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  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 Gramineae, 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 [page 450] 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.* 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.
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
* 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. [page 451]
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.* Three young plants (about a foot in height) of Ipomoea 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
* 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. [page 452]
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.
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, [page 453] 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 Ipomoea 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 [page 454] 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 first chapter that the greater number of seedlings circumnutate largely and rapidly; and as heliotropism consists of modified circumnutation, we are tempted to look at the high development of these two powers in seedlings as intimately connected. Whether there are any plants which circumnutate slowly and to a small extent, and yet are highly heliotropic, we do not know; but there are several, and there is nothing surprising in this fact, which circumnutate largely and are not at all, or only slightly, heliotropic. Of such cases Drosera rotundifolia offers an excellent instance. The stolons of the strawberry circumnutate almost like the stems of climbing plants, and they are not at all affected by a moderate light; but when exposed late in the summer to a somewhat brighter light they were slightly heliotropic; in sunlight, according to De Vries, they are apheliotropic. Climbing plants circumnutate much more widely than any other plants, yet they are not at all heliotropic.
Although the stems of most seedling plants are strongly heliotropic, some few are but slightly heliotropic, without our being able to assign any reason. This is the case with the hypocotyl of Cassia tora, and we were struck with the same fact with some other seedlings, for instance, those of Reseda odorata. With respect to the degree of sensitiveness of the more sensitive kinds, it was shown in the last chapter that seedlings of several species, placed before a north-east window protected by several blinds, and exposed in the rear to the diffused light of the room, moved with unerring certainty towards the window, although [page 455] it was impossible to judge, excepting by the shadow cast by an upright pencil on a white card, on which side most light entered, so that the excess on one side must have been extremely small.
A pot with seedlings of Phalaris Canariensis, which had been raised in darkness, was placed in a completely darkened room, at 12 feet from a very small lamp. After 3 h. the cotyledons were doubtfully curved towards the light, and after 7 h. 40 m. from the first exposure, they were all plainly, though slightly, curved towards the lamp. Now, at this distance of 12 feet, the light was so obscure that we could not see the seedlings themselves, nor read the large Roman figures on the white face of a watch, nor see a pencil line on paper, but could just distinguish a line made with Indian ink. It is a more surprising fact that no visible shadow was cast by a pencil held upright on a white card; the seedlings, therefore, were acted on by a difference in the illumination of their two sides, which the human eye could not distinguish. On another occasion even a less degree of light acted, for some cotyledons of Phalaris became slightly curved towards the same lamp at a distance of 20 feet; at this distance we could not see a circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on white paper, though we could just see a dot 3.56 mm. (.14 inch) in diameter; yet a dot of the former size appears large when seen in the light.*
We next tried how small a beam of light would act; as this bears on light serving as a guide to seedlings whilst they emerge through fissured or encumbered ground. A pot with seedlings of Phalaris was covered
* Strasburger says ('Wirkung des Lichtes auf Schw?rmsporen,' 1878, p. 52), that the spores of Haematococcus moved to a light which only just sufficed to allow middle-sized type to be read. [page 456]
by a tin-vessel, having on one side a circular hole 1.23 mm. in diameter (i.e. a little less than the 1/20th of an inch); and the box was placed in front of a paraffin lamp and on another occasion in front of a window; and both times the seedlings were manifestly bent after a few hours towards the little hole.
A more severe trial was now made; little tubes of very thin glass, closed at their upper ends and coated with black varnish, were slipped over the cotyledons of Phalaris (which had germinated in darkness) and just fitted them. Narrow stripes of the varnish had been previously scraped off one side, through which alone light could enter; and their dimensions were afterwards measured under the microscope. As a control experiment, similar unvarnished and transparent tubes were tried, and they did not prevent the cotyledons bending towards the light. Two cotyledons were placed before a south-west window, one of which was illuminated by a stripe in the varnish, only .004 inch (0.1 mm.) in breadth and .016 inch (0.4 mm.) in length; and the other by a stripe .008 inch in breadth and .06 inch in length. The seedlings were examined after an exposure of 7 h. 40 m., and were found to be manifestly bowed towards the light. Some other cotyledons were at the same time treated similarly, excepting that the little stripes were directed not to the sky, but in such a manner that they received only the diffused light from the room; and these cotyledons did not become at all bowed. Seven other cotyledons were illuminated through narrow, but comparatively long, cleared stripes in the varnish—namely, in breadth between .01 and .026 inch, and in length between .15 and .3 inch; and these all became bowed to the side, by which light entered through the stripes, whether these were directed towards the sky or to one side of [page 457] the room. That light passing through a hole only .004 inch in breadth by .016 in length, should induce curvature, seems to us a surprising fact.
Before we knew how extremely sensitive the cotyledons of Phalaris were to light, we endeavoured to trace their circumnutation in darkness by the aid of a small wax taper, held for a minute or two at each observation in nearly the same position, a little on the left side in front of the vertical glass on which the tracing was made. The seedlings were thus observed seventeen times in the course of the day, at intervals of from half to three-quarters of an hour; and late in the evening we were surprised to find that all the 29 cotyledons were greatly curved and pointed towards the vertical glass, a little to the left where the taper had been held. The tracings showed that they had travelled in zigzag lines. Thus, an exposure to a feeble light for a very short time at the above specified intervals, sufficed to induce well-marked heliotropism. An analogous case was observed with the hypocotyls of Solanum lycopersicum. We at first attributed this result to the after-effects of the light on each occasion; but since reading Wiesner's observations,* which will be referred to in the last chapter, we cannot doubt that an intermittent light is more efficacious than a continuous one, as plants are especially sensitive to any contrast in its amount.
The cotyledons of Phalaris bend much more slowly towards a very obscure light than towards a bright one. Thus, in the experiments with seedlings placed in a dark room at 12 feet from a very small lamp, they were just perceptibly and doubtfully curved towards it after 3 h., and only slightly, yet certainly, after 4 h.
* 'Sitz. der k. Akad. der Wissensch.' (Vienna), Jan. 1880, p. 12. [page 458]
After 8 h. 40 m. the chords of their arcs were deflected from the perpendicular by an average angle of only 16o. Had the light been bright, they would have become much more curved in between 1 and 2 h. Several trials were made with seedlings placed at various distances from a small lamp in a dark room; but we will give only one trial. Six pots were placed at distances of 2, 4, 8, 12, 16, and 20 feet from the lamp, before which they were left for 4 h. As light decreases in a geometrical ratio, the seedlings in the 2nd pot received 1/4th, those in the 3rd pot 1/16th, those in the 4th 1/36th, those in the 5th 1/64th, and those in the 6th 1/100th of the light received by the seedlings in the first or nearest pot. Therefore it might have been expected that there would have been an immense difference in the degree of their heliotropic curvature in the several pots; and there was a well-marked difference between those which stood nearest and furthest from the lamp, but the difference in each successive pair of pots was extremely small. In order to avoid prejudice, we asked three persons, who knew nothing about the experiment, to arrange the pots in order according to the degree of curvature of the cotyledons. The first person arranged them in proper order, but doubted long between the 12 feet and 16 feet pots; yet these two received light in the proportion of 36 to 64. The second person also arranged them properly, but doubted between the 8 feet and 12 feet pots, which received light in the proportion of 16 to 36. The third person arranged them in wrong order, and doubted about four of the pots. This evidence shows conclusively how little the curvature of the seedlings differed in the successive pots, in comparison with the great difference in the amount of light which they received; and it should be noted that there was no [page 459] excess of superfluous light, for the cotyledons became but little and slowly curved even in the nearest pot. Close to the 6th pot, at the distance of 20 feet from the lamp, the light allowed us just to distinguish a dot 3.56 mm. (.14 inch) in diameter, made with Indian ink on white paper, but not a dot 2.29 mm. (.09 inch) in diameter.
The degree of curvature of the cotyledons of Phalaris within a given time, depends not merely on the amount of lateral light which they may then receive, but on that which they have previously received from above and on all sides. Analogous facts have been given with respect to the nyctitropic and periodic movements of plants. Of two pots containing seedlings of Phalaris which had germinated in darkness, one was still kept in the dark, and the other was exposed (Sept. 26th) to the light in a greenhouse during a cloudy day and on the following bright morning. On this morning (27th), at 10.30 A.M., both pots were placed in a box, blackened within and open in front, before a north-east window, protected by a linen and muslin blind and by a towel, so that but little light was admitted, though the sky was bright. Whenever the pots were looked at, this was done as quickly as possible, and the cotyledons were then held transversely with respect to the light, so that their curvature could not have been thus increased or diminished. After 50 m. the seedlings which had previously been kept in darkness, were perhaps, and after 70 m. were certainly, curved, though very slightly, towards the window. After 85 m. some of the seedlings, which had previously been illuminated, were perhaps a little affected, and after 100 m. some of the younger ones were certainly a little curved towards the light. At this time (i.e. after 100 m.) there was a plain difference [page 460] in the curvature of the seedlings in the two pots. After 2 h. 12 m. the chords of the arcs of four of the most strongly curved seedlings in each pot were measured, and the mean angle from the perpendicular of those which had previously been kept in darkness was 19o, and of those which had previously been illuminated only 7o. Nor did this difference diminish during two additional hours. As a check, the seedlings in both pots were then placed in complete darkness for two hours, in order that apogeotropism should act on them; and those in the one pot which were little curved became in this time almost completely upright, whilst the more curved ones in the other pot still remained plainly curved.
Two days afterwards the experiment was repeated, with the sole difference that even less light was admitted through the window, as it was protected by a linen and muslin blind and by two towels; the sky, moreover, was somewhat less bright. The result was the same as before, excepting that everything occurred rather slower. The seedlings which had been previously kept in darkness were not in the least curved after 54 m., but were so after 70 m. Those which had previously been illuminated were not at all affected until 130 m. had elapsed, and then only slightly. After 145 m. some of the seedlings in this latter pot were certainly curved towards the light; and there was now a plain difference between the two pots. After 3 h. 45 m. the chords of the arcs of 3 seedlings in each pot were measured, and the mean angle from the perpendicular was 16o for those in the pot which had previously been kept in darkness, and only 5o for those which had previously been illuminated.
The curvature of the cotyledons of Phalaris towards a lateral light is therefore certainly influenced by the [page 461] degree to which they have been previously illuminated. We shall presently see that the influence of light on their bending continues for a short time after the light has been extinguished. These facts, as well as that of the curvature not increasing or decreasing in nearly the same ratio with that of the amount of light which they receive, as shown in the trials with the plants before the lamp, all indicate that light acts on them as a stimulus, in somewhat the same manner as on the nervous system of animals, and not in a direct manner on the cells or cell-walls which by their contraction or expansion cause the curvature.
It has already been incidentally shown how slowly the cotyledons of Phalaris bend towards a very dim light; but when they were placed before a bright paraffin lamp their tips were all curved rectangularly towards it in 2 h. 20 m. The hypocotyls of Solanum lycopersicum had bent in the morning at right angles towards a north-east window. At 1 P.M. (Oct. 21st) the pot was turned round, so that the seedlings now pointed from the light, but by 5 P.M. they had reversed their curvature and again pointed to the light. They had thus passed through 180o in 4 h., having in the morning previously passed through about 90o. But the reversal of the first half of the curvature will have been aided by apogeotropism. Similar cases were observed with other seedlings, for instance, with those of Sinapis alba.
We attempted to ascertain in how short a time light acted on the cotyledons of Phalaris, but this was difficult on account of their rapid circumnutating movement; moreover, they differ much in sensibility, according to age; nevertheless, some of our observations are worth giving. Pots with seedlings were [page 462] placed under a microscope provided with an eye-piece micrometer, of which each division equalled 1/500th of an inch (0.051 mm.); and they were at first illuminated by light from a paraffin lamp passing through a solution of bichromate of potassium, which does not induce heliotropism. Thus the direction in which the cotyledons were circumnutating could be observed independently of any action from the light; and they could be made, by turning round the pots, to circumnutate transversely to the line in which the light would strike them, as soon as the solution was removed. The fact that the direction of the circumnutating movement might change at any moment, and thus the plant might bend either towards or from the lamp independently of the action of the light, gave an element of uncertainty to the results. After the solution had been removed, five seedlings which were circumnutating transversely to the line of light, began to move towards it, in 6, 4, 7 1/2, 6, and 9 minutes. In one of these cases, the apex of the cotyledon crossed five of the divisions of the micrometer (i.e. 1/100th of an inch, or 0.254 mm.) towards the light in 3 m. Of two seedlings which were moving directly from the light at the time when the solution was removed, one began to move towards it in 13 m., and the other in 15 m. This latter seedling was observed for more than an hour and continued to move towards the light; it crossed at one time 5 divisions of the micrometer (0.254 mm.) in 2 m. 30 s. In all these cases, the movement towards the light was extremely unequal in rate, and the cotyledons often remained almost stationary for some minutes, and two of them retrograded a little. Another seedling which was circumnutating transversely to the line of light, moved towards it in 4 m. after the solution was removed; it then remained [page 463] almost stationary for 10 m.; then crossed 5 divisions of the micrometer in 6 m.; and then 8 divisions in 11m. This unequal rate of movement, interrupted by pauses, and at first with occasional retrogressions, accords well with our conclusion that heliotropism consists of modified circumnutation.
In order to observe how long the after-effects of light lasted, a pot with seedlings of Phalaris, which had germinated in darkness, was placed at 10.40 A.M. before a north-east window, being protected on all other sides from the light; and the movement of a cotyledon was traced on a horizontal glass. It circumnutated about the same space for the first 24 m., and during the next 1 h. 33 m. moved rapidly towards the light. The light was now (i.e. after 1 h. 57 m.) completely excluded, but the cotyledon continued bending in the same direction as before, certainly for more than 15 m., probably for about 27 m. The doubt arose from the necessity of not looking at the seedlings often, and thus exposing them, though momentarily, to the light. This same seedling was now kept in the dark, until 2.18 P.M., by which time it had reacquired through apogeotropism its original upright position, when it was again exposed to the light from a clouded sky. By 3 P.M. it had moved a very short distance towards the light, but during the next 45 m. travelled quickly towards it. After this exposure of 1 h. 27 m. to a rather dull sky, the light was again completely excluded, but the cotyledon continued to bend in the same direction as before for 14 m. within a very small limit of error. It was then placed in the dark, and it now moved backwards, so that after 1 h. 7 m. it stood close to where it had started from at 2.18 P.M. These observations show that the cotyledons of Phalaris, after being exposed to a lateral [page 464] light, continue to bend in the same direction for between a quarter and half an hour.
In the two experiments just given, the cotyledons moved backwards or from the window shortly after being subjected to darkness; and whilst tracing the circumnutation of various kinds of seedlings exposed to a lateral light, we repeatedly observed that late in the evening, as the light waned, they moved from it. This fact is shown in some of the diagrams given in the last chapter. We wished therefore to learn whether this was wholly due to apogeotropism, or whether an organ after bending towards the light tended from any other cause to bend from it, as soon as the light failed. Accordingly, two pots of seedling Phalaris and one pot of seedling Brassica were exposed for 8 h. before a paraffin lamp, by which time the cotyledons of the former and the hypocotyls of the latter were bent rectangularly towards the light. The pots were now quickly laid horizontally, so that the upper parts of the cotyledons and of the hypocotyls of 9 seedlings projected vertically upwards, as proved by a plumb-line. In this position they could not be acted on by apogeotropism, and if they possessed any tendency to straighten themselves or to bend in opposition to their former heliotropic curvature, this would be exhibited, for it would be opposed at first very slightly by apogeotropism. They were kept in the dark for 4 h., during which time they were twice looked at; but no uniform bending in opposition to their former heliotropic curvature could be detected. We have said uniform bending, because they circumnutated in their new position, and after 2 h. were inclined in different directions (between 4o and 11o) from the perpendicular. Their directions were also changed after two additional hours, and again on the following morning. We may [page 465] therefore conclude that the bending back of plants from a light, when this becomes obscure or is extinguished, is wholly due to apogeotropism.*
In our various experiments we were often struck with the accuracy with which seedlings pointed to a light although of small size. To test this, many seedlings of Phalaris, which had germinated in darkness in a very narrow box several feet in length, were placed in a darkened room near to and in front of a lamp having a small cylindrical wick. The cotyledons at the two ends and in the central part of the box, would therefore have to bend in widely different directions in order to point to the light. After they had become rectangularly bent, a long white thread was stretched by two persons, close over and parallel, first to one and then to another cotyledon; and the thread was found in almost every case actually to intersect the small circular wick of the now extinguished lamp. The deviation from accuracy never exceeded, as far as we could judge, a degree or two. This extreme accuracy seems at first surprising, but is not really so, for an upright cylindrical stem, whatever its position may be with respect to the light, would have exactly half its circumference illuminated and half in shadow; and as the difference in illumination of the two sides is the exciting cause of heliotropism, a cylinder would naturally bend with much accuracy towards the light. The cotyledons, however, of Phalaris are not cylindrical, but oval in section; and the longer axis was to the shorter axis (in the one which was measured) as 100 to 70. Nevertheless, no difference could be
* It appears from a reference in Wiesner ('Die Undulirende Nutation der Internodien,' p. 7), that H. Müller of Thurgau found that a stem which is bending heliotropically is at the same time striving, through apogeotropism, to raise itself into a vertical position. [page 466]
detected in the accuracy of their bending, whether they stood with their broad or narrow sides facing the light, or in any intermediate position; and so it was with the cotyledons of Avena sativa, which are likewise oval in section. Now, a little reflection will show that in whatever position the cotyledons may stand, there will be a line of greatest illumination, exactly fronting the light, and on each side of this line an equal amount of light will be received; but if the oval stands obliquely with respect to the light, this will be diffused over a wider surface on one side of the central line than on the other. We may therefore infer that the same amount of light, whether diffused over a wider surface or concentrated on a smaller surface, produces exactly the same effect; for the cotyledons in the long narrow box stood in all sorts of positions with reference to the light, yet all pointed truly towards it.
That the bending of the cotyledons to the light depends on the illumination of one whole side or on the obscuration of the whole opposite side, and not on a narrow longitudinal zone in the line of the light being affected, was shown by the effects of painting longitudinally with Indian ink one side of five cotyledons of Phalaris. These were then placed on a table near to a south-west window, and the painted half was directed either to the right or left. The result was that instead of bending in a direct line towards the window, they were deflected from the window and towards the unpainted side, by the following angles, 35o, 83o, 31o, 43o, and 39o. It should be remarked that it was hardly possible to paint one-half accurately, or to place all the seedlings which are oval in section in quite the same position relatively to the light; and this will account for the differences in the angles. Five coty- [page 467] ledons of Avena were also painted in the same manner, but with greater care; and they were laterally deflected from the line of the window, towards the unpainted side, by the following angles, 44o, 44o, 55o, 51o, and 57o. This deflection of the cotyledons from the window is intelligible, for the whole unpainted side must have received some light, whereas the opposite and painted side received none; but a narrow zone on the unpainted side directly in front of the window will have received most light, and all the hinder parts (half an oval in section) less and less light in varying degrees; and we may conclude that the angle of deflection is the resultant of the action of the light over the whole of the unpainted side.
It should have been premised that painting with Indian ink does not injure plants, at least within several hours; and it could injure them only by stopping respiration. To ascertain whether injury was thus soon caused, the upper halves of 8 cotyledons of Avena were thickly coated with transparent matter,—4 with gum, and 4 with gelatine; they were placed in the morning before a window, and by the evening they were normally bowed towards the light, although the coatings now consisted of dry crusts of gum and gelatine. Moreover, if the seedlings which were painted longitudinally with Indian ink had been injured on the painted side, the opposite side would have gone on growing, and they would consequently have become bowed towards the painted side; whereas the curvature was always, as we have seen, in the opposite direction, or towards the unpainted side which was exposed to the light. We witnessed the effects of injuring longitudinally one side of the cotyledons of Avena and Phalaris; for before we knew that grease was highly injurious to them, several were painted down one side [page 468] with a mixture of oil and lamp-black, and were then exposed before a window; others similarly treated were afterwards tried in darkness. These cotyledons soon became plainly bowed towards the blackened side, evidently owing to the grease on this side having checked their growth, whilst growth continued on the opposite side. But it deserves notice that the curvature differed from that caused by light, which ultimately becomes abrupt near the ground. These seedlings did not afterwards die, but were much injured and grew badly.
Phalaris Canariensis.—Whilst observing the accuracy with which the cotyledons of this plant became bent towards the light of a small lamp, we were impressed with the idea that the uppermost part determined the direction of the curvature of the lower part. When the cotyledons are exposed to a lateral light, the upper part bends first, and afterwards the bending gradually extends down to the base, and, as we shall presently see, even a little beneath the ground. This holds good with cotyledons from less than .1 inch (one was observed to act in this manner which was only .03 in height) to about .5 of an inch in height; but when they have grown to nearly an inch in height, the basal part, for a length of .15 to .2 of an inch above the ground, ceases to bend. As with young cotyledons the lower part goes on bending, after the upper part has become well arched towards a lateral light, the apex would ultimately point to the ground instead of to the light, did not the upper part reverse its curvature and straighten itself, as [page 469] soon as the upper convex surface of the bowed-down portion received more light than the lower concave surface. The position ultimately assumed by young and upright cotyledons, exposed to light entering obliquely from above through a window, is shown in the accompanying figure (Fig. 181); and here it may be seen that the whole upper part has become very nearly straight. When the cotyledons were exposed before a bright lamp, standing on the same level with them, the upper part, which was at first
Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open on one side in front of a south-west window during 8 h. Curvature towards the light accurately traced. The short horizontal lines show the level of the ground.
greatly arched towards the light, became straight and strictly parallel with the surface of the soil in the pots; the basal part being now rectangularly bent. All this great amount of curvature, together with the subsequent straightening of the upper part, was often effected in a few hours.
[After the uppermost part has become bowed a little to the light, its overhanging weight must tend to increase the curvature of the lower part; but any such effect was shown in several ways to be quite insignificant. When little caps of tin-foil (hereafter to be described) were placed on the summits of the cotyledons, though this must have added considerably to their weight, the rate or amount of bending was not thus increased. But the best evidence was afforded by placing pots with seedlings of Phalaris before a lamp in such a position, that the cotyledons were horizontally extended and projected at right angles to the line of light. In the course of 5 ? h. they were directed towards the light with their bases bent at right angles; and this abrupt [page 470] curvature could not have been aided in the least by the weight of the upper part, which acted at right angles to the plane of curvature.
It will be shown that when the upper halves of the cotyledons of Phalaris and Avena were enclosed in little pipes of tin-foil or of blackened glass, in which case the upper part was mechanically prevented from bending, the lower and unenclosed part did not bend when exposed to a lateral light; and it occurred to us that this fact might be due, not to the exclusion of the light from the upper part, but to some necessity of the bending gradually travelling down the cotyledons, so that unless the upper part first became bent, the lower could not bend, however much it might be stimulated. It was necessary for our purpose to ascertain whether this notion was true, and it was proved false; for the lower halves of several cotyledons became bowed to the light, although their upper halves were enclosed in little glass tubes (not blackened), which prevented, as far as we could judge, their bending. Nevertheless, as the part within the tube might possibly bend a very little, fine rigid rods or flat splinters of thin glass were cemented with shellac to one side of the upper part of 15 cotyledons; and in six cases they were in addition tied on with threads. They were thus forced to remain quite straight. The result was that the lower halves of all became bowed to the light, but generally not in so great a degree as the corresponding part of the free seedlings in the same pots; and this may perhaps be accounted for by some slight degree of injury having been caused by a considerable surface having been smeared with shellac. It may be added, that when the cotyledons of Phalaris and Avena are acted on by apogeotropism, it is the upper part which begins first to bend; and when this part was rendered rigid in the manner just described, the upward curvature of the basal part was not thus prevented.
To test our belief that the upper part of the cotyledons of Phalaris, when exposed to a lateral light, regulates the bending of the lower part, many experiments were tried; but most of our first attempts proved useless from various causes not worth specifying. Seven cotyledons had their tips cut off for lengths varying between .1 and .16 of an inch, and these, when left exposed all day to a lateral light, remained upright. In another set of 7 cotyledons, the tips were cut off for a length of only about .05 of an inch (1.27 mm.) and these became bowed towards [page 471] a lateral light, but not nearly so much as the many other seedlings in the same pots. This latter case shows that cutting off the tips does not by itself injure the plants so seriously as to prevent heliotropism; but we thought at the time, that such injury might follow when a greater length was cut off, as in the first set of experiments. Therefore, no more trials of this kind were made, which we now regret; as we afterwards found that when the tips of three cotyledons were cut off for a length of .2 inch, and of four others for lengths of .14, .12, .1, and .07 inch, and they were extended horizontally, the amputation did not interfere in the least with their bending vertically upwards, through the action of apogeotropism, like unmutilated specimens. It is therefore extremely improbable that the amputation of the tips for lengths of from .1 to .14 inch, could from the injury thus caused have prevented the lower part from bending towards the light.
We next tried the effects of covering the upper part of the cotyledons of Phalaris with little caps which were impermeable to light; the whole lower part being left fully exposed before a south-west window or a bright paraffin lamp. Some of the caps were made of extremely thin tin-foil blackened within; these had the disadvantage of occasionally, though rarely, being too heavy, especially when twice folded. The basal edges could be pressed into close contact with the cotyledons; though this again required care to prevent injuring them. Nevertheless, any injury thus caused could be detected by removing the caps, and trying whether the cotyledons were then sensitive to light. Other caps were made of tubes of the thinnest glass, which when painted black served well, with the one great disadvantage that the lower ends could not be closed. But tubes were used which fitted the cotyledons almost closely, and black paper was placed on the soil round each, to check the upward reflection of light from the soil. Such tubes were in one respect far better than caps of tin-foil, as it was possible to cover at the same time some cotyledons with transparent and others with opaque tubes; and thus our experiments could be controlled. It should be kept in mind that young cotyledons were selected for trial, and that these when not interfered with become bowed down to the ground towards the light.
We will begin with the glass-tubes. The summits of nine cotyledons, differing somewhat in height, were enclosed for rather less than half their lengths in uncoloured or transparent [page 472] tubes; and these were then exposed before a south-west window on a bright day for 8 h. All of them became strongly curved towards the light, in the same degree as the many other free seedlings in the same pots; so that the glass-tubes certainly did not prevent the cotyledons from bending towards the light. Nineteen other cotyledons were, at the same time, similarly enclosed in tubes thickly painted with Indian ink. On five of them, the paint, to our surprise, contracted after exposure to the sunlight, and very narrow cracks were formed, through which a little light entered; and these five cases were rejected. Of the remaining 14 cotyledons, the lower halves of which had been fully exposed to the light for the whole time, 7 continued quite straight and upright; 1 was considerably bowed to the light, and 6 were slightly bowed, but with the exposed bases of most of them almost or quite straight. It is possible that some light may have been reflected upwards from the soil and entered the bases of these 7 tubes, as the sun shone brightly, though bits of blackened paper had been placed on the soil round them. Nevertheless, the 7 cotyledons which were slightly............
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