The Power of Movement in Plants

either had proved decidedly advantageous or disad- vantageous, one of them no doubt would soon have prevailed. Asa Gray has described * the peculiar manner of ger- mination of three widely different plants, in which the hypocotyl is hardly at all developed. These were there- fore observed by us in relation to our present subject. Delphinium nudicaule—The elongated petioles of the two cotyledons are confluent (as are sometimes their blades at the base), and they break through the ground as an arch. They thus resemble in a most deceptive manner a hypocotyl. At first they are solid, but after a time become tubular ; and the basal part beneath the ground is enlarged into a hollow chamber, within which the young leaves are developed without any prominent plumule. Externally root- hairs are formed on the confluent petioles, either a little above, or on a level with, the plumule. The first leaf at an early period of its growth and whilst within the chamber is quite straight, but the petiole soon becomes arched; and the swelling of this part (and probably of the blade) splits open one side of the chamber, and the leaf then emerges. The slit was found in one case to be 3:2 mm. in length, and it is seated on the line of confluence of the two petioles. The leaf when it first escapes from the chamber is buried beneath the ground, and now an upper part of the petiole near the blade becomes arched in the usual manner. ‘The second leaf comes out of the slit either straight or somewhat arched, but afterwards the upper part of the petiole,—certainly in some, and we believe in all cases,—arches itself whilst forcing a passage through the soil.

Megarrhiza Californica—-The cotyledons of this Gourd never free themselves from the seed-coats and are hypogean. Their petioles are completely con- fluent, forming a tube which terminates downwards in a little solid point, consisting of a minute radicle and hypocotyl, with the likewise minute plumule enclosed within the base of the tube. This structure was well exhibited in an abnormal specimen, in which one of the two cotyledons failed to produce a petiole, whilst the other produced one consisting of an open semicylinder ending in a sharp point, formed of the parts just described. As soon as the confluent petioles protrude from the seed they bend down, as they are strongly geotropic, and penetrate the ground. The seed itself retains its original position, either on the surface or buried at some depth, as the case may be. If, however, the point of the confluent petioles meets with some obstacle in the soil, as appears to have occurred with the seedlings described and figured by Asa Gray,* the cotyledons are lifted up above the ground. The petioles are clothed with root-hairs like those on a true radicle, and they likewise resemble radicles in becoming brown when immersed in a solution of permanganate of potassium. Our seeds were subjected to a high temperature, and in the course of three or four days the petioles pene- trated the soil perpendicularly to a depth of from 2 to 23 inches; and not until then did the true radicle begin to grow. In one specimen which was closely observed, the petioles in 7 days after their first protrusion attained a length of 2} inches, and the radicle by this time had also become well developed. The plumule, still enclosed within the tube, was now

‘8 inch in length, and was quite straight; but from having increased in thickness it had just begun to split open the lower part of the petioles on one side, along the line of their confluence. By the following morning the upper part of the plumule had arched itself into a right angle, and the convex side or elbow had thus been forced out through the slit. Here then the arching of the plumule ‘plays the same part as in the case of the petioles of the Delphinium. As the plumule continued to grow, the tip became more arched, and in the course of six days it emerged through the 23 inches of superin- cumbeut soil, still retaining its arched form. After reaching the surface it straightened itself in the usual manner. In the accompany- ing figure (Fig. 58, A) we have a sketch of a seedling in this ad- vanced state of development; the ’ surface of the ground being re- Meqarrhiza Californica : presented by the line G-----.. G. Sere Food The germination of the seeds in - reduced to one-half their native Californian home pro- scale: @ cotyledons eds in a rather different manner,

within seed-coats 3 p, i : ve near oe as we infer from an interesting DeTIOIeSs 5 and 7, - Pit ne maids , letter from Mr. Rattan, sent to us pl, plumule; G....4, by Prof. Asa Gray. The petioles surface of Soil. protrude from the seeds soon after the autumnal rains, and penetrate the ground, generally in a vertical direction, to a depth of from 4 to even 6 inches. They were found in this state by Mr. mules still enclosed within the tubes ; and he remarks that if the plumules had been at once developed and had reached the surface (as occurred with our seeds which were exposed to a high temperature), they would surely have been killed by the frost. As it is they lie dormant at some depth beneath the surface, and are thus protected from the cold; and the root- hairs on the petioles would supply them with sufficient moisture. We shall hereafter see that many seedlings are protected from frost, but by a widely different process, namely, by being drawn beneath the surface by the contraction of their radicles. We may, how- ever, believe that the extraordinary manner of germi- nation of Megarrhiza has another and secondary advantage. ‘The radicle begins in a few weeks to enlarge into a little tuber, which then abounds with starch and is only slightly bitter. It would theretore be very liable to be devoured by animals, were it not protected by being buried whilst young and tender, at a depth of some inches beneath the surface. Ultimately it grows to a huge size.

Ipomea leptophylla—tIn most of the species of this genus the hypocotyl is well developed, and breaks through the ground as an arch. But the seeds of the present species in germinating behave like those of Megarrhiza, excepting that the elongated petioles of the cotyledons are not confluent. After they have protruded from the seed, they are united at their lower ends with the undeveloped hypocotyl and un- developed radicle, which together form a point only about *1 inch in length. They are at first highly geotropic, and penetrate the ground to a depth of rather above half an inch. The radicle then begins to grow. On four occasions after the petioles had grown for a short distance vertically downwards, they

were placed in a horizontal position in damp air in the dark, and in the course of 4 hours they again became curved vertically downwards, having passed through 90° in this time. But their sensitiveness to geotropism lasts for only 2,or 3 days; and the terminal part alone, for a length of between ‘2 and -4 inch, is thus sensitive. Although the petioles of our specimens did not penetrate the ground to a greater depth than about 4 inch, yet they continued for some time to grow rapidly, and finally attained the great length of about 3 inches. The upper part is apogeotropic, and there- fore grows vertically upwards, excepting a short portion close to the blades, which at an early period bends downwards and becomes arched, and thus breaks through the ground. Afterwards this portion straightens itself, and the cotyledons then free them- selves from the seed-coats. Thus we here have in different parts of the same organ widely different kinds of movement and of sensitiveness; for the basal part is geotropic, the upper part apogeotropic, and a portion near the blades temporarily and spontaneously arches itself. The plumule is not developed for some little time ; and as it rises between the bases of the parallel and closely approximate petioles of the cotyledons, which in breaking through the ground have formed an almost open passage, it does not require to be arched and is consequently always straight. Whether the plumule remains buried and dormant for a time in its native country, and is thus protected from the cold of winter, we do not know. The radicle, like that of the Megar- rhiza, grows into a tuber-like mass, which ultimately attains a great size. So it is with Ipomea pandurata, the germination of which, as Asa Gray informs us, resembles that of I. leptophyila.

the root-like nature of the petioles. The radicle of a seedling was cut off, as it was completely decayed, and the two now separated cotyledons were planted. They emitted roots from their bases, and continued green and healthy for two months. The blades of both then withered, and on removing the earth the bases of the petioles (instead of the radicle) were found enlarged into little tubers. Whether these would have had the power of producing two in- dependent plants in the following summer, we do not know.

In Quercus virens, according to Dr. Engelmann,* both the cotyledons and their petioles are confluent. The latter grow to a length “of an inch or even more ;” and, if we understand rightly, penetrate the ground, so that they must be geotropic. The nutri- ment within the cotyledons is then quickly transferred to the hypocotyl or radicle, which thus becomes developed into a fusiform tuber. The fact ot tubers being formed by the foregoing three widely distinct plants, makes us believe that their protection from animals at an early age and whilst tender, is one at least of the advantages gained by the remark- able elongation of the petioles of the cotyledons, together with their power of penetrating the ground like roots under the guidance of geotropism.

The following cases may be here given, as they bear on our present subject, though not relating to seed- lings. The flower-stem of the parasitic Lathrea sguamaria, which is destitute of true leaves, breaks through the ground as an arch;f so does the flower- * ¢Transact. St. Louis Acad. ground cannot fail to be greatly Science,’ vol. iv. p. 190. facilitated by the extraordinary + The passage of the flower- quantity of water secreted at. thia etem of the Lathrxa through the period of the year by the subter-

stem of the parasitic and leafless Monotropa hg ypoptt YS. With Helleborus niger, the flower-stems, which rise up independently of the leaves, likewise break through the ground as arches. This is also the case with the greatly elongated flower-stems, as well as with the petioles of Epimedium pinnatum. So it is with the petioles of Ranunculus ficaria, when they have to break through the ground, but when they arise from the summit of the bulb above ground, they are from the first quite straight ; and this is a fact which deserves notice. The rachis of the bracken fern (Pterts aqui- lina), and of some, probably many, other ferns, like- wise rises above ground under the form of an arch. No doubt other analogous instances could be found by careful search. In all ordinary cases of bulbs, rhizomes,

ranean scale-like leaves : not that there is any reason to suppose that the secretion is a special adaptation for this purpose: it probably follows from the ereat quantity of sap absorbed in the early spring by the parasitic roots, After a long period without any rain, the earth had become light- coloured and very dry, but it was dark coloured and damp, even in parts quite wet. for a distance of al least six inches all round cach flower-stem. The wateris secreted by glands (described by Cohn, ‘Bericht. Bot. Sect. der Schle- sisc::en Gesell.” 1876, p. 113) wlich line the longitudinal channels running through evch scale-like leaf. A large plant was dug up, washed so as to remove the earth, left for some time to drain, and then placed in the evening on a dry glass-plate, covered with a hel \l-vlass, and by next morning it hail seoreted a large pvol of water. The pl:te was wiped dry, and in the course of the succecding 7 or 8 hours

another little pool was secreted, and after 16 additional hours several large drops, A smaller plant was washed and placed in a large jar, which was left inclined for an hour, by which time no more water drained off. The jar was then placed upright and closed: after 23 hours twodrachms of water were collected from the hottom, and a little more after 25 additional hours. The flower- stems were now cut off, for they do not secrete, and the subter- ranean part of the plant was found tu weigh 106°8 grams (I6LL grains), and the water secr-ted during the 48. hours weighed 11°9 grams (183 grains),—that is, one-ninth of the whole weight of the plant, excluding the flower- stems. We should rememb:r that plants in a state of nature would probably secrete in 48 hours much more than the above large amount, for their routs would continue all the time absorbing sap from the plant on which they were para sitic.

root-stocks, &c., buried beneath the ground, the surface is broken by a cone formed by the young imbricated leaves, the combined growth of which gives them force sufficient for the purpose. With germinating monocotyledonous seeds, of * which, however, we did not observe a large number, the plumules, for instance, those of Asparagus and Canna, are straight whilst breaking through the ground. With the Graminez, the sheath-like cotyledons are likewise straight ; they, however, terminate in a sharp crest, which is white and somewhat indurated ; and this structure obviously facilitates their emergence from the soil: the first true leaves escape from the sheath through a slit beneath the chisel-like apex and at right angles to it. In the case of the onion (Allium cepa) we again meet with an arch; the leaf-like coty- ledon being abruptly bowed, when it breaks through the ground, with the apex still enclosed within the seed-coats. The crown of the arch, as previously described, is developed into a white conical pro- tuberance, which we may safely believe to be a special adaptation for this office.

The fact of so many organs of different kinds— hypocotyls and epicotyls, the petioles of some coty- ledons and of some first leaves, the cotyledons of the onion, the rachis of some ferns, and some flower- stems—being all arched whilst they break through the ground, shows how just are Dr. Haberlandt’s” remarks on the importance of the arch to seedling plants. He attributes its chief importance to the upper, young, and more tender parts of the hypocotyl

* «Die Schutzeinrichtunzen in though our observations lead 1% Aer Evutwickelung der Keim- to differ on some poiuts from the pilanze,’ 1877. We have learned author. much irom this interestiug essay, or epicotyl, being thus saved from abrasion and pressure whilst breaking through the ground. But we think that some importance may be attributed to the increased force gained by the hypocotyl, epicotyl, or other organ by being at first arched ; for both legs of the arch increase in length, and both have points of resistance as long as the tip remains enclosed within the seed-coats; and thus the crown of the arch is pushed up through the earth with twice as much force as that which a straight hypocotyl, &c., could exert.

all the work has to be done by the basal leg. In the case of the epicotyl of the common bean, the basal leg (the apex having freed itself from the seed- coats) grew upwards with a force sufficient to lift a thin plate of zinc, loaded with 12 ounces. Two more ounces were added, and the 14 ounces were lifted up to a very little height, and then the epicotyl yielded and bent to one side. With respect to the primary cause of the arching process, we long thought in the case of many seedlings that this might be attributed to the manner in which the hypocotyl or epicotyl was packed and curved within the seed-coats ; and that the arched shape thus acquired was merely retained until the parts in question reached the surface of the ground. But it is doubtful whether this is the whole of the truth in any case. For instance, with the common bean, the epicotyl or plumule is bowed into an arch whilst breaking through the seed-coats, as shown in Fig. 59 (p. 92). The plumule first frotrudes as a solid knob (e in A), which after twenty-four hours’ growth is seen (e in B) to be the crown of an arch. Nevertheless, with several beans which germinated in damp air, and had other. wise been treated in an unnatural manner, little

plumules were developed in the axils of the petioles of both cotyledons, and these were as perfectly arched as the normal plumule; yet they had not been sub- jected to any confinement or pressure, for the seed- coats were completely ruptured, and they grew in the open air. This proves that the plumule has an innate or spontaneous tendency to arch itself. In some other cases the hypocotyl or epicotyl pro- trudes from the seed at first only slightly bowed; but the bowing afterwards increases independently of any constraint. The arch is thus made narrow, with the two legs, which are sometimes much elongated, parallel and close together, and thus it becomes well fitted for breaking through the ground.

With many kinds of plants, the radicle, whilst still enclosed within the seed and likewise after its first pro- trusion, lies in a straight line with the future hypocotyl and with the longitudinal axis of the cotyledons. This is the case with Cucurbita ovifera; nevertheless, in whatever position the seeds were buried, the hypocotyl always came up arched in one particular direction. Seeds were planted in friable peat at a depth of about an inch in a vertical position, with the end from which the radicle protrudes downwards. Therefore all the parts occupied the same relative positions which they would ultimately hold after the seedlings had risen clear above the surface. Notwithstanding this fact, the hypocotyl arched itself; and as the arch grew upwards through the peat, the buried seeds were turned either upside down, or were laid horizontally, being afterwards dragged above the ground. Ulti- mately the hypocotyl straightened itself in the usual manner; and now after all these movements the several parts occupied the same position relatively to one another and to the centre of the earth, which they

had done when the seeds were first buried. But it may be argued in this and other such cases that, as the hypocotyl grows up through the soil, the seed wll almost certainly be tilted to one side; and ther from the resistance which it must offer during its further elevation, the upper part of the hypocotyl will be doubled down and thus become arched. This view seems the more probable, because with Ranunculus ficaria only the petioles of the leaves which forced a passage through the earth were arched; and not those which arose from the summits of the bulbs above the ground. Nevertheless, this explanation does not apply to the Cucurbita, for when germinating seeds were suspended in damp air in various positions by pins passing through the cotyledons, fixed to the inside of the lids of jars, in which case the hypo- cotyls were not subjected to any friction or constraint, yet the upper part became spontaneously arched. This fact, moreover, proves that it is not the weight of the cotyledons which causes the arching. Seeds of Helianthus annuus and of two species of Ipomca (those of I. bona noe being for the genus large and heavy) were pinned in the same manner, and the hypocotyls became spontaneously arched ; the racdicles, which had been vertically dependent, assumed in consequence a horizontal position. In the case of Ipomea leptophylla it is the petioles of the cotyledons which become arched whilst rising through the ground; and this occurred spontaneously when the seeds were fixed to the lids of jars.

It may, however, be suggested with some degree of probability that the arching was aboriginally caused by mechanical compulsion, owing to the confinement of the parts in question within the seed-coats, or to friction whilst they were being dragged upwards. But if this is so, we must admit from the cases just given, that a tendency in the upper part of the several specified organs to bend downwards and thus to be- come arched, has now become with many plants firmly inherited. The arching, to whatever cause it may be due, is the result of modified circumnutation, through increased growth along the convex side of the part; such growth being only temporary, for the part always straightens itself subsequently by increascd growth along the concave side, as will hereafter be described.

It is a curious fact that the hypocotyls of some plants, which are but little developed and which never raise their cotyledons above the ground, never- theless inherit a slight tendency to arch themselves, although this movement is not of the least use to them. We refer to a movement observed by Sachs in the hypocotyls of the bean and some other Legumi- nosz, and which is shown in the accompanying figure (Fig. 59), copied from his Essay.* The hypocotyl and radicle at first grow perpendicularly downwards, as at A, and then bend, often in the course of 24 hours, into the position shown at B. As we shall here- after often have to recur to this movement, we will, for brevity sake, call it “Sachs’ curvature.” At first sight it might be thought that the altered position of the radicle in B was wholly due to the outgrowth of the epicotyl (e), the petiole (p) serving as a hinge; and it is probable that this is partly the cause; but the hypocotyl and upper part of the radicle themselves become slightly curved.

The above movement in the bean was repeatedly seen by us; but our observations were made chiefly on Phaseolus multiflorus, the cotyledons of which are like- wise hypogean. Some seedlings with well-developed radicles were first immersed in a solution of perman- ganate of potassium; and, judging from the changes of colour (though these were not very clearly defined), the hypocotyl is about 3 inch in length. Straight, thin, black lines of this length were now drawn from the bases of the short petioles along the hypocotyls

Vieta faba: germinating sceds, suspended in damp air: A, with radicle growing perpendicularly downwards; B, the same bean after 24 hours and after the radicle has curved itself; 7, radicle; A, short hypocotyl ; ¢, epicotyi appearing asa knob in A and as an arch in B; ap, petiole of the cotyledon, the latter enclosed within the seed-coats, of 23 germinating seeds, which were pinned to the lids of jars, generally with the hilum downwards, and with their radicles pointing to the centre of the earth. After an interval of from 24 to 48 hours the black lines on the hypocotyls of 16 out of the 23 seedlings became distinctly curved, but in very various degiees (namely, with radii between 20 and

8) mm. on Sachs’ cyclometer) in the same relative direction as shown at B in Fig. 59. As geotropism will obviously tend to check this curvature, seven seeds were allowed to germinate with proper pre- cautions for their growth in a klinostat,* by which means geotropism was eliminated. The position of the hypocotyls was observed during four successive days, and they continued to bend towards the hilum and lower surface of the seed. On the fourth day they were deflected by an average angle of 63° from a ling perpendicular to the lower surface, and were therefore considerably more curved than the hypocotyl and radicle in the bean at B (Fig. 59), though in the same relative direction.

It will, we presume, be admitted that all leguminous plants with hypogean cotyledons are descended from forms which once raised their cotyledons above the ground in the ordinary manner; and in doing so, it is certain that their hypocotyls would have been abruptly arched, as in the case of every other dicotyledonous plant. This is especially clear in the case of Phaseolus, for out of five species, the seedlings of which we observed, namely, P. multiflorys, caracalla, vulgaris, Hernandesii and Roaxburghit (inhabitants of the Old and New Worlds), the three last-named species have well-developed hypocotyls which break through the ground as arches. Now, if we imagine a seedling of the common bean or of P. multzflorus, to behave as its progenitors once did, the hypocotyl (A, Fig. 59), in whatever position the,seed may have been buried, would become so much arched that the upper part would be doubled down parallel to the lower part; and

* An instrument devised by on which the plant under obscrva- Ssc.s, consisting essentially of a tion 1s sunported : see * Wiirzburg slowly revolving borizontal axis. Arbeiten,’ 1879, p. 200. this is exactly the kind of curvature which actually occurs in these two plants, though, to a much less degree. Therefore we can hardly doubt that their short hypocotyls have retained Jy inheritance a ten- dency to curve themselves in the same manner as they did at a former period, when this movement was highly important to them for breaking through the ground, though now rendered useless by the cotyledons being hypogean. Judimentary structures are in most cases highly variable, and we might expect that rudimentary or obsolete actions would be equally so; and Sachs’ curvature varies extremely in amount, and sometimes altogether fails. This is the sole instance known to us of the inheritance, though in a feeble degree, of movements which have become superfluous from changes which the species has undergone.

Rudimentary Cotyledons—A few remarks on this subject may be here interpolated. It is well known that some dicotyle- donous plants produce only a single cotyle- don; for instance, cer- tain species of Ranun- culus, Corydalis, Che- rophyllum; and we will here endeavour to show that the loss of one or both cotyle- dons is apparently due Vitrus aurantium: two young seedlings: to a store of nutri- ce, larger cotyledon; ¢’, smaller cotyle- é A ‘ don; A, thickened hypocotyl; r,radicle. ment being laid up in In A the epicotyl is still arched, in B it some other part, as in has become erect. ee the hypocotyl or one of the two cotyledons, or one of the secondary radicles

With the orange (Citrus awrantium) the cotyledons are hypogean, and one is larger than the other, as may be seen in A (Fig. 60). In B the inequality is rather greater, and the stem has grown between the points of insertion of the two petioles, so that they do not stand opposite to one another; in another case the separation amounted to one-fifth of an inch. The smaller cotyledon of one seedling was extremely thin, and not half the length of the larger one, so that it was clearly becoming rudimen- tary.* In all these seedlings the hypocotyl was enlarged or swollen. With Abronia wmbellata one of the cotyledons is quite rudimen- tary, as may be seen (c’) in Fig. 61. In this specimen it consisted of a little green flap, ~,th inch in length, destitute of a petiole and covered with glands like those on the fully developed cotyledon (e). At first it stood opposite to the larger cotyledon ; but as the petiole of the latter increased in length and grew in the same line with the hypocotyl (h), the rudiment appeared in older seedlings as if mls % ™dile. seated some way down the hypocotyl. With Abronia arenarva there is a similar rudiment, which in one

Abronia umbelluta. seed- ling twice natural size: c, cotyledon; oc’, rudi- mentary cotyledon ; A, enlarged hypocotyl, with a heel or projec- tion (4') at the lower * In Pachira aquatica, as de- ecribed by Mr. R. I. Lynch (‘Journal Linn. Soc. Bot.’ vol. xvii. 1878, p. 147), one of the hypogean cotyledons is of im- wense size; the other is small and soon falls off; the pair de not siways stand opposite. In anctiier and very different water-plant, Trapa nutans, one of the cotyle- dous, filled with farinaceous natter, is much larger than the other, which is seurcely visible, as is stated by Aug. de Cundolle, ‘Physiologie Vég.’ tom. ii. p. 834,

apecimen was only ;1,th and in another ;j,th inch in length; it ultimately appeared as if seated halfway down the hypocotyl. In both these species the hypo- cotyl is so much enlarged, especially at a very early age, that it might almost be called a corm. The lower end forms a heel or projection, the use of which will hereafter be described. In Cyclamen Persicum the hypocotyl, even whilst still within the seed, is enlarged into a regular corm,* and only a single cotyledon is at first developed (see former Fig. 57.) With Ranunculus ficaria two cotyledons are never produced, and here one of the secondary radicles is developed at an early age into a so-called bulb.t Again, certain species of Cherophyllum and Corydalis produce only a single cotyledon ;{ in the former the hypocotyl, and in the latter the radicle is enlarged, according to Irmisch, into a bulb.