The Power of Movement in Plants

We are therefore fully justified, as it seems to us, in believing that whenever light enters laterally, it is the 1879), Sachs has discussed the the organs of plants stand with manner in which geotropism and respect to the direction of the heliotropism are affected by dif- incident force. ferences in the angles at which 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 pos- sible 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 de- scribe 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 regu- lating cause is a difference in the intensity of the light, coming from above, at different periods of the twenty-four hours; whilst with heliotropie and aphe- liotropic movements, it is a difference in the intensity of the light on the two sides of the plant. Transversal-heliotropismus (of Frank*) or Déiahelio- troysm.—The 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

* *Die natiirliche Wagerechte Frage iiber Transver-al-Geo-und Richtung von Pflanzenth ilen,’ Heliotropismus,” ‘ Bot. Zeitung, 18'0 See also some interesting 1873, p. 17 e¢ seq. articles by the same author, * Zur of diaheliotropism than that offered by many seed- lings, the cotyledons of which are extended hori- zontally. 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 jn the degree of illumination of their upper and lower surfaces, and if they were heliotropic they would bend quickly upwards. It must not, how- ever, 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 ex- posed at right angles to the light; but if the hypo- cotyl 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 in- duce 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 move- ment 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. ; In our observations on the cotyledons of seedling plants, we often felt surprise at their persistent hori- zontal position during the day, and were convinced before we had read Frank’s essay, that some special explanution was necessary. De Vries has shown* 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 trans- versely with respect to the light.

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 coty- ledon nearest to the light bends in an opposed direc- tion or downwards, and both place themselves as nearly as possible at right angles to the light. The move- ment, therefore, deserves a distinct name. As coty- ledons 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, dia- heliotropism 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 dia- heliotropic position, in direct opposition to heliotro- pism. This return into their diurnal position, which often requires an angular movement of 90°, is analo- gous 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* in the different posi- tions of those leaves or cotyledons which oscillate largely up and down during the day; and yet they recover their horizontal or diaheliotropic position.

We may therefore conclude that diaheliotropic movements cannot be fully explained by the direct action of light, gravitation, weight, &c., any more * See former note, in reference to Sachs’ remarks on this st hject. than can the nyctitropic movements of cotyledons and leaves. In the latter case they place themselvez 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 alter- nations of light and darkness. In the case of diahelio- tropism, 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 circumnu- tation 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 hori- zontal 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 ;* and apogeotropism would account for the above rising movement, which is com- mon tosomany 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 sen- sitiveness to gravitation and on their power of resist- ing 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 dif- ferent 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

* According to Frank (‘Die nat. Wagerechte Richtung von Pianzentheilen.’ 1870, p. 46) the root-leaves of many plants, kept in darkness, rise up anil even be- eome vertical; and so it is in some cases with shoots. (See Rauwen- hoff, ‘Archives Nceérlandaises,’ tom. xii. p. 32.) These movements indicate apogeotropism ; but when organs have been long kept in the dark, the amouut of water and of mineral matter which they con- tain is so much altered, and their regular growth is so much dis- turbed, that it is perhaps rash to infer from their movements what would occur under normal con- ditions. (See Godlewski, ‘ Bot Zeitung,’ Feb. 14th, 1879.)

Nevertheless, they did not rise nearly so ae : night, as when subjected to apogeotropism. a = not possible, or even probable, that leaves SBC Cou ledons, which have moved upwards a0 the knee: through the action of apogeotropism during countless generations, may inherit a tendency to this movement ? We have seen that the hypocotyls of several Legu- minous 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.

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 morm- ing,—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 import- ance to the plant to keep its leaves horizontal. The cotyledons of Anoda Wrightii, of one variety of Gossypium, and of several species of Ipomea, remain horizontal in the evening whilst they are very young; as they grow a little older they curve a little down- wards, 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 Ipomeea, it was proved that the downward movement did not depend on the weight of the cotyledons; but from the fact of the move- ment 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 circum- nutating movement should be in a downward direction. The so-called Diurnal Sleep of Leaves, or Parahelio- tropism.—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* 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 down- wards. We have observed the same movement, when the sun shone brightly on the leaflets of an Australian Acacia. Those of Amphicarpwa 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, uni- foliate, 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 2h. to

* Pfeffer gives the names and dates of several ancient writers in hia * Die Periodischen Bewegungen, 1875, p. 62. 446 Cuar, VIIL 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 1}h.; and now all the leaves and leaflets had re- assumed 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 453° 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.

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* believes that it is protected -by the most diversified means, such as the presence of hairs, colouring matter, &c., 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.

* Die Niaturlicher Einrich- tungen zum Schutze des Chloro- phylls” &c., 1876. Pringsheim has recently observed under the microscope the destruction of ehjorophyll in a few minutes by the ection of concentrated light from the sun, in the presence of oxygen. See, also, Stahl on the protection of chlorophyll from intense light, in ‘Bot. Zeituny, 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 U. Urtegesit. A similar movement may frv- quently be observed with the leaflets of Averrhoa bilimbi (a member of the Oxalidz); 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 mevement may be seen closely to resemble that (Fig. 183) bv

Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine; but the leaflets are sometimes more depressed than is here shown. Figure much reduced. 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 acetoselia 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 __

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 pwrpose 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 modi- fied cireumnutation; but this certainly was the case with the Averrhoa, and probably with the other species, a3 their leaves were continually circumnntating.

Uses of heliotropism—Inscctivorous and climbing plants not heliotropic —Same organ helictropic ut one age and not at another—Extra- ordinary sensitiveness of some plants to light—The effects of light de not correspond with its intensity— Effects of previous illumination —Time required for the action of light—Aftcr-effects of light— Apogeotropism acts as soon as light tails—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 eftects—Cotyledons of | halaris, manner of bending— Results of the exclusion of light from their tips—Effects trans- m.tted beneath the surface of the ground—Lateral illumination of the tip determines the direction of the curvature of the base—Coty- ledons of Avena, curvature of basal part due to the illumination of upper part—S'milar results with the hypocotyls of Brassica and Beta—Radicles of Sinapis apheliotropic, due to tte sensitiveness of their tips—Concluding remarks and summary of chapter—Means by which cireumnutation 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 coty- ledons of some Graminez, for instance, those of Pha- laris, 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 ex- tremely 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 ne 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 apogeo- tropism 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 Dionza, 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 decom- posing 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 capreclata * According to F. Kurtz(‘Ver- tonia Californica are strongly handl. dvs Bot. Vereins der Pro- apheliotropic. We failed to detect vinz Brandenburg,’ Bd. xx. 1878) this movemcnt in a plant which the leaves or pitchers of Darling- we possessed for a short time.

and of Snulaz 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 in- vestigation, for it implies that heliotropism can be readily eliminated. When twining plants are exposed to a lateral light, their stems go on revolving or cir- cumnutating about the same spot, without any evident deflection towards the light; but we thought that we might detect some trace of heliotropism by com- paring 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 Ipomea cwxrulea and four of I. purpwrea, 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 2pm. on June 17th. After a few observations we concluded that we could safely estimate the time

* Some erroneous statements nvmber of observiitions, for we did are unfortunately given on this not then know at how unequal subject, in ‘The Movements and a rate the stems. and tendrils of Habits of Climbing Plants,’ 1875, climbing plants rometimes travel pp. 28, 82, 40, and 53. Conclusions in different parte of the same re- were drawn from an insufticient volution. 452 SENSITIVENESS TO LIGHT. Cise te taken by cach semicircle, within a limit of error of at most 5 minutes. Although the rate of movement im 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 semicirelos from the light each in 73°5 minutes. It may, there- fore, 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 semi- circle 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 pro- bable limit of error. During the three days of expos sure, 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 thar’ 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 teaderey. With Lonicera brachypoda the semicircles from and to the light differed considerably in time; for 5 semi- circles 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 circum- stances 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 Ipomea cxrulea 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 aphelio- tropic, 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 Tropwolum 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 apheliotropie, is connected with their habit of climbing.

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 circum- nutate largely and rapidly; and as heliotropism con- sists of modified cireumnutation, 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 helio- tropic, 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

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 com- pletely 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 dis- tance 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 dis- tinguish. 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 cir- cular 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 ; us 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 cove-ed * Strasburger says (© Wirkung Hamatococcus moved to a hght des Lichites auf Schwarmsporen, = which only just sufficed to allow 1878, p. 52), that the spores of middle-sized type to be read, by a tin-vessel, having on one side a circular hole 1:23 mm. in diameter (i.e. a little less than the th of an inch) ; and the box was placed in front of a paraffin Jamp 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.