The figure, which was described during 10 h., apparently represents two irregular and unequal ellipses or circles. The actual amount of movement of the apex, in the line not influenced by the light, was .11 of an inch, and in that thus influenced .37 of an inch. With a seedling 2 inches in height it was obvious, even without the aid of any tracing, that the uppermost part of the stem bent successively to all points of the compa.s.s, like the stem of a twining plant. A little increase in the power of circ.u.mnutating and in the flexibility of the stem, would convert the common asparagus into a twining plant, as has occurred with one species in this genus, namely, A.
scandens.
Phalaris Canariensis (Gramineae).--With the Gramineae the part which first rises above the ground has been called by some authors the pileole; and various views have been expressed on its h.o.m.ological nature. It is considered by some great authorities to be a cotyledon, which term we will use without venturing to express any opinion on the subject.* It consists in the present case of a slightly flattened reddish sheath, terminating upwards in a sharp white edge; it encloses a true green leaf, which protrudes from the sheath through a slit-like orifice, close beneath and at right angles to the sharp edge on the summit. The sheath is not arched when it breaks through the ground.
The movements of three rather old seedlings, about 1 inch in height, shortly before the protrusion of the leaves, were first traced. They were illuminated exclusively from above; for, as will hereafter be shown, they are excessively sensitive to the * We are indebted to the Rev. G. Henslow for an abstract of the views which have been held on this subject, together with references.
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action of light; and if any enters even temporarily on one side, they merely bend to this side in slightly zigzag lines. Of the three tracings one alone (Fig. 49) is here given. Had the observations been more frequent during the 12 h. two oval figures would have been described with their longer axes at right angles to one another. The actual amount of movement of the apex from side to side was about .3 of an inch. The figures described by the other two seedlings resembled to a certain extent the one here given.
Fig. 49. Phalaris Canariensis: circ.u.mnutation of a cotyledon, with a mark placed below the apex, traced on a horizontal gla.s.s, from 8.35 A.M. Nov.
26th to 8.45 A.M. 27th. Movement of apex magnified 7 times, here reduced to one-half scale.
A seedling which had just broken through the ground and projected only 1/20th of an inch above the surface, was next observed in the same manner as before. It was necessary to clear away the earth all round the seedling to a little depth in order to place a mark beneath the apex. The figure (Fig. 50) shows that the apex moved to one side, but changed its course ten times in the course of the ten hours of observation; so that there can be no doubt about its circ.u.mnutation. The cause of the general movement in one direction could hardly be attributed to the entrance of lateral light, as this was carefully guarded against; and we suppose it was in some manner connected with the removal of the earth round the little seedling.
Fig. 50. Phalaris Canariensis: circ.u.mnutation of a very young cotyledon, with a mark placed below the apex, traced on a horizontal gla.s.s, from 11.37 A.M. to 9.30 P.M. Dec. 13th. Movement of apex greatly magnified, here reduced to one-fourth of original scale.
Lastly, the soil in the same pot was searched with the aid of a lens, and the white knife-like apex of a seedling was found on an exact level with that of the surrounding surface. The soil was removed all round the apex to the depth of a quarter of an inch, the seed itself remaining covered. The pot, protected from lateral light, was placed under the micro- [page 64]
scope with a micrometer eye-piece, so arranged that each division equalled 1/500th of an inch. After an interval of 30 m. the apex was observed, and it was seen to cross a little obliquely two divisions of the micrometer in 9 m. 15 s.; and after a few minutes it crossed the same s.p.a.ce in 8 m. 50s.
The seedling was again observed after an interval of three-quarters of an hour, and now the apex crossed rather obliquely two divisions in 10 m. We may therefore conclude that it was travelling at about the rate of 1/50th of an inch in 45 minutes. We may also conclude from these and the previous observations, that the seedlings of Phalaris in breaking through the surface of the soil circ.u.mnutate as much as the surrounding pressure will permit. This fact accounts (as in the case before given of the asparagus) for a circular, narrow, open s.p.a.ce or crack being distinctly visible round several seedlings which had risen through very fine argillaceous sand, kept uniformly damp.
Fig. 51. Zea mays: circ.u.mnutation of cotyledon, traced on horizontal gla.s.s, from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead magnified on an average about 25 times.
Zea mays (Gramineae).--A gla.s.s filament was fixed obliquely to the summit of a cotyledon, rising .2 of an inch above the ground; but by the third morning it had grown to exactly thrice this height, so that the distance of the bead from the mark below was greatly increased, consequently the tracing (Fig. 51) was much more magnified on the first than on the second day. The upper part of the cotyledon changed its course by at least as much as a rectangle six times on each of the two days. The plant was illuminated by an obscure light from vertically above. This was a necessary precaution, as on the previous day we had traced the movements of cotyledons placed in a deep box, the inner side of which was feebly illuminated on one side from a distant north-east window, and at each observation by a wax taper held for a minute or two on the same side; and the result was that the cotyledons travelled all day long to this side, though making in their course some conspicuous flexures, from which fact alone we might have [page 65]
concluded that they were circ.u.mnutating; but we thought it advisable to make the tracing above given.
Radicles.--Gla.s.s filaments were fixed to two short radicles, placed so as to stand almost upright, and whilst bending downwards through geotropism their courses were strongly zigzag; from this latter circ.u.mstance circ.u.mnutation might have been inferred, had not their tips become slightly withered after the first 24 h., though they were watered and the air kept very damp. Nine radicles were next arranged in the manner formerly described, so that in growing downwards they left tracks on smoked gla.s.s-plates, inclined at various angles between 45o and 80o beneath the horizon. Almost every one of these tracks offered evidence in their greater or less breadth in different parts, or in little bridges of soot being left, that the apex had come alternately into more and less close contact with the gla.s.s. In the accompanying figure (Fig. 52) we have an accurate copy of one such track. In two instances alone (and in these the plates were highly inclined) there was some evidence of slight lateral movement.
We presume therefore that the friction of the apex on the smoked surface, little as this could have been, sufficed to check the movement from side to side of these delicate radicles.
Fig. 52. Zea mays: track left on inclined smoked gla.s.s-plate by tip of radicle in growing downwards.
Avena sativa (Gramineae).--A cotyledon, 1 inch in height, was placed in front of a north-east window, and the movement of the apex was traced on a horizontal gla.s.s during two days. It moved towards the light in a slightly zigzag line from 9 to 11.30 A.M. on October 15th; it then moved a little backwards and zigzagged much until 5 P.M., after which hour, and curing the night, it continued to move towards the window. On the following morning the same movement was continued in a nearly straight line until 12.40 P.M., when the sky remained until 2.35 extraordinarily dark from thunder-clouds.
During this interval of 1 h. 55 m., whilst the light was obscure, it was interesting to observe how circ.u.mnutation overcame heliotropism, for the apex, instead of continuing to move towards the window in a slightly zigzag line, reversed its course four times, making two small narrow ellipses. A diagram of this case will be given in the chapter on Heliotropism.
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A filament was next fixed to a cotyledon only 1/4 of an inch in height, which was illuminated exclusively from above, and as it was kept in a warm greenhouse, it grew rapidly; and now there could be no doubt about its circ.u.mnutation, for it described a figure of 8 as well as two small ellipses in 5 hours.
Nephrodium molle (Filices).--A seedling fern of this species came up by chance in a flowerpot near its parent. The frond, as yet only slightly lobed, was only .16 of an inch in length and .2 in breadth, and was supported on a rachis as fine as a hair and .23 of an inch in height. A very thin gla.s.s filament, which projected for a length of .36 of an inch, was fixed to the end of the frond. The movement was so highly magnified that the figure (Fig. 53) cannot be fully trusted; but the frond was constantly moving in a complex manner, and the bead greatly changed its course eighteen times in the 12 hours of observation. Within half an hour it often returned in a line almost parallel to its former course. The greatest amount of movement occurred between 4 and 6 P.M. The circ.u.mnutation of this plant is interesting, because the species in the genus LyG.o.dium are well known to circ.u.mnutate conspicuously and to twine round any neighbouring object.
Fig. 53. Nephrodium molle: circ.u.mnutation of very young frond, traced in darkness on horizontal gla.s.s, from 9 A.M. to 9 P.M. Oct. 30th. Movement of bead magnified 48 times.
Selaginella Kraussii (?) (Lycopodiaceae).--A very young plant, only .4 of an inch in height, had sprung up in a pot in the hot-house. An extremely fine gla.s.s filament was fixed to the end of the frond-like stem, and the movement of the bead traced on a horizontal gla.s.s. It changed its course several times, as shown in Fig. 54, whilst observed during 13 h. 15 m., and returned at night to a point not far distant from that whence it had started in the morning. There can be no doubt that this little plant circ.u.mnutated.
Fig. 54. Selaginella Kraussii (?): circ.u.mnutation of young plant, kept in darkness, traced from 8.45 A.M. to 10 P.M. Oct. 31st.
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CHAPTER II.
GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
Generality of the circ.u.mnutating movement--Radicles, their circ.u.mnutation of service--Manner in which they penetrate the ground--Manner in which hypocotyls and other organs break through the ground by being arched-- Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons- -Circ.u.mnutation of hypocotyls and epicotyls whilst still buried and arched- -Their power of straightening themselves--Bursting of the seed-coats-- Inherited effect of the arching process in hypogean hypocotyls-- Circ.u.mnutation of hypocotyls and epicotyls when erect--Circ.u.mnutation of cotyledons--Pulvini or joints of cotyledons, duration of their activity, rudimentary in Oxalis corniculata, their development--Sensitiveness of cotyledons to light and consequent disturbance of their periodic movements- -Sensitiveness of cotyledons to contact.
THE circ.u.mnutating movements of the several parts or organs of a considerable number of seedling plants have been described in the last chapter. A list is here appended of the Families, Cohorts, Sub-cla.s.ses, etc., to which they belong, arranged and numbered according to the cla.s.sification adopted by Hooker.* Any one who will consider this list will see that the young plants selected for observation, fairly represent the whole vegetable series excepting the lowest cryptogams, and the movements of some of the latter when mature will hereafter be described. As all the seedlings which were observed, including Conifers, Cycads and Ferns, which belong to the most ancient
* As given in the "General System of Botany," by Le Maout and Decaisne, 1873.
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types amongst plants, were continually circ.u.mnutating, we may infer that this kind of movement is common to every seedling species.
SUB-KINGDOM I.--Phaenogamous Plants.
Cla.s.s I.--DICOTYLEDONS.
Sub-cla.s.s I.--Angiosperms.
Family. Cohort.
14. Cruciferae. II. PARIETALES.
26. Caryophylleae. IV. CARYOPHYLLALES.
36. Malvaceae. VI MALVALES.
41. Oxalideae. VII. GERANIALES.
49. Tropaeoleae. DITTO 52. Aurantiaceae. DITTO 70. Hippocastaneae. X. SAPINDALES.
75. Leguminosae. XI. ROSALES.
106. Cucurbitaceae. XII. Pa.s.sIFLORALES.
109. Cacteae. XIV. FICOIDALES.
122. Compositae. XVII. ASTRALES.
135. Primulaceae. XX. PRIMULALES.
145. Asclepiadeae. XXII. GENTIa.n.a.lES.
151. Convolvulaceae. XXIII. POLEMONIALES.
154. Boragineae. DITTO 156. Nolaneae. DITTO 157. Solaneae. XXIV. SOLa.n.a.lES.
181. Chenopodieae. XXVII. CHENOPODIALES.
202. Euphorbiaceae. x.x.xII. EUPHORBIALES.
211. Cupuliferae. x.x.xVI. QUERNALES.
212. Corylaceae. DITTO
Sub-cla.s.s II.--Gymnosperms.
223. Coniferae.
224. Cycadeae.
Cla.s.s II.--MONOCOTYLEDONS.
2. Cannaceae. II. AMOMALES.
34. Liliaceae. XI. LILIALES.
41. Asparageae. DITTO 55. Gramineae. XV. GLUMALES.
SUB-KINGDOM II.--Cryptogamic Plants.
1. Filices. I. FILICALES.
6. Lycopodiaceae. DITTO [page 69]
Radicles.--In all the germinating seeds observed by us, the first change is the protrusion of the radicle, which immediately bends downwards and endeavours to penetrate the ground. In order to effect this, it is almost necessary that the seed should be pressed down so as to offer some resistance, unless indeed the soil is extremely loose; for otherwise the seed is lifted up, instead of the radicle penetrating the surface. But seeds often get covered by earth thrown up by burrowing quadrupeds or scratching birds, by the castings of earth-worms, by heaps of excrement, the decaying branches of trees, etc., and will thus be pressed down; and they must often fall into cracks when the ground is dry, or into holes.