The Power of Movement in Plants

In the several foregoing cases one of the cotyledons is delayed in its development, or reduced in size, or rendered rudimentary, or quite aborted; but in other cases both cotyledons are represented by mere rudi- ments. With Opuntia basidaris this is.not the case, for both cotyledons are thick and large, and the hypocotyl shows at first no signs of enlargement; but afterwards, when the cotyledons have withered and dis- articulated themselves, it becomes thickened, and from its tapering form, together with its smooth, tough, brown skin, appears, when ultimately drawn down to some depth into the soil, like a root. On the other

des Plantes 7’ Europe, tom i. 1841, p- 149) of the germination of the seeds of several species of Cory- dalis, that the bulb or tubercule begins to be formed at an ex: tremely early age. hand, with several other Cactew, the hypocotyl is from the first much enlarged, and both cotyledons are almost or quite rudimentary. Thus with Cereus Land- beckat two little triangular projections, representing the cotyledons, are narrower than the hypocotyl, which is pear-shaped, with the point downwards. In Rhipsalcs cassytha the cotyledons are represented by mere points on the enlarged hypocotyl. In Echinocactus viridescens the hypocotyl is globular, with two little prominences on its summit. In Pilocereus Houlletéi the hypocotyl, much swollen in the upper part, is merely notched on the summit ; and each side of the notch evidently repre- sents a cotyledon. Stapelia sarpedon, a member of the very distinct family of the Asclepiadez, is fleshy like a cactus ; and here again the upper part of the flattened hypocotyl is much thickened and bears two minute coty- ledons, which, measured internally, were only ‘15 inch in length, and in breadth not equal to one-fourth of the diameter of the hypocotyl in its narrow axis; yet these minute cotyledons are probably not quite useless, for when the hypocotyl breaks through the ground in the form of an arch, they are closed or pressed against one another, and thus protect the plumule. They after- wards open.

From the several cases now given, which refer to widely distinct plants, we may infer that there is some close connection between the reduced size of one or both cotyledons and the formation, by the enlargement of the hypocotyl or of the radicle, of a so-called bulb. But it may be asked, did the cotyledons first tend to abort, or did a bulb first begin to be formed? As all dieotyledons naturally produce two well-developed cotyledons, whilst the thickness of the hypocotyl and of the radicle differs much in different plants, it seems probable that these latter organs first became from»

some cause thickened—in several instances apparently in correlation with the fleshy nature of the mature plant—so as to contain a store of nutriment sufficient for the seedling, and then that one or both cotyledons, from being superfluous, decreased in size. It is not surprising that one cotyledon alone should sometimes have been thus affected, for with certain plants, for instance the cabbage, the cotyledons are at first of unequal size, owing apparently to the manner in which they are packed within the seed. It does not, how- ever, follow from the above connection, that whenever a bulb is formed at an early age, one or both coty- ledons will necessarily become superfluous, and conse- quently more or less rudimentary. Finally, these cases offer a good illustration of the principle of com- pensation or balancement of growth, or, as Goethe expresses it, “in order to spend on one side, Nature is forced to economise on the other side.” Circumnutation and other movements of Hypocotyls and Epicotyls, whilst still arched and buried beneath the ground, and whilst breaking through 7.—<According to the position in which a seed may chance to have been buried, the arched hypocotyl or epicoty] will begin to protrude in a horizontal, a more or less inclined, or in a vertical plane. Except when already standing vertically upwards, both legs of the’ arch are acted on from the earliest period by apo- geotropism. Consequently they both bend upwards, until the arch becomes vertical. During the whole of this process, even before the arch has broken through the ground, it is continually trying to circumnutate to a slight extent; as it likewise does if it happens at first to stand vertically up,—all which cases have been observed and described, more or less fully, in the last chapter. After the arch has grown to some

height upwards, the basal part ceases to circumnutate, whilst the upper part continues to do so. That an arched hypocotyl or epicotyl, with the twe legs fixed in the ground, should be able to cir- cumnutate, seemed to us, until we had read Prof. Wiesner’s observations, an inexplicable fuct. He has shown* in the case of certain seedlings, whose tips are bent downwards (or which nutate), that whilst the posterior side of the upper or dependent portion grows quickest, the anterior and opposite side of the basal portion of the same internode grows quickest; these two portions being separated by an indifferent zone, where the growth is equal on all sides. There may even be more than one indifferent zone in the same internode; and the opposite sides of the parts above and below each such zone grow quickest. This pecu- liar manner of growth is called by Wiesner “un- dulatory nutation.” Circumnutation depends on one side of an organ growing quickest (probably preceded by increased turgescence), and then another side, generally almost the opposite one, growing quickest. Now if we look at an arch like this f} and suppose the whole of one side—we will say the whole convex side of both legs—to increase in length, this would not cause the arch to bend to either side. But if the -outer side or surface of the left leg were to increase in length the arch would be pushed over to the right, and this would be aided by the inner side of the right leg increasing in length. If afterwards the process were reversed, the arch would be pushed over to the opposite or left side, and so on alternately,— that is, it would circumnutate. As an arched hypo-

* «Die undulirende Nutation Also published separately see der Internodien, Akad. der Wis- p. 32, sench. (Vienna), Jan. 17th, 1878, cotyl, with the two legs fixed in the ground, certainly circumnutates, and as it consists of a single internode, we may conclude that it grows in the manner de- scribed by Wiesner. It may be added, that the crown of the arch does not grow, or grows very slowly, for it does not increase much in breadth, whilst the arch itself increases greatly in height.

The circumnutating movements of arched hypo- cotyls and epicotyls can hardly fail to aid them in breaking through the ground, if this be damp and soft; though no doubt their emergence depends mainly on the force exerted by their longitudinal growth. Although the arch circumnutates only to a slight extent and probably with little force, yet it is able to move the soil near the surface, though it may not be able to do so at a moderate depth. A pot with seeds of Solanum palinacanthum, the tall arched hypo- cotyls of which had emerged and were growing rather slowly, was covered with fine argillaceous sand kept damp, and this at first closely surrounded the bases of the arches; but soon a narrow open crack was formed round each of them, which could be accounted for only by their having pushed away the sand on all sides ; for no such cracks surrounded some little sticks and pins which had been driven into the sand. It has already been stated that the cotyledons of Phalaris and Avena, the plumules of Asparagus and the hypo- cotyls of Brassica, were likewise able to displace the same kind of sand, either whilst simply circumnu- tating or whilst bending towards a lateral light.

As long as an arched hypocotyl or epicotyl remains buried beneath the ground, the two legs cannot sepa- rate from one another, except to a slight extent from the yielding of the soil; but as soon as the arel rises above the ground, or at an earlier period if the pressure of the surrounding earth be artificially removed, the arch immediately begins to straighten itself. This no doubt is due to growth along the whole inner surface of both legs of the arch; such growth being checked or prevented, as long as the two legs of the arch are firmly pressed together. When the earth is removed all round an arch and the two legs are tied together at their bases, the growth on the under side of the crown causes it after a time to become much flatter and broader than naturally occurs. The straightening process consists of a mo- dified form of circumnutation, for the lines described during this process (as with the hypocotyl of Brassica, . and the epicotyls of Vicia and Corylus) were often plainly zigzag and sometimes looped. After hypo- cotyls or epicotyls have emerged from the ground, they quickly become perfectly straight. No trace is Jeft of their former abrupt curvature, excepting in the case of Alliwm cepa, in which the cotyledon rarely becomes quite straight, owing to the protuberance developed on the crown of the arch.

The increased growth along the inner surface of the arch which renders it straight, apparently begins in the basal leg or that which is united to the radicle; for this leg, as we often observed, is first bowed back- wards from the other leg. This movement facilitates the withdrawal of the tip of the epicotyl or of the cotyledons, as the case may be, from within the seed- coats and from the ground. But the cotyledons often emerge from the ground still tightly enclosed within the seed-coats, which apparently serve to protect them. The seed-coats are afterwards ruptured and cast off by the swelling of the closely conjoined cotyledons, and not by any movement or their separation from one another.

Cucurbitacee, the seed-coats are ruptured by a curious contrivance, described by M. Flahault.* A heel or peg is developed on one side of the summit of the radicle or base of the hypocotyl; and this holds down the lower half of the seed-cvats (the radicle being fixed into the ground) whilst the continued growth of the arched hypocotyl forces up- wards the upper half, and tears asunder the seed-coats at one end, and the cotyledons are then easily withdrawn. The accompanying figure (Fig. 62) will render this description intelligible. Forty- one seeds of Cucurbita ovifera were laid on friable peat and were covered by a layer about an inch in thickness, not much pressed down, so that the cotyledons in being dragged up were subjected to very little friction, yet forty of Parcs ae: them came up naked, the seed-

nating seed, showing the Coats being left buried in the peat. cast off in tne course of two or three days by the swelling of the cotyledons. Until this occurs light is excluded, and the cotyledons cannot decompose car- bonie acid; but no one probably would have thought that the advantage thus gained bya little earlier cast ing off of the seed-coats would be sufficient to account for the development of the peg. Yet, according te M. Flahault, seedlings which have been preventet from casting their seed-coats whilst beneath the ground, are inferior to those which have emerged with their cotyledons naked and ready to act.

The peg is developed with extraordinary rapidity . for it could only just be distinguished in two seed- lings, having radicles ‘35 inch in length, but after an interval of only 24 hours was well developed in both. It is formed, according to Flahault, by the enlargement of the layers of the cortical parenchyma at the base of the hypocotyl. If, however, we judge by the effects of a solution of permanganate of potassium, it is developed on the exact line of junction between the hypocotyl and radicle; for the flat lower surface, as well as the edges, were coloured brown like the radicle; whilst the upper slightly inelined surface was left uncoloured like the hypocotyl, excepting indeed in one out of 33 im- mersed seedlings in which a large part of the upper sur- face was coloured brown. Secondary roots sometimes spring from the lower surface of the peg, which thus seems in all respects to partake of the nature of the radicle. The peg is always developed on the side which becomes concave by the arching of the hypocotyl; and it would be of no service if it were formed on any other side. It is also always developed with the flat lower side, which, as just stated, forms a part of the radicle, at right angles to it, and in a horizontal plane. This fact was clearly shown by burying some of the thin flat seeds in the same position as in Fig. 62, excepting that they were not laid on their flat broad sides, but with one edge downwards. Nine seeds were thus planted, and the peg was developed in the

same position, relatively to the radicle, as in the figure; consequently it did not rest on the flat tip ot the lower half of the seed-coats, but was inserted like a wedge between the two tips. As the arched hypocotyl grew upwards it tended to draw up the whole seed, and the peg necessarily rubbed against both tips, but did not hold either down. The result was, that the cotyledons of five out of the nine seeds thus placed were raised above the ground still enclosed within their seed-coats. Four seeds were buried with the end from which the radicle protrudes pointing vertically downwards, and owing to the peg being always developed in the same position, its apex alone came into contact with, and rubbed against the tip on one side; the result was, that the cotyledons of all four emerged still within their seed-coats. These cases show us how the peg acts in co-ordination with the position which the flat, thin, broad seeds would almost always occupy when naturally sown. When the tip of the lower half of the seed-coats was cut off, Flahault found (as we did likewise) that the peg could not act, since it had nothing to press on, and the cotyledons were raised above the ground with their seed-coats not cast off. Lastly, nature shows us the use of the peg; for in the one Cucurbitaceous genus known to us, in which the cotyledons are hypogean and do not cast their seed-coats, namely, Megarrhiza, there is no vestige of a peg. This structure seems to be present in most of the other genera in the family, judging from Flahault’s statements; we found it well-developed and properly acting in Trichosanthes anguina, in which we hardly expected to find it, as the cotyledons are some- what thick and fleshy. Few cases can be advanced of a structure better adapted for a special purpose than the present one.

With Mimosa pudica the radicle protrudes from a small hole in the sharp edge of the seed; and on its summit, where united with the hypocotyl, a transverse ridge is developed at an early age, which clearly aids in splitting the tough seed-coats; but it does not aid in casting them off, as this is subsequently effected by the swelling of the cotyledons after they have been raised above the ground. The ridge or heel therefore acts rather differently from that of Cucurbita. Its lower surface and the edges were coloured brown by the permanganate of potassium, but not the upper surface. It is a singular fact that after the ridge has done its work and has escaped from the seed-coats, it is developed into a frill all round the summit of the radicle.*

At the base of the enlarged hypocotyl of Abronia umbellata, where it blends into the radicle, there is a projection or heel which varies in shape, but its out- line is too angular in our former figure (Fig. 61). The radicle first protrudes from a small hole at one end of the tough, leathery, winged fruit. At this period the upper part of the radicle is packed within the fruit paralle] to the hypocotyl, and the single cotyledon is doubled back parallel to the latter. The swelling of these three parts, and especially the rapid development of the thick heel between the hypocotyl and radicle at the point where they are doubled, ruptures the tough fruit at the upper end and allows the arched hypocotyl to emerge ; and this seems to be the function of the heel. A seed was cut out of the fruit and

* Our attention was culled to this case by a brief statement by Nobbe in his ‘Handbuch der Samenkunde,’ 1876. p. 215, where a figure is also given of a seedling of Martynia with a heel or ridge at the junction of the radicle and hypocotyl. This seed possesses a very hard and tough coat, and would be likely to require aid in bursting and freeing the cutyle dons. allowed to: germinate in damp air, and now a thin flat disc was developed all round the base of the hypocotyl and grew to an extraordinary breadth, like the frill described under Mimosa, but somewhat broader. Flahault says that with Mirabilis, a member of the same family with Abronia, a heel or collar is developed all round the base of the hypocotyl, but more on one side than on the other; and that it frees the coty- fedons from their seed-coats. We observed only old seeds, and these were ruptured by the absorption of moisture, independently of any aid from the heel and before the protrusion of the radicle; but it does not follow from our experience that fresh and tough fruits would behave in a like manner.

In concluding this section of the present chapter it may be convenient to summarise, under the form of an illustration, the usual movements of the hypocotyls and epicotyls of seedlings, whilst breaking through the ground and immediately afterwards. We may suppose a man to be thrown down on his hands and knees, and at the same time to one side, by a load of hay falling on him. He would first endeavour to get his arched back upright, wriggling at the same time in all directions to free himself a little from the surrounding pressure; and this may represent the combined effects of apogeotropism and circumnutation, when a seed is so buried that the arched hypocotyl or epicotyl protrudes at first in a horizontal or inclined plane. The man, still wriggling, would then raise his arched back as high as he could; and this may represent the growth and continued circumnutation of an arched hypocotyl or epicotyl, before it has reached the surface of the ground. As svon as the man felt himself at all free, he would raise the upper part of his body, whilst still on

his knees and still wriggling; and this may represent the bowing backwards of the basal leg of the arch, which in most cases aids in the withdrawal of the cotyledons from the buried and ruptured seed-coats, and the subsequent straightening of the whole hypo- cotyl or epicotyl—circumnutation still continuing. Circumnutation of Hypocotyls and Epicotyls, when erect.—The hypocotyls, epicotyls, and first shoots of the many seedlings observed by us, after they had become straight and erect, circumnutated continuously. The diversified figures described by them, often during two successive days, have been shown in the woodcuts in the last chapter. It should be recollected that the dots were joined by straight lines, so that the figures are angular; but if the observations had been made every few minutes the lines would have been more or less curvilinear, and irregular ellipses or ovals, or perhaps occasionally circles, would have been formed. The direction of the longer axes of the ellipses made during the same day or on successive days generally changed completely, so as to stand at right angles to one another. The number of irregular ellipses or circles made within a given time differs much with different species. Thus with Brassica oleracea, Cerinthe major, and Cucurbita ovifera about four such figures were completed in 12 h.; whereas with Solanum palina- canthum and Opuntia basilaris, scarcely more than one. The figures likewise differ greatly in size; thus they were very small and in some degree doubtful in Stapelia, and large in Brassica, &c. The ellipses described by Lathyrus nissolia and Brassica were narrow, whilst those made by the Oak were broad. The figures are often complicated by small loops and zigzag lines.

As most seedling plants before the development of true leaves are of low, sometimes very low stature, the extreme amount of movement from side to side of their circumnutating stems was small; that of the hypocotyl of Githago segetum was about ‘2 of an inch, and that of Cucurdita ovifera about ‘28. A very young shoot of Lathyrus nissolia moved about 14, that of an American oak -2, that of the common nut only ‘04, and a rather tall shoot of the Asparagus ‘11 of an inch. The extreme amount of movement of the sheath-like cotyledon of Phalaris Canartensis was ‘3 of an inch; but it did not move very quickly, the tip crossing on one occasion five divisions of the micrometer, that is, 7} 5th of an inch, in 22m.5s. A seedling Nolana prostrata travelled the same distance inl0m. 388s. Seedling cabbages circumutated much more quickly, for the tip of a cotyledon crossed i}oth of an inch on the micrometer in 3 m. 20s.; and this rapid movement, accompanied by incessant oscil- lations, was a wonderful spectacle when beheld under the microscope. ®

The absence of light, for at least a day, does not interfere in the least with the circumnutation of the hypocotyls, epicotyls, or young shoots of the various dicotyledonous seedlings observed by us ; nor with that of the young shoots of some monocotyledons. The circumnutation was indeed much plainer in darkness than in light, for if the light was at all lateral the stem bent towards it in a more or less zigzag course. Finally, the hypocotyls of many seedlings are drawn during the winter into the ground, or even beneath it so that they disappear. This remarkable process, which apparently serves for their protection, has been fully described by De Vries.* He shows that

it is effected by the contraction of the parenchyma- cells of the root. But the hypocotyl itself in some cases contracts greatly, and although at first smooth becomes covered with zigzag ridges, as we observed with Githago segetum. How much of the drawing down and burying of the hypocotyl of Opuntia basilaris was due to the contraction of this part and how much to that of the radicle, we did not observe. Circumnutation of Cotyledons.— With all the dico- tyledonous seedlings described in the last chapter, the cotyledons were in constant movement, chiefly in a ver- tical plane, and commonly once up and once down in the course of the 24 hours. But there were many excep- tions to such simplicity of movement; thus the cotyle- dons of Ipomcea cxrulea moved 18 times either upwards or downwards in the course of 16h.18m. Those of Oxalis rosea moved in the same manner 7 times in the course of 24 h.; and those of Cassta tora described 5 irregular ellipses in 9 h. The cotyledons of some individuals of Mimosa pudica and of Lotus Jacobeeus moved only once up and down in 24 h., whilst those of others performed within the same period an additional small oscillation. Thus with different species, and with different individuals of the same species, there were many gradations from a single diurnal move- ment to oscillations as complex as those of the Tpomcea and Cassia. The opposite cotyledons on the same seedling move to a certain extent independently of one another. This was conspicuous with those of Oxalis sensitiva, in which one cotyledon might be seen during the daytime rising up until it stood vertically, whilst the opposite one was sinking down. Although the movements of cotyledons were gene- rally in nearly the same vertical plane, yet their upward and downward courses never exactly coin

cided; so that ellipses, more or less narrow, were described, and the cotyledons may safely be said tc have circumnutated. Nor could this fact be accounted for by the mere increase in length of the cotyledons through growth, for this by itself would not induce any lateral movement. That there was lateral move- ment in some instances, as with the cotyledons of the cabbage, was evident ; for these, besides moving up and down, changed their course from right to left 12 times in ]4h.15m. With Solanum lycopersicum the cotyledons, after falling in the forenoon, zigzagged from side to side between 12 and 4 p.m, and then commenced rising. The cotyledons of Lupinus luteus are so thick (about ‘08 of an inch) and fleshy,* that they seemed little likely to move, and*were there- fore observed with especial interest; they certainly moved largely up and down, and as the line traced was zigzag there was some lateral movement. The nine cotyledons of a seedling Pinus pinaster plainly circum- nutated ; and the figures described approached more nearly to irregular circles than to irregular ovals or ellipses, The sheath-like cotyledons of the Gra- mines circumnutate, that is, move to all sides, as plainly as do the hypocotyls or epicotyls of any dico- tyledonous plants. Lastly, the very young fronds of a Fern and of a Selaginella circumnutated.

In a large majority of the cases which were care- fully observed, the cotyledons sink a little downwards in the forenoon, and rise a little in the afternoon or evening. They thus stand rather more highly inclined during the night than during the mid-day, at which * The cotyledons, though bright &¢, 1877, p. 95), on the gradations grecn, resemble to a certain ex- in the Legun,inose between sub- tent hypogean ones; sce the in- aérial and subterranean cotvle teresting discussion by Haber- dons. tandt (‘Die Schutzcinrichtungen,’

time they are expanded almost horizontally. The circumnutating movement is thus at least partially periodic, no doubt in connection, as we shall hereafter see, with the daily alternations of light and darkness. The cotyledons of several plants move up so much at night as to stand nearly or quite vertically; and in this latter case they come into close contact with one another. On the other hand, the cotyledons of a few. plants sink almost or quite vertically down at night; and in this latter case they clasp the upper part of the hypocotyl. In the same genus Oxalis the cotyledons of certain species stand vertically up, and those of other species vertically down, at night. In all such cases the cotyledons may be said to sleep, for they act in the same manner as do the leaves of many sleeping plants. This is a movement for a special purpose, and will therefore be considered in a future chapter devoted to this subject. :

In order to gain some rude notion of the proportional number of cases in which the cotyledons of dico- tyledonous plants (hypogean ones being of course excluded) changed their position in a conspicuous manner at night, one or more species in several genera were cursorily observed, besides those described in the last chapter. Altogether 153 genera, included in as many families as could be procured, were thus observed by us. The cotyledons were looked at in the middle of the day and again at night; and those were noted as sleeping which stood either vertically or at an angle of at least 60’ above or beneath the horizon. Of such genera there were 26; and in 21 of them the cotyledons of some of the species rose, and in only 6 sank at night; and some of these latter eases are rather doubtful from causes to be explained in the chapter on the sleep of cotyledons. When

cotyledons which at noon were nearly horizontal, stood at night at more than 20° and less than 60° above the horizon, they were recorded as “ plainly raised ;” and of such genera there were 38. We did not meet with any distinct instances of cotyledons periodically sink- ing only a few degrees at night, although no doubt such occur. We have now accounted for 64 genera out of the 153, and there remain 89 in which the cotyledons did not change their position at night by as much as 20°—that is, in a conspicuous manner which could easily be detected by the unaided eye and by memory; but it must not be inferred from this statement that these cotyledons did not move at all, for in several cases a rise of a few degrees was re- corded, when they were carefully observed. The number 89 might have been a little increased, for the cotyledons remained almost horizontal at night in some species in a few genera, for instance, Trifo- lium and Geranium, which are included amongst the sleepers, such genera might therefore have been added to the 89. Again, one species of Oxalis generally raised its cotyledons at night more than 20° and less than 60° above the horizon ; so that this genus might have been included under two heads. But as several species in the same genus were not often observed, such double entries have been avoided.

In a future chapter it will be shown that the leaves of many plants which do not sleep, rise a few degrees in the evening and during the early part of the night; and it will be convenient to defer until then the consideration of the periodicity of the movements of cotyledons. On the Pulvini or Joints of Cotyledons.—With several of the seedlings described in this and the last chapter, the summit of the petiole is developed into a pulvinus, cushion, or joint (as this organ has been variously called), like that with which many leaves are provided. Tt consists of a mass of small cells usually of a pale colour from the absence of chlorophyll, and with its outline more or less convex, as shown in the annexed figure. In the case of Oxalis sensitiva two-thirds of the Fig. 63, petiole, and in that of Mi- mosa pudica, apparently the 5 whole of the short sub- petioles of the leaflets have been converted into pulvini. With pulvinated leaves (ie. those provided with a pul- vinus) their periodical move- ments depend, according to Pfeffer,* on the cells of the pulvinus alternately expand- ing more quickly on one side than on the other; whereas the similar movements of leaves not provided with pul- vini, depend on their growth as goss : longitudinal section : : of a pulvinus on the summ‘t being alternately more rapid of the petiole of a cotyledon, on one side than on the drawn with the camera lucida, magnified 75 times: p, p, pe- other. As long as a leaf tiole; f, fibro-vascular bundle; provided with a pulvinus is ae of PIRde a young and continues to grow, its movement depends on both these causes combined ;t and if the view now held by many botanists be sound, namely, that growth is always preceded by the expan- sion of the growing cells, then the difference between the movements induced by the aid of pulvini and

without such aid, is reduced to the expansion of the cells not being followed by growth in the first case, and being so followed in the second case. Dots were made with Indian ink along the midrib of both pulvinated cotyledons of a rather old seedling of Oxalis Valdiviana; their distances were repeatedly measured with an eye-piece micrometer during 83 days, ard they did not exhibit the least trace of increase. It is therefore almost certain that the pulvinus itself was not then growing. Nevertheless, during this whole time and for ten days afterwards, these coty- ledons rose vertically every night. In the case of some seedlings raised from seeds purchased under the name of Oxalis floribunda, the cotyledons continued for a long time to move vertically down at night, and the movement apparently depended exclusively on the pulvini, for their petioles were of nearly the same length in young, and in old seedlings which had pro- duced true leaves. With some species of Cassia, on the other hand, it was obvious without any measure- ment that the pulvinated cotyledons continued to increase greatly in length during some weeks; so that here the expansion of the cells of the pulvini and the growth of the petiole were probably combined m causing their prolonged periodic movements. It was equally evident that the cotyledons of many plants, not provided with pulvini, increased rapidly in length ; and their periodic movements no doubt were exclu- sively due to growth.

In accordance with the view that the periodic movements of all cotyledons depend primarily on the expansion of the cells, whether or not followed by growth, we can understand the fact that there is but little difference in the kind or form of movement in the two sets of cases. This may be seen by com: paring the diagrams given in the last chapter. Thus the movements of the cotyledons of Brassica oleracea and of Ipomea cerulea, which are not provided with pulvini, are as complex as those of Oxalis and Cassia which are thus provided. The pulvinated cotyledons of some individuals of Mimosa pudica and Lotus Jacobeus made only a single oscillation, whilst those of other individuals moved twice up and down in the course of 24 hours; so it was occasionally with the cotyledons of Cucurbita ovifera, which are destitute of a pulvinus. The movements of pulvinated cotyledons are generally larger in extent than those without a pulvinus; nevertheless some of the latter moved through an angle of 90°. There is, however, one important difference in the two sets of cases; the nocturnal movements of cotyledons without pulvini, for instance, those in the Crucifere, Cucurbitaces, Githago, and Beta, never last even for a week, to any conspicuous degree. Pulvinated cotyledons, on the other hand, continue to rise at night for a much longer period, even for more than a month, as we shall now show. But the period no doubt depends largely on the temperature to which the seedlings are exposed and their consequent rate of development.

Oxalis Valdiviana.—Some cotyledons which had lately opened and were horizontal on March 6th at noon, stood at night ver- tically up; on the 18th the first true leaf was formed, and was embraced at night by the cotyledons; on April 9th, after an in- terval of 35 days, six leaves were developed, and yet the coty- Iedons rose almost vertically at night. The cotyledons of another seedling, which when first observed had already pro- duced a leaf, stood vertically at night and continued to do so for 11 additional days. After 16 days from the first observation two leaves were developed, and the cotyledons were still greatly raised at night. After 21 days the cotyledons during the day were deflected beneath the horizon, but at night were raised 4 3°