance of, to seedling plants, 87, 88; emergence of hypocotyls or epicotyls in the form of an, 553

Asplenium trichomanes, movement in the fruiting fronds, 257, n. —~, heliotropic movement and cir- cumnutation of cotyledon, 421,422 Bary, de, on the effect of the Aici- dium on the silver fir, 188 Batilin, Prof, on the nyctitropic movements of leaves, 283; on tlie slecp of leaves of Sida napea, 322; on Polygonum aviculare, 387; on the effect of sunshine ou leafle!s of Oxalis aretosella, 447 Beta vulgaris, circumnutation of hy pocoty! of seedlings, 52

Bignonia capreolata, apheliotropic movement of tendrils, 432, 450 Brassica oleracea, movement of buried and arched hypocotyl, 13, — , of a cotyledon with hypvcotyl secured to a stick, 19, 20 Bryophyllum (vel Calanchoe) calyei- num, movement of leaves, 237 Candolle, A. de, on Trapa natans, 95; on sensitiveness of coty- ledons. 127 tora, circumnutation of coty- ledons and hypocotyls, 34, 35, Cells, table of the measurement of, in the pulvini of Ozalis corniculata, 120; changes in,

Ciesielski, on the sensitiveness of the tip of the radicles, 4, 523 Circumuutation, meaning explained, 1; modified, 263-279; and helio- tropism, relation between, 435; of paramount importance to every plant, 547 Cohn, on the water secreted by Lathrea squamaria, 86, n.; on the movement of leaflets of UOxa- lis, 447 Corylus avellana, circumuutation of young shoot, cmitted from the epicotyl, 55, 56 Cycas pectinata, circumnutation of young leaf, whilst emerging from the ground, 58

—,, downward apheliotropic move- ment of a flower peduncle, 433- Darlingtonia Californica, its leaves or pitchers apheliotropic, 450, n. Darwin, Charles, on Maurandia semper flurens, 225; on the Swedish turnip, 230, 2.; movements of climbing plants, 266. 271; the heliotropic movement of the ten- drils of Bignonia capreolata, 433 ; revolution of climbing plants, 451; on the curling of a tendril, Delpino, on cotyledons of Cheero- phyllum and Corydalis, 96, 2.

Delphinium nudicaule, mode of breaking through the ground, 80 Diahcliotropisrn, 5; or Transversal- Heliotropismus of Frank, 419; infinenced by epinasty, 439; by weight and apogeotropism, Dionea muscipula, circumnutation of young expanding leaf, 239, Duchartre on Tephrosia caribea, 354; on the nyctitropic movemcnt of the Cassia, 369 Duval-Jouve, on the movements of Bryophyllum calycinum, 237; of the narrow leaves of the Grami- nex, +13 Dyer, Mr. Thiselton, on the leaves of Crotolaria, 340 ; on Cassia flori- bunda, 369, n., on the absorbent hairs on the buried flower-heads of Trifolium subterraneum, 517

Elfving, F., on the rhizomes of Sparganium ramosum, 189; on the diageotropic movement in the rhizomes of some plants, 521 Epicotyl. or plumule, 5; manner of breaking through the ground, 77; emerges from the ground under the form of an arch, 553 crista-galli, effect of tem perature on sleep of leaves, tropic movement of leaflets, 367 Euphorbia jacquinexflora, nycti- tropic movement of leaves, 388 Frank, Dr. A. B., (he terms Helio- tropism and Geotropism, first

on by geotropism, 70, n.; on the stolons of Fragaria, 215; periodic and nyctitropic movements of leaves, 284; on the root-leaves of plants kept in durkness, 443; on pulvini, 485; on natural selection in connection with Seep tin heliotropism, &c., Geotropism, 5; effect of, on the primary radicle, 196; the reverse of apogeotropism, 512: effect on the tips of radicles, 548 Gray, Asa, on Delphinium nudi- caule, 80; on Megarrhiza Cali- fornica, 81; on the movements in the fruiting fron:Is of Asplenium trichomanes, 257; on the Amphi- carpea monoica, 520 ; on the Ipomea Jalappa, 557

Gressner, Dr. H., on the cotyledons of Cyclamen Persicum, 46, 77° on hypocotyl of the same, 96 Flaberlandt, Dr., on the protube- rance on the hypocotyl of Allium, 59; the importance of the arch to seedling plauts, 87; sub- aerial and subterranean cotyle- dons, 110, n.; the arched hypo- cotyl, 55+ Hematixylon Campechianum, uov- turnal movement of leaves, 368, Helianthemum prostratum, geotro- pic movement of fluwer-heads, Heliotropism, 5; uses of, 449; a modified form of circumnutution,

Henslow, Rev. G., on the coty- ledons of Hhalaris Canariensis, Hofmeister, on the curious move- ment of Spirogyra, 3, 259, n.; of the leaves of Pistia strativtes, 255; of cotyledons at night, 297; of petals, 414 Wooker, Sir J., on the effect of light on the pitchers of Sarracenia, Hypocotyl, 5; manner of break- ing through the ground, 77; emerges under the form of an arch, 553 nutation and other movements when arched.98; power of straight- ening themselves, 100; rupture of the seed-coats, 102-106; ilus- tration of, 106; circumnutation when erect, 107; when in dark Hyponasty, 6, 267

Insectivorous and climbing plants not heliotropic, 450; influence of light on, 488 ——., difference in sensitiveness to gravitation in different parts, —— purpurea (vel Pharbitis his- pida), nocturnal movement of cotyledons, 305, 312 Klinostat, the, an instrument de- vised by Sachs to eliminate geo- tropism, 93 Kraus, Dr. Carl, on the underground shoots of Triticum repens, 189; on Cannabis sativa, 250, 307, 312; on the movements of leaves, Tuthyrus nissolia, circumnuta- tion of stem of young seedling,

Leguminose, sleep of cotyledons, 308 ; sleeping species, 340 Light, movements excited by: 418, 563; influence on most vegetable tissues, 486; acts on plantas on the nervous system of animals, Linneus, ‘Somnus Plantarum,’ 280; on plants sleeping, 320; on the leaves of Sida abutilon, 324; on Géinothera mollissima, Loomis, Mr., on the movements in the fruiting fronds of Asplentum trichomanes, 257 Maranta arundinacea, nyctitropic movement of leaves, 389-391 —, after much agitation do not

Martins, on radiation at night, 284, n. Masters, Dr. Maxwell, on the lead- ing shoots of the Contferx, 211 Maurandia semperflorens, circumnu- tation of peduncle, 225 Mechan, Mr., on the effect of an Acidium on Portulaca oleracea, —— messanensis, sleep of leaves on full-grown and young plants, —— officinalis, effect of exposure of leaves at night, 290, 296 —, circumnutation and nyeti- tropic movement of main petiol3, Mimosa albida, circumnutation and nyctitropie movement of pinna,

Mirabilis jalapa and longiflora, nocturnal movements of cotyle- dons, 307 Mohl, on heliotropism in ten- drils, stems, and twining plants, Momentum-like movement, the ac- cumulated effects of apogeo- tropi-m, 508 Monotropa hypopitys, mode of breaking through the ground, 8€ Morren, on the movements of stamens of Sparmannia and Cereus, 226 Miiller, Fritz, on Cassia tora, 34; on the circumnutation of Linum usitatissimum, 203; movements of the flower-stems of an Alisma,