Moquin-Tandon[353] alludes to a case of this kind in a species of pine (_Pinus_), in which a branch ended in four unequal divisions, which were strongly curved from without inwards, then became united in pairs, these latter in their turn blending into a single ma.s.s.

In the case of some beeches growing in the forest of Verzy, near Rheims, the trunks of the trees are contorted in every direction, and, at a height of from fifteen to twenty feet, a number of branches are also given off, also much contorted, and occasionally intergrafted, so that it seems as if a heavy weight had been placed on the trees and literally flattened them. Similar malformations may occasionally be met with in the branches of the oak, and commonly in the weeping ash.

M. Fournier[354] mentions the stems of _Ruscus aculeatus_ rolled in a circle, others twisted spirally.

The phenomenon is not confined to woody plants, but has been met with in chicory, in _Antirrhinum_, and other herbaceous species.

It is very difficult in some cases to separate these instances of irregular torsion from those in which the twisting takes place in a more or less regular spiral direction. In the former case the fibres of the plant are only indirectly involved, but in the latter the fibres themselves are coiled spirally from right to left, or _vice versa_ (spiral torsion), while not unfrequently both conditions may be met with at the same time.

The leaves also are subject to similar deformities, of which a notable ill.u.s.tration has been recorded in the case of the date palm, _Phoenix dactylifera_, originally observed by Goethe, and figured and described by Jaeger;[355] the leaves are folded and twisted in every direction, in consequence of the fibrous band or cord which surrounds the leaves, and which generally breaks as the leaflets increase in size, remaining from some cause or other unbroken, and thus serving to restrain the growth.

A similar irregularity of growth occurs, not unfrequently, in the case of crocus leaves, when in the course of their growth, as they push their way through the soil, their progress becomes checked either by a stone or even by frost.

=Spiral torsion.=--Growth in a spiral direction, and the arrangement of the various organs of the plant in a spiral manner, are among the most common of natural phenomena in plants.[356] Fibres are coiled spirally in the minute vessels of flowering plants, and are not wholly wanting even among fungi. The leaf-organs are very generally spirally arranged; the leaf-stalks are often so twisted as to bring leaves on one plane which otherwise would occupy several. In the leaf itself we have a spiral twist taking place constantly in _Alstroemeria_, in _Avena_, and other plants. A similar tendency is manifested in the flower-stalks, as in _Cyclamen_ and _Vallisneria_, and the whole inflorescence, as in _Spiranthes_. Even the bark and wood of trees is often disposed spirally. This is very noticeable in some firs, and in the bark of the sweet chestnut (_Castanea_), of _Thuja occidentalis_, and other trees.

The knaurs or excrescences which are sometimes found on the roots or stems of trees afford other ill.u.s.trations of this universal tendency.

These bodies consist of a number of embryo buds, which, from some cause or other, are incapable of lengthening. On examination every rudimentary or undeveloped bud may be seen to be surrounded by densely crowded fibres arranged spirally.

The axes of nearly all twining plants are themselves twisted, and twisted in a direction corresponding to the spontaneous revolving movement exhibited by these plants, as in the hop, the convolvulus, pa.s.sion flower, &c., the degree of twisting being dependent to a great extent on the roughness of the surface around which the stem twines[357].

Considered as an exceptional occurrence, it occurs frequently in certain plants, and, when it affects the stem or branches, necessarily causes some changes in the arrangement of the parts attached to them; thus, spiral torsion of the axial organs is generally accompanied by displacement of the leaves, whorled leaves becoming alternate, and opposite or whorled leaves becoming arranged on one side of the stem only. Frequently also this condition is a.s.sociated with fasciation, or, at least, with a distended or dilated state. An ill.u.s.tration of this in _Asparagus_ has been figured at p. 14.

Very often the leaves are produced in a spiral line round the stem, as in a specimen of _Dracocephalum speciosum_ described and figured by C.

Morren. The leaves of this plant are naturally rectiserial and decussate, but, in the twisted stem the leaves were curviserial, and arranged according to the 5/13 plan. Now, referring to the ordinary notation of alternate leaves, we shall have the first leaf covered by the fifth, with two turns of the spiral; since decussate leaves result from two conjugate lines, the formula will be necessarily 2/5. The fraction 5/13 hence comes regularly into the 2/5 series (2/5, 3/8, 5/13). Thus, the leaves in a.s.suming a new phyllotaxy, take one quite a.n.a.logous to the normal one.

One of the most curious instances that have fallen under the writer's own observation occurred in the stem of _Dipsacus fullonum_. (See 'Proceedings of the Linnean Society,' March 6, 1855, vol. ii, p. 370).

The stem was distended, and hollow, and twisted on itself; its fibres, moreover, were arranged in an oblique or spiral direction; the branches or leaf-stalks, which usually are arranged in an opposite and decussate manner, were, in this case, disposed in a linear series, one over the other, following the line of curvature of the stem. When the course of the fibres was traced from the base of one of the stalks, upward around the stem, a spiral was found to be completed at the base of the second stalk, above that which was made the starting point. Now, if opposite leaves depend on the shortened condition of the internode between the two leaves, then, in the teazel-stem just described, each turn of the spiral would represent a lengthened internode; and, if the fibres of this specimen could be untwisted, and made to a.s.sume the vertical direction, and, at the same time, the internodes were shortened, the result would be the opposition of the branches and the decussation of the pairs; this explanation is borne out by the similar twisting which takes place so frequently in the species of _Galium_ and other _Rubiaceae_.

[Ill.u.s.tration: FIG. 172.--Twisted stem of _Dipsacus fullonum_.]

G. Franc[358] was one of the first to notice this twisting in _Galium_, and M. Duchartre,[359] in mentioning a similar instance, gives the following explanation of the appearance which will be found to apply to most of these cases. In the normal stem of _Galium Mollugo_ the branches are opposite in each verticil and crossed in the two successive ones. The stem is four-angled, each angle having a nerve.

Each of these nerves, springing from the origin of a branch in one whorl, terminates in the interval which separates the point of origin of the two branches in the whorl next above it. In the deformed stem one of the nerves corresponds to the insertion of a branch, its neighbour is in the adjoining vacant s.p.a.ce; hence it results that four nerves correspond to two branches and to two consecutive inters.p.a.ces, and hence the a.n.a.logy between a single normal internode provided with its two branches and its four nerves. What confirms this inference is that the nerve, which begins at the point of origin of a branch, after making one spiral turn round the stem, terminates in the interval that separates the two following branches, just as in a branch of the normal stem it ends in the upper whorl between the two next branches. The torsion, then, in this _Galium_ caused the separation of the two opposite branches of the same verticil, and placed them one above another, and this being reproduced in all the whorls, all the branches come to be arranged on the same longitudinal line. The leaves are susceptible of the same explanation; they are inserted in groups of three or four in one arc round the origin of each branch. In the malformation each series or group of four leaves, with its central branch, is equivalent to half a whorl of the natural plant with its axillary branch. In other words, the malformation consists in a torsion of the stem, which separates each whorl into two distinct halves; these half-whorls, with their axillary branches, are placed on a single longitudinal series one above another.

This case is quoted at some length, as it is an admirable example of a very common form of malformation in these plants.

In some parts of Holland where madder is cultivated a similar deformation is particularly frequent. The leaves, however, are not always grouped in the way in which they were described by M. Duchartre, but more commonly form a single continuous line; when arranged in leaf-whorls it generally happens that some of the leaves are turned downwards, while others are erect. It has been said that this condition occurs particularly frequently in plants growing in damp places. It is certainly true that spiral torsion of the stem is specially frequent in the species of _Equisetum_, most of which grow in such spots. In these plants either the whole of the upper part of the stem is thus twisted, or a portion only: thus Reinsch[360] cites a case in _Equisetum Telmateia_, where the upper and lower portions of the stem were normal, while the intermediate portion was twisted spirally. In this instance the whorl next beneath the spiral had twenty-eight branchlets, and that immediately above it thirty. Along the course of the spire there were two hundred and three; dividing this latter number by the mean of the two preceding, it was seen that the spire included the const.i.tuents of seven ordinary verticils.

[Ill.u.s.tration: FIG. 173.--Stem of _Galium_ spirally twisted. From a specimen communicated by Mr. Darwin.]

Here also may be mentioned a curious bamboo, the stem of which is preserved in the British Museum, and in which the internodes, on the exterior, and the corresponding diaphragms and cavities within are spiral or oblique in direction.

The root is also subject to the same malformation, the inducing cause being usually some obstruction to downward growth, as when a plant has been grown in a small pot, and becomes, as gardeners say, pot-bound.

[Ill.u.s.tration: FIG. 174.--Showing "pot-bound" root twisted spirally (from the 'Gard. Chron.,' 1849).]

The axial portion of the flower, the thalamus, is also occasionally twisted in a spiral direction, the lateral parts of the flower being in consequence displaced. Morren spoke of this displacement of the floral organs as "speiranthie."[361]

Morren draws a distinction between spiral-torsion or spiralism and the less regular torsion spoken of in the preceding section; in the former case not only is the axis twisted, but its const.i.tuent fibres also. The condition in question in some cases seems to be inherited in the seedling plants.

The following is a list of the plants in which spiral torsion of the stem or branches has been most frequently observed. (See also under Fasciation and Contortion.)

Hesperis matronalis.

Dianthus barbatus.

Pyrus Malus.

torminalis.

Cercis siliquastrum!

Punica Granatum.

Robinia pseudacacia!

Rubia tinctorum.

Dipsacus fullonum!

pilosus.

Gmelini.

Scabiosa arvensis.

*Valeriana officinalis!

dioica!

Galium aparine!

* Mollugo!

verum!

Hippuris vulgaris!

Veronica spicata.

longifolia.

Hyssopus officinalis.

Thymus Serpyllum.

Lamium purpureum!

Dracocephalum speciosum.

Mentha aquatica.

Mentha viridis.

Fraxinus vulgaris!

Sambucus nigra.

Zinnia.

Phylica.

Beta.

Rumex, sp.

Ulmus campestris.

Casuarina rigida.

Abies excelsa!

Lilium Martagon!

candidum.

*Asparagus officinalis!

Sagittaria sagittifolia.

Epipactis pal.u.s.tris.

Tritic.u.m repens!

Lolium perenne!

Phleum pratense.

Juncus conglomeratus!

Scirpus lacustris.

Equisetum Telmateia.

limosum.

fluviatile.

arvense!