Means of observation—Apogeotropism—Cytisus—Verbena—Beta—Gradual   conversion of the movement of circumnutation into apogeotropism in Rubus,
  Lilium, Phalaris, Avena, and Brassica—Apogeotropism retarded by
  heliotropism—Effected by the aid of joints or pulvini—Movements of
  flower-peduncles of Oxalis—General remarks on apogeotropism—Geotropism—
  Movements of radicles—Burying of seed-capsules—Use of process—Trifolium
  subterraneum—Arachis—Amphicarpaea—Diageotropism—Conclusion
OUR object in the present chapter is to show that geotropism, apogeotropism, and diageotropism are modified forms of circumnutation. Extremely fine filaments of glass, bearing two minute triangles of paper, were fixed to the summits of young stems, frequently to the hypocotyls of seedlings, to flower-peduncles, radicles, etc., and the movements of the parts were then traced in the manner already described on vertical and horizontal glass-plates. It should be remembered that as the stems or other parts become more and more oblique with respect to the glasses, the figures traced on them necessarily become more and more magnified. The plants were protected from light, excepting whilst each observation was being made, and then the light, which was always a dim one, was allowed to enter so as to interfere as little as possible with the movement in progress; and we did not detect any evidence of such interference.
 
When observing the gradations between circumnu- [page 494] tation and heliotropism, we had the great advantage of being able to lessen the light; but with geotropism analogous experiments were of course impossible. We could, however, observe the movements of stems placed at first only a little from the perpendicular, in which case geotropism did not act with nearly so much power, as when the stems were horizontal and at right angles to the force. Plants, also, were selected which were but feebly geotropic or apogeotropic, or had become so from having grown rather old. Another plan was to place the stems at first so that they pointed 30 or 40o beneath the horizon, and then apogeotropism had a great amount of work to do before the stem was rendered upright; and in this case ordinary circumnutation was often not wholly obliterated. Another plan was to observe in the evening plants which during the day had become greatly curved heliotropically; for their stems under the gradually waning light very slowly became upright through the action of apogeotropism; and in this case modified circumnutation was sometimes well displayed.
 
[Apogeotropism.—Plants were selected for observation almost by chance, excepting that they were taken from widely different families. If the stem of a plant which is even moderately sensitive to apogeotropism be placed horizontally, the upper growing part bends quickly upwards, so as to become perpendicular; and the line traced by joining the dots successively made on a glass-plate, is generally almost straight. For instance, a young Cytisus fragrans, 12 inches in height, was placed so that the stem projected 10o beneath the horizon, and its course was traced during 72 h. At first it bent a very little downwards (Fig. 182), owing no doubt to the weight of the stem, as this occurred with most of the other plants observed, though, as they were of course circumnutating, the short downward lines were often oblique. After three-quarters of an hour the stem began to curve upwards, quickly during the first two hours, but much more slowly during the afternoon and night, [page 495] and on the following day. During the second night it fell a little, and circumnutated during the following day; but it also moved a short distance to the right, which was caused by a little light having been accidentally admitted on this side. The stem was now inclined 60o above the horizon, and had therefore risen 70o. With time allowed it would probably have become upright, and no doubt would have continued circumnutating. The sole remarkable feature in the figure here given is the straightness of the course pursued. The stem, however, did not move upwards at an equable rate, and it sometimes stood almost or quite still. Such periods probably represent attempts to circumnutate in a direction opposite to apogeotropism.
 
Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10o beneath
to 60o above horizon, traced on vertical glass, from 8.30 A.M. March 12th
to 10.30 P.M. 13th. The subsequent circumnutating movement is likewise
shown up to 6.45 A.M. on the 15th. Nocturnal course represented, as usual,
by a broken line. Movement not greatly magnified, and tracing reduced to
two-thirds of original scale.
 
 The herbaceous stem of a Verbena melindres (?) laid horizontally, rose in
7 h. so much that it could no longer be observed on the vertical glass
which stood in front of the plant. The long line which was traced was
almost absolutely straight. After the 7 h. it still continued to rise, but
now circumnutated slightly. On the following day it stood upright, and
circumnutated regularly, as shown in Fig. 82, given in the fourth chapter.
The stems of several other plants which were highly sensitive to
apogeotropism rose up in almost straight lines, and
[page 496]
then suddenly began to circumnutate. A partially etiolated and somewhat old
hypocotyl of a seedling cabbage (2 3/4 inches in height) was so sensitive
that when placed at an angle of only 23o from the perpendicular, it became
vertical in 33 minutes. As it could not have been strongly acted upon by
apogeotropism in the above slightly inclined position, we expected that it
would have circumnutated, or at least have moved in a zigzag course.
Accordingly, dots were made every 3 minutes; but, when these were joined,
the line was nearly straight. After this hypocotyl had become upright it
still moved onwards for half an hour in the same general direction, but in
a zigzag manner. During the succeeding 9 h. it circumnutated regularly, and
described 3 large ellipses. In this case apogeotropism, although acting at
a very unfavourable angle, quite overcame the ordinary circumnutating
movement.
Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19o beneath horizon to a vertical position, with subsequent circumnutation, traced on a vertical and on a horizontal glass-plate, from 8.28 A.M. Sept. 28th to 8.40 A.M. 29th. Figure reduced to one-third of original scale.
 
The hypocotyls of Beta vulgaris are highly sensitive to apogeotropism. One was placed so as to project 19o beneath the horizon; it fell at first a very little (see Fig. 183), no doubt owing to its weight; but as it was circumnutating the line was [page 497] oblique. During the next 3 h. 8 m. it rose in a nearly straight line, passing through an angle of 109o, and then (at 12.3 P.M.) stood upright. It continued for 55 m. to move in the same general direction beyond the perpendicular, but in a zigzag course. It returned also in a zigzag line, and then circumnutated regularly, describing three large ellipses during the remainder of the day. It should be observed that the ellipses in this figure are exaggerated in size, relatively to the length of the upward straight line, owing to the position of the vertical and horizontal glass-plates. Another and somewhat old hypocotyl was placed so as to stand at only 31o from the perpendicular, in which position apogeotropism acted on it with little force, and its course accordingly was slightly zigzag.
 
The sheath-like cotyledons of Phalaris Canariensis are extremely sensitive to apogeotropism. One was placed so as to project 40o beneath the horizon. Although it was rather old and 1.3 inch in height, it became vertical in 4 h. 30 m., having passed through an angle of 130o in a nearly straight line. It then suddenly began to circumnutate in the ordinary manner. The cotyledons of this plant, after the first leaf has begun to protrude, are but slightly apogeotropic, though they still continue to circumnutate. One at this stage of development was placed horizontally, and did not become upright even after 13 h., and its course was slightly zigzag. So, again, a rather old hypocotyl of Cassia tora (1 1/4 inch in height) required 28 h. to become upright, and its course was distinctly zigzag; whilst younger hypocotyls moved much more quickly and in a nearly straight line.
 
When a horizontally placed stem or other organ rises in a zigzag line, we may infer from the many cases given in our previous chapters, that we have a modified form of circumnutation; but when the course is straight, there is no evidence of circumnutation, and any one might maintain that this latter movement had been replaced by one of a wholly distinct kind. This view seems the more probable when (as sometimes occurred with the hypocotyls of Brassica and Beta, the stems of Cucurbita, and the cotyledons of Phalaris) the part in question, after bending up in a straight course, suddenly begins to circumnutate to the full extent and in the usual manner. A fairly good instance of a sudden change of this kind—that is, from a nearly straight upward movement to one of circumnutation—is shown in Fig. 183; but more striking instances were occasionally observed with Beta, Brassica, and Phalaris.
 
We will now describe a few cases in which it may be [page 498] seen how gradually circumnutation becomes changed into apogeotropism, under circumstances to be specified in each instance.
 
Rubus idaeus (hybrid).—A young plant, 11 inches in height, growing in a pot, was placed horizontally; and the upward movement was traced during nearly 70 h.; but the plant, though growing vigorously, was not highly sensitive to apogeotropism, or it was not capable of quick movement, for during the above time it rose only 67o. We may see in the diagram (Fig. 184) that during the first day of 12 h. it rose in a nearly straight line. When placed horizontally, it was evidently circumnutating, for it rose at first a little, notwithstanding the weight of the stem, and then sank down; so that it did not start on its permanently upward course until 1 h. 25 m. had elapsed. On the second day, by which time it had risen considerably, and when apogeotropism acted on it with somewhat less power, its course during 15 ? h. was clearly zigzag, and the rate of the upward movement was not equable. During the third day, also of 15 ? h., when apogeotropism acted on it with still less power, the stem plainly circumnutated, for it moved during this day 3 times up and 3 times down, 4 times to the left and 4 to the right. But the course was so complex that it could hardly be traced on the glass. We can, however, see that the successively formed irregular ellipses rose higher and higher. Apogeotropism continued to act on the fourth morning, as the stem was still rising, though it now stood only 23o from the perpendicular. In this diagram the several stages may be followed by which an almost rectilinear, upward, apogeotropic course first becomes zigzag, and then changes into a circumnutating movement, with most of the successively formed, irregular ellipses directed upwards.
 
Fig 184: Rubus idaeus (hybrid): apogeotropic movement of stem, traced on a vertical glass during 3 days and 3 nights, from 10.40 A.M. March 18th to 8 A.M. 21st. Figure reduced to one-half of the original scale.
 
Lilium auratum.—A plant 23 inches in height was placed [page 499] horizontally, and the upper part of the stem rose 58o in 46 h., in the manner shown in the accompanying diagram (Fig. 185). We here see that during the whole of the second day of 15 ? h., the stem plainly circumnutated whilst bending upwards through apogeotropism. It had still to rise considerably, for when the last dot in the figure was made, it stood 32o from an upright position.
 
Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on a vertical glass during 2 days and 2 nights, from 10.40 A.M. March 18th to 8 A.M. 20th. Figure reduced to one-half of the original scale.
 
Phalaris Canariensis.—A cotyledon of this plant (1.3 inch in height) has already been described as rising in 4 h. 30 m. from 40o beneath the horizon into a vertical position, passing through an angle of 130o in a nearly straight line, and then abruptly beginning to circumnutate. Another somewhat old cotyledon of the same height (but from which a true leaf had not yet protruded), was similarly placed at 40o beneath the horizon. For the first 4 h. it rose in a nearly straight course (Fig. 186), so that by 1.10 P.M. it was highly inclined, and now apogeotropism acted on it with much less power than before, and it began to zigzag. At 4.15 P.M. (i.e. in 7 h. from the commencement) it stood vertically, and afterwards continued to circumnutate in the usual manner about the same spot. Here then we have a graduated change from a straight upward apogeotropic course into circumnutation, instead of an abrupt change, as in the former case.
 
Avena sativa.—The sheath-like cotyledons, whilst young, are strongly apogeotropic; and some which were placed at 45o beneath the horizon rose 90o in 7 or 8 h. in lines almost absolutely straight. An oldish cotyledon, from which the first leaf began to [page 500] protrude whilst the following observations were being made, was placed at 10o beneath the horizon, and it rose only 59o in 24h. It behaved rather differently from any other plant, observed by us, for during the first 4 ? h. it rose in a line not far from straight; during the next 6 ? h. it circumnutated, that is, it descended and again ascended in a strongly marked zigzag course; it then resumed its upward movement in a moderately straight line, and, with time allowed, no doubt would have become upright. In this case, after the first 4 ? h., ordinary circumnutation almost completely conquered for a time apogeotropism.
 
Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon, traced on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to 9 A.M. 20th. Figure here reduced to one-fifth of original scale.
 
Brassica oleracea.—The hypocotyls of several young seedlings placed horizontally, rose up vertically in the course of 6 or 7 h. in nearly straight lines. A seedling which had grown in darkness to a height of 2 1/4 inches, and was therefore rather old and not highly sensitive, was placed so that the hypocotyl projected at between 30o and 40o beneath the horizon. The upper part alone became curved [page 501] upwards, and rose during the first 3 h. 10 m. in a nearly straight line (Fig. 187); but it was not possible to trace the upward movement on the vertical glass for the first 1 h. 10 m., so that the nearly straight line in the diagram ought to have been much longer. During the next 11 h. the hypocotyl circumnutated, describing irregular figures, each of which rose a little above the one previously formed. During the night and following early morning it continued to rise in a zigzag course, so that apogeotropism was still acting. At the close of our observations, after 23 h. (represented by the highest dot in the diagram) the hypocotyl was still 32o from the perpendicular. There can be little doubt that it would ultimately have become upright by describing an additional number of irregular ellipses, one above the other.
 
Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced on vertical glass, from 9.20 A.M., Sept. 12th to 8.30 A.M. 13th. The upper part of the figure is more magnified than the lower part. If the whole course had been traced, the straight upright line would have been much longer. Figure here reduced to one-third of the original scale.
 
Apogeotropism retarded by Heliotropism.—When the stem of any plant bends during the day towards a lateral light, the movement is opposed by apogeotropism; but as the light gradually wanes in the evening the latter power slowly gains the upper hand, and draws the stem back into a vertical position. Here then we have a good opportunity for observing how apogeotropism acts when very nearly balanced by an opposing force. For instance, the plumule of Tropaeolum majus (see former Fig. 175) moved towards the dim evening light in a slightly zigzag line until 6.45 P.M., it then returned on its course until [page 502] 10.40 P.M., during which time it zigzagged and described an ellipse of considerable size. The hypocotyl of Brassica oleracea (see former Fig. 173) moved in a straight line to the light until 5.15 P.M., and then from the light, making in its backward course a great rectangular bend, and then returned for a short distance towards the former source of the light; no observations were made after 7.10 P.M., but during the night it recovered its vertical position. A hypocotyl of Cassia tora moved in the evening in a somewhat zigzag line towards the failing light until 6 P.M., and was now bowed 20o from the perpendicular; it then returned on its course, making before 10.30 P.M. four great, nearly rectangular bends and almost completing an ellipse. Several other analogous cases were casually observed, and in all of them the apogeotropic movement could be seen to consist of modified circumnutation.
 
Apogeotropic Movements effected by the aid of joints or pulvini.—Movements of this kind are well known to occur in the Gramineae, and are effected by means of the thickened bases of their sheathing leaves; the stem within being in this part thinner than elsewhere.* According to the analogy of all other pulvini, such joints ought to continue circumnutating for a long period, after the adjoining parts have ceased to grow. We therefore wished to ascertain whether this was the case with the Gramineae; for if so, the upward curvature of their stems, when extended horizontally or laid prostrate, would be explained in accordance with our view—namely, that apogeotropism results from modified circumnutation. After these joints have curved upwards, they are fixed in their new position by increased growth along their lower sides.
 
Lolium perenne.—A young stem, 7 inches in height, consisting of 3 internodes, with the flower-head not yet protruded, was selected for observation. A long and very thin glass filament was cemented horizontally to the stem close above the second joint, 3 inches above the ground. This joint was subsequently proved to be in an active condition, as its lower side swelled much through the action of apogeotropism (in the manner described by De Vries) after the haulm had been fastened down for 24 h. in a horizontal position. The pot was
 
* This structure has been recently described by De Vries in an interesting article, 'Ueber die Aufrichtung des gelagerten Getreides,' in 'Landwirthschaftliche Jahrbücher,' 1880, p. 473. [page 503]
 
so placed that the end of the filament stood beneath the 2-inch object glass of a microscope with an eye-piece micrometer, each division of which equalled 1/500 of an inch. The end of the filament was repeatedly observed during 6 h., and was seen to be in constant movement; and it crossed 5 divisions of the micrometer (1/100 inch) in 2 h. Occasionally it moved forwards by jerks, some of which were 1/1000 inch in length, and then slowly retreated a little, afterwards again jerking forwards. These oscillations were exactly like those described under Brassica and Dionaea, but they occurred only occasionally. We may therefore conclude that this moderately old joint was continually circumnutating on a small scale.
 
Alopecurus pratensis.—A young plant, 11 inches in height, with the flower-head protruded, but with the florets not yet expanded, had a glass filament fixed close above the second joint, at a height of only 2 inches above the ground. The basal internode, 2 inches in length, was cemented to a stick to prevent any possibility of its circumnutating. The extremity of the filament, which projected about 50o above the horizon, was often observed during 24 h. in the same manner as in the last case. Whenever looked at, it was always in movement, and it crossed 30 divisions of the micrometer (3/50 inch) in 3 ? h.; but it sometimes moved at a quicker rate, for at one time it crossed 5 divisions in 1 ? h. The pot had to be moved occasionally, as the end of the filament travelled beyond the field of vision; but as far as we could judge it followed during the daytime a semicircular course; and it certainly travelled in two different directions at right angles to one another. It sometimes oscillated in the same manner as in the last species, some of the jerks forwards being as much as 1/1000 of an inch. We may therefore conclude that the joints in this and the last species of grass long continue to circumnutate; so that this movement would be ready to be converted into an apogeotropic movement, whenever the stem was placed in an inclined or horizontal position.
 
Movements of the Flower-peduncles of Oxalis carnosa, due to apogeotropism and other forces.—The movements of the main peduncle, and of the three or four sub-peduncles which each main peduncle of this plant bears, are extremely complex, and are determined by several distinct causes. Whilst the flowers are expanded, both kinds of peduncles circumnutate about the same spot, as we have seen (Fig. 91) in the fourth chapter. But soon after the flowers have begun to wither the sub- [page 504] peduncles bend downwards, and this is due to epinasty; for on two occasions when pots were laid horizontally, the sub-peduncles assumed the same position relatively to the main peduncle, as would have been the case if they had remained upright; that is, each of them formed with it an angle of about 40o. If they had been acted on by geotropism or apheliotropism (for the plant was illuminated from above), they would have directed themselves to the centre of the earth. A main peduncle was secured to a stick in an upright position, and one of the upright sub-peduncles which had been observed circumnutating whilst the flower was expanded, continued to do so for at least 24 h. after it had withered. It then began to bend downwards, and after 36 h. pointed a little beneath the horizon. A new figure was now begun (A, Fig. 188), and the sub-peduncle was traced descending in a zigzag line from 7.20 P.M. on the 19th to 9 A.M. on the 22nd. It now pointed almost perpendicularly downwards, and the glass filament had to be removed and fastened transversely across the base of the young capsule. We expected that the sub-peduncle would have been motionless in its new position; but it continued slowly to swing, like a pendulum, from side to side, that is, in a plane at right angles to that in which it had descended. This circumnutating movement was observed from 9 A.M. on 22nd to 9 A.M. 24th, as shown at B in the diagram. We were not able to observe this particular sub-peduncle any longer; but it would certainly have gone on circumnutating until the capsule was nearly ripe (which requires only a short time), and it would then have moved upwards.
 
The upward movement (C, Fig. 188) is effected in part by the whole sub-peduncle rising in the same manner as it had previously descended through epinasty—namely, at the joint where united to the main peduncle. As this upward movement occurred with plants kept in the dark and in whatever position the main peduncle was fastened, it could not have been caused by heliotropism or apogeotropism, but by hyponasty. Besides this movement at the joint, there is another of a very different kind, for the sub-peduncle becomes upwardly bent in the middle part. If the sub-peduncle happens at the time to be inclined much downwards, the upward curvature is so great that the whole forms a hook. The upper end bearing the capsule, thus always places itself upright, and as this occurs in darkness, and in whatever position the main peduncle may have been secured, [page 505] the upward curvature cannot be due to heliotropism or hyponasty, but to apogeotropism.
 
Fig. 188. Oxalis carnosa: movements of flower-peduncle, traced on a vertical glass: A, epinastic downward movement; B, circumnutation whilst depending vertically; C, subsequent upward movement, due to apogeotropism and hyponasty combined. [page 506]
 
In order to trace this upward movement, a filament was fixed to a sub-peduncle bearing a capsule nearly ripe, which was beginning to bend upwards by the two means just described. Its course was traced (see C, Fig 188) during 53 h., by which time it had become nearly upright. The course is seen to be strongly zigzag, together with some little loops. We may therefore conclude that the movement consists of modified circumnutation.
 
The several species of Oxalis probably profit in the following manner by their sub-peduncles first bending downwards and then upwards. They are known to scatter their seeds by the bursting of the capsule; the walls of which are so extremely thin, like silver paper, that they would easily be permeated by rain. But as soon as the petals wither, the sepals rise up and enclose the young capsule, forming a perfect roof over it as soon as the sub-peduncle has bent itself downwards. By its subsequent upward movement, the capsule stands when ripe at a greater height above the ground by twice the length of the sub-peduncle, than it did when dependent, and is thus able to scatter its seeds to a greater distance. The sepals, which enclose the ovarium whilst it is young, present an additional adaptation by expanding widely when the seeds are ripe, so as not to interfere with their dispersal. In the case of Oxalis acetosella, the capsules are said sometimes to bury themselves under loose leaves or moss on the ground, but this cannot occur with those of O. carnosa, as the woody stem is too high.
 
Oxalis acetosella.—The peduncles are furnished with a joint in
 
Fig. 189. Oxalis acetosella: course pursued by the upper part of a peduncle, whilst rising, traced from 11 A.M. June 1st to 9 A.M. 3rd. Figure here reduced to one-half of the original scale.
 
the middle, so that the lower part answers to the main peduncle, [page 507] and the upper part to one of the sub-peduncles of O. carnosa. The upper part bends downwards, after the flower has begun to wither, and the whole peduncle then forms a hook; that this bending is due to epinasty we may infer from the case of O. carnosa. When the pod is nearly ripe, the upper part straightens itself and becomes erect; and this is due to hyponasty or apogeotropism, or both combined, and not to heliotropism, for it occurred in darkness. The short, hooked part of the peduncle of a cleistogamic flower, bearing a pod nearly ripe, was observed in the dark during three days. The apex of the pod at first pointed perpendicularly down, but in the course of three days rose 90o, so that it now projected horizontally. The course during the two latter days is shown in Fig. 189; and it may be seen how greatly the peduncle, whilst rising, circumnutated. The lines of chief movement were at right angles to the plane of the originally hooked part. The tracing was not continued any longer; but after two additional days, the peduncle with its capsule had become straight and stood upright.]
 
Concluding Remarks on Apogeotropism.—When apogeotropism is rendered by any means feeble, it acts, as shown in the several foregoing cases, by increasing the always present circumnutating movement in a direction opposed to gravity, an............