themselves into a spire, and according to Erasmus Darwin (‘Botanic Garden,’ Canto., iii. p. 126), the pods forcibly penetrate the earth.
See also Grenier and Godron, ‘Flore de France,’ tom. ii. p. 459.
Fig. 179. Cyclamen Persicum: downward apheliotropic movement of a flower-peduncle, greatly magnified (about 47 times?), traced on a In order to observe the nature of this movement, a peduncle bearing a large pod which had reached and rested on the ground, was lifted a little up and secured to a stick. A filament was fixed across the pod with a mark beneath, and its movement, greatly magnified, was traced on a horizontal glass during 67 h. The plant was illuminated during the day from above. A copy of the tracing is given on p. 434 (Fig. 179); and there can be no doubt that the descending movement is one of modified circumnutation, but on an extremely small scale. The observation was repeated on another pod, which had partially buried itself in sawdust, and which was lifted up a quarter of an inch above the surface; it described three very small circles in 24 h. Considering the great length and thinness of the peduncles and the lightness of the pods, we may conclude that they would not be able to excavate saucer-like depressions in sand or sawdust, or bury themselves in moss, etc., unless they were aided by their continued rocking or circumnutating movement.
Relation between Circumnutation and Heliotropism.—Any one who will look at the foregoing diagrams, showing the movements of the stems of various plants towards a lateral and more or less dimmed light, will be forced to admit that ordinary circumnutation and heliotropism graduate into one another. When a plant is exposed to a dim lateral light and continues during the whole day bending towards it, receding late in the evening, the movement unquestionably is one of heliotropism. Now, in the case of Tropaeolum (Fig. 175) the stem or epicotyl obviously circumnutated during the whole day, and yet it continued at the same time to move heliotropically; this latter movement being effected by the apex of each successive elongated figure or ellipse standing nearer to the light than the previous one. In the case of Cassia (Fig. 177) the comparison of the movement of the hypocotyl, when exposed to a dim lateral light and to darkness, is very instructive; as is that between the ordinary circumnutating movement of a seedling Brassica (Figs. 172, 173), or that of Phalaris (Figs. 49, 174), and their heliotropic movement towards a window protected by blinds. In both these cases, and in many others, it was interesting to notice how gradually the stems began to circumnutate as the light waned in the evening. We have therefore many kinds of gradations from a movement towards the light, which must be considered as one of circumnutation very slightly modified and still consisting of ellipses or circles,—though a movement more or less strongly zigzag, with loops or ellipses occasionally formed,—to a nearly straight, or even quite straight, heliotropic course.
A plant, when exposed to a lateral light, though this may be bright, commonly moves at first in a zigzag line, or even directly from the light; and this no doubt is due to its circumnutating at the time in a direction either opposite to the source of the light, or more or less transversely to it. As soon, however, as the direction of the circumnutating movement nearly coincides with that of the entering light, the plant bends in a straight course towards the light, if this is bright. The course appears to be rendered more and more rapid and rectilinear, in accordance with the degree of brightness of the light—firstly, by the longer axes of the elliptical figures, which the plant continues to describe as long as the light remains very dim, being directed more or less accurately towards its source, and by each successive ellipse being described nearer to the light. Secondly, if the light is only somewhat dimmed, by the acceleration and increase of the movement towards it, and by the retardation or arrestment of that from the light, some lateral movement being still retained, for the light will interfere less with a movement at right angles to its direction, than with one in its own direction.[4] The result is that the course is rendered more or less zigzag and unequal in rate. Lastly, when the light is very bright all lateral movement is lost; and the whole energy of the plant is expended in rendering the circumnutating movement rectilinear and rapid in one direction alone, namely, towards the light.
[4] In his paper, ‘Ueber orthotrope und plagiotrope Pflanzentheile’ (‘Arbeiten des Bot. Inst. in Würzburg,’ Band ii. Heft ii. 1879), Sachs has discussed the manner in which geotropism and heliotropism are affected by differences in the angles at which the organs of plants stand with respect to the direction of the incident force.
The common view seems to be that heliotropism is a quite distinct kind of movement from circumnutation; and it may be urged that in the foregoing diagrams we see heliotropism merely combined with, or superimposed on, circumnutation. But if so, it must be assumed that a bright lateral light completely stops circumnutation, for a plant thus exposed moves in a straight line towards it, without describing any ellipses or circles. If the light be somewhat obscured, though amply sufficient to cause the plant to bend towards it, we have more or less plain evidence of still-continued circumnutation. It must further be assumed that it is only a lateral light which has this extraordinary power of stopping circumnutation, for we know that the several plants above experimented on, and all the others which were observed by us whilst growing, continue to circumnutate, however bright the light may be, if it comes from above. Nor should it be forgotten that in the life of each plant, circumnutation precedes heliotropism, for hypocotyls, epicotyls, and petioles circumnutate before they have broken through the ground and have ever felt the influence of light.
We are therefore fully justified, as it seems to us, in believing that whenever light enters laterally, it is the movement of circumnutation which gives rise to, or is converted into, heliotropism and apheliotropism. On this view we need not assume against all analogy that a lateral light entirely stops circumnutation; it merely excites the plant to modify its movement for a time in a beneficial manner. The existence of every possible gradation, between a straight course towards a lateral light and a course consisting of a series of loops or ellipses, becomes perfectly intelligible. Finally, the conversion of circumnutation into heliotropism or apheliotropism, is closely analogous to what takes place with sleeping plants, which during the daytime describe one or more ellipses, often moving in zigzag lines and making little loops; for when they begin in the evening to go to sleep, they likewise expend all their energy in rendering their course rectilinear and rapid. In the case of sleep-movements, the exciting or regulating cause is a difference in the intensity of the light, coming from above, at different periods of the twenty-four hours; whilst with heliotropic and apheliotropic movements, it is a difference in the intensity of the light on the two sides of the plant.
cause of leaves placing themselves more or less transversely to the light, with their upper surfaces directed towards it, has been of late the subject of much controversy. We do not here refer to the object of the movement, which no doubt is that their upper surfaces may be fully illuminated, but the means by which this position is gained. Hardly a better or more simple instance can be given of diaheliotropism than that offered by many seedlings, the cotyledons of which are extended horizontally. When they first burst from their seed-coats they are in contact and stand in various positions, often vertically upwards; they soon diverge, and this is effected by epinasty, which, as we have seen, is a modified form of circumnutation.
After they have diverged to their full extent, they retain nearly the same position, though brightly illuminated all day long from above, with their lower surfaces close to the ground and thus much shaded.
There is therefore a great contrast in the degree of illumination of their upper and lower surfaces, and if they were heliotropic they would bend quickly upwards. It must not, however, be supposed that such cotyledons are immovably fixed in a horizontal position. When seedlings are exposed before a window, their hypocotyls, which are highly heliotropic, bend quickly towards it, and the upper surfaces of their cotyledons still remain exposed at right angles to the light; but if the hypocotyl is secured so that it cannot bend, the cotyledons themselves change their position. If the two are placed in the line of the entering light, the one furthest from it rises up and that nearest to it often sinks down; if placed transversely to the light, they twist a little laterally; so that in every case they endeavour to place their upper surfaces at right angles to the light. So it notoriously is with the leaves on plants nailed against a wall, or grown in front of a window. A moderate amount of light suffices to induce such movements; all that is necessary is that the light should steadily strike the plants in an oblique direction. With respect to the above twisting movement of cotyledons, Frank has given many and much more striking instances in the case of the leaves on branches which had been fastened in various positions or turned upside down.
[5] ‘Die natürliche Wagerechte Richtung von Pflanzentheilen,’ 1870.
See also some interesting articles by the same author, “Zur Frage über seq.
In our observations on the cotyledons of seedling plants, we often felt surprise at their persistent horizontal position during the day, and were convinced before we had read Frank’s essay, that some special explanation was necessary. De Vries has shown[6] that the more or less horizontal position of leaves is in most cases influenced by epinasty, by their own weight, and by apogeotropism. A young cotyledon or leaf after bursting free is brought down into its proper position, as already remarked, by epinasty, which, according to De Vries, long continues to act on the midribs and petioles. Weight can hardly be influential in the case of cotyledons, except in a few cases presently to be mentioned, but must be so with large and thick leaves. With respect to apogeotropism, De Vries maintains that it generally comes into play, and of this fact we shall presently advance some indirect evidence. But over these and other constant forces we believe that there is in many cases, but we do not say in all, a preponderant tendency in leaves and cotyledons to place themselves more or less transversely with respect to the light.
[6] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp.
223–277.
In the cases above alluded to of seedlings exposed to a lateral light with their hypocotyls secured, it is impossible that epinasty, weight and apogeotropism, either in opposition or combined, can be the cause of the rising of one cotyledon, and of the sinking of the other, since the forces in question act equally on both; and since epinasty, weight and apogeotropism all act in a vertical plane, they cannot cause the twisting of the petioles, which occurs in seedlings under the above conditions of illumination. All these movements evidently depend in some manner on the obliquity of the light, but cannot be called heliotropic, as this implies bending towards the light; whereas the cotyledon nearest to the light bends in an opposed direction or downwards, and both place themselves as nearly as possible at right angles to the light. The movement, therefore, deserves a distinct name.
As cotyledons and leaves are continually oscillating up and down, and yet retain all day long their proper position with their upper surfaces directed transversely to the light, and if displaced reassume this position, diaheliotropism must be considered as a modified form of circumnutation. This was often evident when the movements of cotyledons standing in front of a window were traced. We see something analogous in the case of sleeping leaves or cotyledons, which after oscillating up and down during the whole day, rise into a vertical position late in the evening, and on the following morning sink down again into their horizontal or diaheliotropic position, in direct opposition to heliotropism. This return into their diurnal position, which often requires an angular movement of 90°, is analogous to the movement of leaves on displaced branches, which recover their former positions. It deserves notice that any force such as apogeotropism, will act with different degrees of power[7] in the different positions of those leaves or cotyledons which oscillate largely up and down during the day; and yet they recover their horizontal or diaheliotropic position.
[7] See former note, in reference to Sachs’ remarks on this subject.
We may therefore conclude that diaheliotropic movements cannot be fully explained by the direct action of light, gravitation, weight, etc., any more than can the nyctitropic movements of cotyledons and leaves. In the latter case they place themselves so that their upper surfaces may radiate at night as little as possible into open space, with the upper surfaces of the opposite leaflets often in contact. These movements, which are sometimes extremely complex, are regulated, though not directly caused, by the alternations of light and darkness. In the case of diaheliotropism, cotyledons and leaves place themselves so that their upper surfaces may be exposed to the light, and this movement is regulated, though not directly caused, by the direction whence the light proceeds. In both cases the movement consists of circumnutation modified by innate or constitutional causes, in the same manner as with climbing plants, the circumnutation of which is increased in amplitude and rendered more circular, or again with very young cotyledons and leaves which are thus brought down into a horizontal position by epinasty.
We have hitherto referred only to those leaves and cotyledons which occupy a permanently horizontal position; but many stand more or less obliquely, and some few upright. the cause of these differences of position is not known; but in accordance with Wiesner’s views, hereafter to be given, it is probable that some leaves and cotyledons would suffer, if they were fully illuminated by standing at right angles to the light.
We have seen in the second and fourth chapters that those cotyledons and leaves which do not alter their positions at night sufficiently to be said to sleep, commonly rise a little in the evening and fall again on the next morning, so that they stand during the night at a rather higher inclination than during the middle of the day. It is incredible that a rising movement of 2° or 3°, or even of 10° or 20°, can be of any service to the plant, so as to have been specially acquired. It must be the result of some periodical change in the conditions to which they are subjected, and there can hardly be a doubt that this is the daily alternations of light and darkness. De Vries states in the paper before referred to, that most petioles and midribs are apogeotropic;[8] and apogeotropism would account for the above rising movement, which is common to so many widely distinct species, if we suppose it to be conquered by diaheliotropism during the middle of the day, as long as it is of importance to the plant that its cotyledons and leaves should be fully exposed to the light. The exact hour in the afternoon at which they begin to bend slightly upwards, and the extent of the movement, will depend on their degree of sensitiveness to gravitation and on their power of resisting its action during the middle of the day, as well as on the amplitude of their ordinary circumnutating movements; and as these qualities differ much in different species, we might expect that the hour in the afternoon at which they begin to rise would differ much in different species, as is the case. Some other agency, however, besides apogeotropism, must come into play, either directly or indirectly, in this upward movement. Thus a young bean (Vicia faba), growing in a small pot, was placed in front of a window in a klinostat; and at night the leaves rose a little, although the action of apogeotropism was quite eliminated. Nevertheless, they did not rise nearly so much at night, as when subjected to apogeotropism. Is it not possible, or even probable, that leaves and cotyledons, which have moved upwards in the evening through the action of apogeotropism during countless generations, may inherit a tendency to this movement? We have seen that the hypocotyls of several Leguminous plants have from a remote period inherited a tendency to arch themselves; and we know that the sleep-movements of leaves are to a certain extent inherited, independently of the alternations of light and darkness.
[8] According to Frank (‘Die nat. Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 46) the root-leaves of many plants, kept in darkness, rise up and even become vertical; and so it is in some cases with shoots. (See Rauwenhoff, ‘Archives Néerlandaises,’ tom. xii. p.
32.) These movements indicate apogeotropism; but when organs have been long kept in the dark, the amount of water and of mineral matter which they contain is so much altered, and their regular growth is so much disturbed, that it is perhaps rash to infer from their movements what would occur under normal conditions. (See Godlewski, ‘Bot. Zeitung,’ Feb. 14th, 1879.)
In our observations on the circumnutation of those cotyledons and leaves which do not sleep at night, we met with hardly any distinct cases of their sinking a little in the evening, and rising again in the morning,—that is, of movements the reverse of those just discussed. We have no doubt that such cases occur, inasmuch as the leaves of many plants sleep by sinking vertically downwards. How to account for the few cases which were observed must be left doubtful. The young leaves of Cannabis sativa sink at night between 30° and 40° beneath the horizon; and Kraus attributes this to epinasty in conjunction with the absorption of water. Whenever epinastic growth is vigorous, it might conquer diaheliotropism in the evening, at which time it would be of no importance to the plant to keep its leaves horizontal. The cotyledons of Anoda Wrightii, of one variety of Gossypium, and of several species of Ipomœa, remain horizontal in the evening whilst they are very young; as they grow a little older they curve a little downwards, and when large and heavy sink so much that they come under our definition of sleep. In the case of the Anoda and of some species of Ipomœa, it was proved that the downward movement did not depend on the weight of the cotyledons; but from the fact of the movement being so much more strongly pronounced after the cotyledons have grown large and heavy, we may suspect that their weight aboriginally played some part in determining that the modification of the circumnutating movement should be in a downward direction.
The so-called Diurnal Sleep of Leaves, Or Paraheliotropism.—This is another class of movements, dependent on the action of light, which supports to some extent the belief that the movements above described are only indirectly due to its action. We refer to the movements of leaves and cotyledons which when moderately illuminated are diaheliotropic; but which change their positions and present their edges to the light, when the sun shines brightly on them. These movements have sometimes been called diurnal sleep, but they differ wholly with respect to the object gained from those properly called nyctitropic; and in some cases the position occupied during the day is the reverse of that during the night.
It has long been known[9] that when the sun shines brightly on the leaflets of Robinia, they rise up and present their edges to the light; whilst their position at night is vertically downwards. We have observed the same movement, when the sun shone brightly on the leaflets of an Australian Acacia. Those of Amphicarpæa monoica turned their edges to the sun; and an analogous movement of the little almost rudimentary basal leaflets of Mimosa albida was on one occasion so rapid that it could be distinctly seen through a lens. the elongated, unifoliate, first leaves of Phaseolus Roxburghii stood at 7 A.M. at 20° above the horizon, and no doubt they afterwards sank a little lower. At noon, after having been exposed for about 2 h. to a bright sun, they stood at 56° above the horizon; they were then protected from the rays of the sun, but were left well illuminated from above, and after 30 m. they had fallen 40°, for they now stood at only 16° above the horizon. Some young plants of Phaseolus Hernandesii had been exposed to the same bright sunlight, and their broad, unifoliate, first leaves now stood up almost or quite vertically, as did many of the leaflets on the trifoliate secondary leaves; but some of the leaflets had twisted round on their own axes by as much as 90° without rising, so as to present their edges to the sun. The leaflets on the same leaf sometimes behaved in these two different manners, but always with the result of being less intensely illuminated. These plants were then protected from the sun, and were looked at after 1½ h.; and now all the leaves and leaflets had reassumed their ordinary sub-horizontal positions. The copper-coloured cotyledons of some seedlings of Cassia mimosoides were horizontal in the morning, but after the sun had shone on them, each had risen 45½° above the horizon. the movement in these several cases must not be confounded with the sudden closing of the leaflets of Mimosa pudica, which may sometimes be noticed when a plant which has been kept in an obscure place is suddenly exposed to the sun; for in this case the light seems to act, as if it were a touch.
[9] Pfeffer gives the names and dates of several ancient writers in From Prof. Wiesner’s interesting observations, it is probable that the above movements have been acquired for a special purpose. the chlorophyll in leaves is often injured by too intense a light, and Prof. Wiesner[10] believes that it is protected by the most diversified means, such as the presence of hairs, colouring matter, etc., and amongst other means by the leaves presenting their edges to the sun, so that the blades then receive much less light. He experimented on the young leaflets of Robinia, by fixing them in such a position that they could not escape being intensely illuminated, whilst others were allowed to place themselves obliquely; and the former began to suffer from the light in the course of two days.
[10] ‘Die Näturlichen Einrichtungen zum Schutze des Chlorophylls,’ etc., 1876. Pringsheim has recently observed under the microscope the destruction of chlorophyll in a few minutes by the action of concentrated light from the sun, in the presence of oxygen. See, also, Stahl on the protection of chlorophyll from intense light, in ‘Bot.
Zeitung,’ 1880.
In the cases above given, the leaflets move either upwards or twist laterally, so as to place their edges in the direction of the sun’s light; but Cohn long ago observed that the leaflets of Oxalis bend downwards when fully exposed to the sun. We witnessed a striking instance of this movement in the very large leaflets of O. Ortegesii. A similar movement may frequently be observed with the leaflets of Averrhoa bilimbi (a member of the Oxalidæ); and a leaf is here represented (Fig. 180) on which the sun had shone. A diagram (Fig. 134) was given in the last chapter, representing the oscillations by which a leaflet rapidly descended under these circumstances; and the movement may be seen closely to resemble that (Fig. 133) by which it assumed its nocturnal position. It is an interesting fact in relation to our present subject that, as Prof. Batalin informs us in a letter, dated February, 1879, the leaflets of Oxalis acetosella may be daily exposed to the sun during many weeks, and they do not suffer if they are allowed to depress themselves; but if this be prevented, they lose their colour and wither in two or three days. Yet the duration of a leaf is about two months, when subjected only to diffused light; and in this case the leaflets never sink downwards during the day.
Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine; but the leaflets are sometimes more depressed than is here shown. Figure much reduced.
As the upward movements of the leaflets of Robinia, and the downward movements of those of Oxalis, have been proved to be highly beneficial to these plants when subjected to bright sunshine, it seems probable that they have been acquired for the special purpose of avoiding too intense an illumination. As it would have been very troublesome in all the above cases to have watched for a fitting opportunity and to have traced the movement of the leaves whilst they were fully exposed to the sunshine, we did not ascertain whether paraheliotropism always consisted of modified circumnutation; but this certainly was the case with the Averrhoa, and probably with the other species, as their leaves were continually circumnutating.
CHAPTER IX.
SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
Uses of heliotropism—Insectivorous and climbing plants not heliotropic—Same organ heliotropic at one age and not at another—Extraordinary sensitiveness of some plants to light—The effects of light do not correspond with its intensity—Effects of previous illumination—Time required for the action of light—After-effects of light—Apogeotropism acts as soon as light fails—Accuracy with which plants bend to the light—This dependent on the illumination of one whole side of the part—Localised sensitiveness to light and its transmitted effects—Cotyledons of Phalaris, manner of bending—Results of the exclusion of light from their tips—Effects transmitted beneath the surface of the ground—Lateral illumination of the tip determines the direction of the curvature of the base—Cotyledons of Avena, curvature of basal part due to the illumination of upper part—Similar results with the hypocotyls of Brassica and Beta—Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips—Concluding remarks and summary of chapter—Means by which circumnutation has been converted into heliotropism or apheliotropism.
No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated. They are thus enabled to decompose carbonic acid. But the sheath-like cotyledons of some Gramineæ, for instance, those of Phalaris, are not green and contain very little starch; from which fact we may infer that they decompose little or no carbonic acid. Nevertheless, they are extremely heliotropic; and this probably serves them in another way, namely, as a guide from the buried seeds through fissures in the ground or through overlying masses of vegetation, into the light and air. This view is strengthened by the fact that with Phalaris and Avena the first true leaf, which is bright green and no doubt decomposes carbonic acid, exhibits hardly a trace of heliotropism. The heliotropic movements of many other seedlings probably aid them in like manner in emerging from the ground; for apogeotropism by itself would blindly guide them upwards, against any overlying obstacle.
Heliotropism prevails so extensively among the higher plants, that there are extremely few, of which some part, either the stem, flower-peduncle, petiole, or leaf, does not bend towards a lateral light. Drosera rotundifolia is one of the few plants the leaves of which exhibit no trace of heliotropism. Nor could we see any in Dionaea, though the plants were not so carefully observed. Sir J.
Hooker exposed the pitchers of Sarracenia for some time to a lateral light, but they did not bend towards it.[1] We can understand the reason why these insectivorous plants should not be heliotropic, as they do not live chiefly by decomposing carbonic acid; and it is much more important to them that their leaves should occupy the best position for capturing insects, than that they should be fully exposed to the light.
[1] According to F. Kurtz (‘Verhandl. des Bot. Vereins der Provinz Brandenburg,’ Bd. xx. 1878) the leaves or pitchers of Darlingtonia Californica are strongly apheliotropic. We failed to detect this movement in a plant which we possessed for a short time.
Tendrils, which consist of leaves or of other organs modified, and the stems of twining plants, are, as Mohl long ago remarked, rarely heliotropic; and here again we can see the reason why, for if they had moved towards a lateral light they would have been drawn away from their supports. But some tendrils are apheliotropic, for instance those of Bignonia capreolata and of Smilax aspera; and the stems of some plants which climb by rootlets, as those of the Ivy and Tecoma radicans, are likewise apheliotropic, and they thus find a support. The leaves, on the other hand, of most climbing plants are heliotropic; but we could detect no signs of any such movement in those of Mutisia clematis.
As heliotropism is so widely prevalent, and as twining plants are distributed throughout the whole vascular series, the apparent absence of any tendency in their stems to bend towards the light, seemed to us so remarkable a fact as to deserve further investigation, for it implies that heliotropism can be readily eliminated. When twining plants are exposed to a lateral light, their stems go on revolving or circumnutating about the same spot, without any evident deflection towards the light; but we thought that we might detect some trace of heliotropism by comparing the average rate at which the stems moved to and from the light during their successive revolutions.[2] Three young plants (about a foot in height) of Ipomœa caerulea and four of I.
purpurea, growing in separate pots, were placed on a bright day before a north-east window in a room otherwise darkened, with the tips of their revolving stems fronting the window. When the tip of each plant pointed directly from the window, and when again towards it, the times were recorded. This was continued from 6.45 A.M. till a little after 2 P.M. on June 17th. After a few observations we concluded that we could safely estimate the time taken by each semicircle, within a limit of error of at most 5 minutes.
Although the rate of movement in different parts of the same revolution varied greatly, yet 22 semicircles to the light were completed, each on an average in 73.95 minutes; and 22 semicircles from the light each in 73.5 minutes. It may, therefore, be said that they travelled to and from the light at exactly the same average rate; though probably the accuracy of the result was in part accidental. In the evening the stems were not in the least deflected towards the window. Nevertheless, there appears to exist a vestige of heliotropism, for with 6 out of the 7 plants, the first semicircle from the light, described in the early morning after they had been subjected to darkness during the night and thus probably rendered more sensitive, required rather more time, and the first semicircle to the light considerably less time, than the average. Thus with all 7 plants, taken together, the mean time of the first semicircle in the morning from the light, was 76.8 minutes, instead of 73.5 minutes, which is the mean of all the semicircles during the day from the light; and the mean of the first semicircle to the light was only 63.1, instead of 73.95 minutes, which was the mean of all the semicircles during the day to the light.
[2] Some erroneous statements are unfortunately given on this subject, in ‘The Movements and Habits of Climbing Plants,’ 1875, pp. 28, 32, 40, and 53. Conclusions were drawn from an insufficient number of observations, for we did not then know at how unequal a rate the stems and tendrils of climbing plants sometimes travel in different parts of the same revolution.
Similar observations were made on Wistaria Sinensis, and the mean of 9 semicircles from the light was 117 minutes, and of 7 semicircles to the light 122 minutes, and this difference does not exceed the probable limit of error. During the three days of exposure, the shoot did not become at all bent towards the window before which it stood. In this case the first semicircle from the light in the early morning of each day, required rather less time for its performance than did the first semicircle to the light; and this result, if not accidental, appears to indicate that the shoots retain a trace of an original apheliotropic tendency. With Lonicera brachypoda the semicircles from and to the light differed considerably in time; for 5 semicircles from the light required on a mean 202.4 minutes, and 4 to the light, 229.5 minutes; but the shoot moved very irregularly, and under these circumstances the observations were much too few.
It is remarkable that the same part on the same plant may be affected by light in a widely different manner at different ages, and as it appears at different seasons. The hypocotyledonous stems of Ipomœa caerulea and purpurea are extremely heliotropic, whilst the stems of older plants, only about a foot in height, are, as we have just seen, almost wholly insensible to light. Sachs states (and we have observed the same fact) that the hypocotyls of the Ivy (Hedera helix) are slightly heliotropic; whereas the stems of plants grown to a few inches in height become so strongly apheliotropic, that they bend at right angles away from the light. Nevertheless, some young plants which had behaved in this manner early in the summer again became distinctly heliotropic in the beginning of September; and the zigzag courses of their stems, as they slowly curved towards a north-east window, were traced during 10 days. The stems of very young plants of Tropaeolum majus are highly heliotropic, whilst those of older plants, according to Sachs, are slightly apheliotropic. In all these cases the heliotropism of the very young stems serves to expose the cotyledons, or when the cotyledons are hypogean the first true leaves, fully to the light; and the loss of this power by the older stems, or their becoming apheliotropic, is connected with their habit of climbing.
Most seedling plants are strongly heliotropic, and it is no doubt a great advantage to them in their struggle for life to expose their cotyledons to the light as quickly and as fully as possible, for the sake of obtaining carbon. It has been shown in the