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Following Augustin Pyranius De Candolle, botanists have applied the term cohesion to the coalescence of parts of the same organ or of members of the same whorl; for instance, to the union of the sepals in a gamosepalous calyx, or of the petals in a gamopetalous corolla. It may arise either from a union between organs originally distinct, or more frequently from a want of separation between parts, which under general circumstances become divided during their development. Nothing is more common as a normal occurrence, while viewed as a teratological phenomenon it is also very frequent. For the purposes of convenience it admits of subdivision into those cases wherein the union takes place between the branches of the same plant, or between the margins of the same leaf-organ, or between those of different members of the same whorl.

Cohesion between the axes of the same plant.—This cohesion may occur in various manners. Firstly. The branches of the main stem may become united one to the other. Secondly. Two or more stems become joined together. Thirdly. The branches become united to the stem; or, lastly, the roots may become fused one with another.

Fig. 2—Cohesion of two branches in Dipsacus sylvestris.

The first of these is most commonly met with, doubtless owing to the number of the branches and the facilities for their union. An illustration of it is afforded by the figure (fig. 2), showing cohesion affecting the branches of a teazle (Dipsacus sylvestris). Union of the branches may be the result of an original cohesion of the buds, while in other cases the fusion does not take place until after development has proceeded to some extent. Of this latter kind illustrations are common where the branches are in close approximation; if the bark be removed by friction the two surfaces are very likely to become united (natural grafting). Such a union of the branches is very common in the ivy, the elder, the beech, and other plants. It may take place in various directions, lengthwise, obliquely, or transversely, according to circumstances. This mode of union belongs, perhaps, rather to the domain of pathology than of teratology. Some of the instances that have been recorded of very large trees, such as the chestnut of Mount Ætna, are really cases where fusion has taken place between several of the branches, or suckers, thrown out from the same original stem.[10] The same process of grafting occurs sometimes in the roots, as in Taxus baccata mentioned by Moquin, and also in the aerial roots of many of the tropical climbing plants, such as Clusia rosea, &c.

Fig. 3.—Fasciation in Lettuce.

Fasciation.—In the preceding instances of union between the branches, &c., the actual number of the fused parts is not increased; but if it happen that an unusual number of buds be formed in close apposition, so that they are liable to be compressed during their growth, union is very likely to take place, the more so from the softness of the young tissues. In this way it is probable that what is termed fasciation is brought about. This is one of the most common of all malformations, and seems to affect certain plants more frequently than others. In its simplest form it consists of a flat, ribbon-like expansion of the stem or branch; cylindrical below, the branches gradually lose their pristine form, and assume the flattened condition.

Fig. 4.—Fasciation in Asparagus.

Fig. 5.—Fasciated branch of Pinus Pinaster.

Very generally the surface is striated by the prominence of the woody fibres which, running parallel for a time, converge or diverge at the summit according to the shape of the branch. If the rate of growth be equal, or nearly so, on both sides, the stem retains its straight direction, but it more generally happens that the growth on one side is more rapid and more vigorous than on the other, and hence arises that curvature of the fasciated branch so commonly met with, e.g. in the ash (Fraxinus), wherein it has been likened to a shepherd's crook. It is probable that almost any plant may present this change. It occurs alike in herbaceous and in woody plants, originating in the latter case while the branches are still soft. It may be remarked that, in the case of herbaceous plants, the fasciation always affects the principal stem, while, on the other hand, in the case of trees and shrubs the deformity occurs most frequently in the branches; thus, while in the former it may be said that the whole of the stem is more or less affected, in the latter it is rare to see more than one or two branches of the same tree thus deformed. It is a common thing for the fasciated branch to divide at the summit into a number of subdivisions. These latter may be deformed like the parent branch, or they may resume the ordinary aspect of the twigs.

Fig. 6.—Fasciation and spiral torsion in the stem of Asparagus.

Sometimes the flattened stem is destitute of buds, at other times, these organs are scattered irregularly over its surface or are crowded together in a sort of crest along the apex. When, as often happens, the deformity is accompanied with a twisting of the branch spirally, the buds may be placed irregularly, or in other cases along the free edge of the spiral curve. In a specimen of Bupleurum falcatum mentioned by Moquin the spiral arrangement of the leaves was replaced by a series of perfect whorls, each consisting of five, six, seven, or eight segments, and there was a flower-stalk in the axil of each leaf.

When flowers are borne on these fasciated stems they are generally altered in structure; sometimes the thalamus itself becomes more or less fasciated or flattened, and the different organs of the flower are arranged on an elliptical axis. A case of this nature is described by Schlechtendal ('Bot. Zeit.,' 1857, p. 880), in Cytisus nigricans, and M. Moquin-Tandon describes an instance in the vine in one flower of which sepals, petals, stamens, and ovary were abortive, while the receptacle was hypertrophied and fasciated, and bore on its surface a few adventitious buds.[11] The pedicels of Streptocarpus Rexii have also been observed in a fasciated state.[12]

It has been occasionally observed that the fasciated condition is hereditary; thus, Moquin relates that some seeds of a fasciated Cirsium reproduced the same condition in the seedlings,[13] while a similar tendency is inherited in the case of the cockscomb (Celosia).

With reference to the nature of the deformity in question there is a difference of opinion; while most authors consider it to be due to the causes before mentioned, Moquin was of opinion that fasciation was due to a flattening of a single stem or branch. Linnæus, on the other hand, considered such stems to be the result of the formation of an unusual number of buds, the shoots resulting from which became coherent as growth proceeded:—"Fasciata dici solet planta cum plures caules connascuntur, ut unus ex plurimis instar fasciæ evadat et compressus" (Linn., 'Phil. Bot.,' 274). A similar opinion was held by J. D. Major in a singular book entitled 'De Plantâ, Monstrosa, Gottorpiensi,' Schleswig, 1665, wherein the stem of a Chrysanthemum is depicted in the fasciated condition.

Fig. 7.—Fasciation in the scape of the Dandelion (Leontodon Taraxacum).

The striæ, which these stems almost invariably present, exhibit the lines of junction, and the spiral or other curvatures and contraction, which are so often met with, may be accounted for by the unequal growth of one portion of the stem as contrasted with that of another. Against this view Moquin cites the instances of one-stemmed plants, such as Androsace maxima, but, on the other hand, those herbaceous plants having usually but a single stem not unfrequently produce several which may remain distinct, but not uncommonly become united together. Prof. Hincks[14] cites cases of this kind in Primula vulgaris, Hieracium aureum, and Ranunculus bulbosus. I have myself met with several cases of the kind in Primula veris, in the Polyanthus, in the Daisy, and in the Leontodon Taraxacum, in which latter a fusion of two or more flower-stems bearing at the top a composite flower, and made up of two, three, four, or more flowers combined together, and containing all the organs that would be present in the same flowers if separate, is very common.

Moquin's second objection is founded upon the fact that, in certain fasciated stems, the branches are not increased in number or altered in arrangement from what is usual; but however true this may be in particular cases, it is quite certain that in the majority of instances a large increase in the number of leaves and buds is a prominent characteristic of fasciated stems.

Another argument used by the distinguished French botanist to show that fasciated stems are not due to cohesion of two or more stems, is founded on the fact that a transverse section of a fasciated stem generally shows an elliptical outline with but a single central canal. On the other hand, if two branches become united and a transverse section be made, the form of the cut surface would be more or less like that of the figure 8[symbol: 8 turned 90°], although in old stems this may give place to an elliptical outline, but even then traces of two medullary canals may be found. This argument is very deceptive, for the appearance of the transverse section must depend, not only on the intimacy of their union, but also on the internal structure of the stems themselves. When two flowers cohere without much pressure they exhibit uniting circles somewhat resembling the figure of 8[symbol: 8 turned 90°], but when more completely combined they have an outline of a very elongated figure, and something similar is to be expected in herbaceous stems. Even the elongated pith of a transversely cut, woody, fasciated stem only marks the intimate union of several branches, and Prof. Hincks, whose views the writer entirely shares, has noticed instances of the union of two, and of only two, stems where the internal appearance was the same as in other fasciations.

Moquin, moreover, raises the objection that it is unlikely that several branches should become united lengthwise in one plane only, and, further, that in the greater number of fasciations all the other branches which should be present are to be found—not one is wanting, not one has disappeared, as might have been anticipated had fusion taken place. In raising this objection, Moquin seems not sufficiently to have considered the circumstance that the buds in these cases are in one plane from the first, and are all about equal in point of age and size.

The last objection that Moquin raises to the opinion that fasciation is the result of a grafting process is, that in such a case, examples should be found wherein the branches are incompletely fused, and where on a transverse section traces of the medullary canals belonging to each branch should be visible. The arrangement of leaves or buds on the surface should also in such a case indicate a fusion of several spiral cycles or whorls. To this it may be replied that such cases are met with very frequently indeed. A figure is given by De Candolle[15] of a stem of Spartium junceum having several branches only imperfectly fasciated.

Fasciated stems, then, seem to be best explained, as is stated by Prof. Hincks, "on the principle of adhesion arising in cases where from superabundant nourishment, especially if accompanied by some check or injury, numerous buds have been produced in close proximity, and the supposition that these growths are produced by the dilatation of a single stem is founded on a false analogy between fasciated stems and certain other anomalous growths."

It will not, of course, be forgotten that this fasciated condition occurs so frequently in some plants as almost to constitute their natural state, e.g. Sedum cristatum, Celosia, &c. This condition may be induced by the art of the gardener—"Fit idem arte, si plures caules enascentes cogantur penetrare coarctatum spatium et parturiri tanquam ex angusto utero, sic sæpe in Ranunculo, Beta, Asparago, Hesperide Pinu, Celosiâ, Tragopogone, Scorzonerâ Cotula fœtida," Linnæus op. cit.

Plot, in his 'History of Oxfordshire,' considers fasciation to arise from the ascent of too much nourishment for one stalk and not enough for two, "which accident of plants," says Plot, the German virtuosi ('Misc. Curios. Med. Physic. Acad. Nat. Cur.,' Ann. i, Observ. 102,) "think only to happen after hard and late winters, by reason whereof, indeed, the sap, being restrained somewhat longer than ordinary, upon sudden thaws may probably be sent up more forcibly, and so produce these fasciated stalks, whereas the natural and graduated ascent would have produced them but single." Prof. Hincks' explanation is, however, more near to the truth, and his opinion is borne out by the frequency with which this change is met with in certain plants which are frequently forced on during their growth, as lettuce, asparagus, endive, &c., all of which are very subject to this change. In the 'Transactions of the Horticultural Society of London,' vol. iv, p. 321, Mr. Knight gives an account of the cultivation of the cockscomb, so as to ensure the production of the very large flower-stalks for which this plant is admired. The principal points in the culture were the application of a large quantity of stimulating manure and the maintenance of a high temperature. One of them so grown measured eighteen inches in width.

The list which is appended is intended to show those plants in which fasciation has been most frequently observed. It makes no pretension to be complete, but is sufficiently so for the purpose indicated: the * denotes the especial frequency of the change in question; the ! indicates that the writer has himself seen the plant, so marked, affected in this way. The remainder have been copied from various sources.

Exogens.

α. Herbaceous.

 Ranunculus tripartitus.*bulbosus!Philonotis.

 Delphinium elatum.*sp.!

 Hesperis matronalis.

 *Cheiranthus Cheiri!

 *Matthiola incana!

 *Brassica oleracea! var. pl. inflor.

 Linum usitatissimum!

 Althæa rosea!

 Lavatera trimestris.

 Geranii sp.

 Tropæolum majus!

 Viola odorata inflor.!

 Reseda odorata!

 Fragaria vesca.

 Ervum lens.

 Trifolium resupinatum.repens!pratense!

 Saxifraga mutata.irrigua.

 Bupleurum falcatum.

 Bunium flexuosum.

 *Sedum reflexum!cristatum!

 Epilobium augustifolium!

 Momordica Elaterium!

 Gaura biennis.

 Cotula fœtida.

 Barkhausia taraxacifolia.

 Carlina vulgaris!

 Apargia autumnalis.

 *Leontodon Taraxacum inflor.!

 Centaurea Scabiosa.

 *Cichorium Intybus!

 Hieracium Pilosella.aureum.umbellatum.

 *Chrysanthemum Leucanthemum.indicum!

 Anthemis nobilis.arvensis.

 Cirsium lanceolatum.

 Conyza squarrosa!

 Inula dysenterica!

 Tragopogon porrifolium.

 Cnicus palustris.

 Carduus arvensis!

 Helianthus tuberosus!annuus.

 Cineraria palustris.

 Helianthus sp.!

 Dahlia variabilis.

 Bellis perennis inflor.!

 Coreopsis sp.!

 Crepis virens.

 Lactuca sativa!

 Zinnia elegans.

 *Campanula medium!rapunculoides.thyrsoidea.

 Dipsacus pilosus.fullonum.silvestris.

 Knautia arvensis.

 Phyteuma orbiculare.

 Jasione montana.

 *Linaria purpurea!

 Antirrhinum majus!

 Veronica amethystea.

 Veronica maritima.sp.

 Russellia juncea!

 Digitalis purpurea!

 Ajuga pyramidalis.

 Hyssopus officinalis.

 Dracocephalum moldavicum.

 Myosotis scorpioides.

 Echium pyrenaicum.simplex.

 Stapeliæ sp.

 Lysimachia vulgaris!

 Androsace maxima.

 Primula veris inflor.!denticulata inflor.!

 Polemonium cœruleum.

 Convolvulus sepium!arvensis!

 Plantago media.

 *Euphorbia Characias.exigua.*Cyparissias.

 Suæda maritima.

 *Celosia sp.

 Beta vulgaris inflor.!

 Phytolacca sp.

β. Woody.

 Berberis vulgaris.

 Hibiscus syriacus!

 Acer pseudo-platanus!

 Dodonæa viscosa.

 Sterculia platanifolia.

 Euonymus japonicus!

 Vitis vinifera inflor.!

 Spartium Scoparium!

 Spartium junceum!

 Cytisus Laburnum.nigricans.

 Chorozema ilicifolium.

 Amorpha sp.

 Phaseolus sp.

 Prunus sylvestris.Laurocerasus!

 Rosa sp.!

 Spiræa sp.!

 Cotoneaster microphylla!

 Ailanthus glandulosus.

 *Fraxinus Ornus!*excelsior!

 Melia Azedarach.

 Xanthoxylum sp.!

 Sambucus nigra.!

 Aucuba japonica.

 Erica sp. cult.

 Jasminum nudiflorum!officinale!

 Olea europœa.

 Punica Granatum.

 Ilex aquifolium!

 Daphne indica.

 Daphne odora.

 Suæda fruticosa.

 Ulmus campestris.

 Alnus incana.

 Salix vitellina, &c.!

 Thuja orientalis.

 Pinus pinaster!sylvestris!

 Abies excelsa!

 Taxus baccata.

 Larix europœa.

Endogens.

 Lilium Martagon.candidum!

 *Fritillaria imperialis!

 Asparagus officinalis!

 Hyacinthus orientalis!

 Tamus communis!

 Narcissi sp.!

 Gladiolus sp.

 Zea Mays.

 Filices.

See also—Moquin-Tandon, 'Elem. Ter. Veget.,' p. 146; C. O. Weber, 'Verhandl. Nat. Hist.,' Vereins, f. d. Preuss., Rheinl. und Westphal., 1860, p. 347, tab. vii; Hallier, 'Phytopathol.,' p. 128; Boehmer, 'De plantis Fasciatis,' Wittenb., 1752.

Cohesion of foliar organs.—This takes place in several ways, and in very various degrees; the simplest case is that characterised by the cohesion of the margins of the same organ, as in the condition called perfoliate in descriptive works, and which is due either to a cohesion of the margins of the basal lobes of the leaf, or to the development of the leaf in a sheathing or tubular manner. As an abnormal occurrence, I have met with this perfoliation in a leaf of Goodenia ovata. The condition in question is often loosely confounded with connation, or the union of two leaves by their bases. In other cases the union takes place between the margins of two or more leaves.

Cohesion of margins of single organs.—The leaves of Hazels may often be found with their margins coherent at the base, so as to become peltate, while in other cases, the disc of the leaf is so depressed that a true pitcher is formed. This happens also in the Lime Tilia, in which genus pitcher- or hood-like leaves (folia cucullata) may frequently be met with. There are trees with leaves of this character in the cemetery of a Cistercian Monastery at Sedlitz, on which it is said that certain monks were once hung: hence the legend has arisen, that the peculiar form of the leaf was given in order to perpetuate the memory of the martyred monks. ('Bayer. Monogr. Tiliæ,' Berlin, 1861.) It is also stated that this condition is not perpetuated by grafting.

Fig. 8.—Pitcher-shaped leaf of Pelargonium.

I have in my possession a leaf of Antirrhinum majus, and also a specimen of Pelargonium, wherein the blade of the leaf is funnel-like, and the petiole is cylindrical, not compressed, and grooved on the upper surface, as is usually the case. A comparison of the leaves of Pelargonium peltatum with those of P. cucullatum ('Cav. Diss.,' tab., 106) will show how easy the passage is from a peltate to a tubular leaf. In these cases the tubular form may rather be due to dilatation than to cohesion. M. Kickx[16] mentions an instance of the kind in the leaves of a species of Nicotiana, and also figures the leaf of a rose in which two opposite leaflets presented themselves in the form of stalked cups. Schlechtendal[17] notices something of the same kind in the leaf of Amorpha fruticosa; Treviranus[18] in that of Aristolochia Sipho.

M. Puel[19] describes a leaf of Polygonatum multiflorum, the margins of which were so completely united together, as only to leave a circular aperture at the top, through which passed the ends of the leaves. The Rev. Mr. Hincks, at the meeting of the British Association at Newcastle (1838), showed a leaf of a Tulip, whose margins were so united that the whole leaf served as a hood, and was carried upwards by the growing flower like the calyptra of a Moss.

The margins of the stipules are also occasionally united, so as to form a little horn-shaped tube. I have met with instances of this kind in the common white clover, Trifolium repens, where on each side of the base of the petiole the stipules had the form just indicated. That the bracts also may assume this condition, may be inferred from the peculiar horn-like structures of Marcgraavia, which appear to originate from the union of the margins of the reflected leaf.

Tubular petals occur normally in some flowers, as Helleborus, Epimedium, Viola, &c., and as an exceptional occurrence I have seen them in Ranunculus repens, while in Eranthis hyemalis transitions may frequently be seen between the flat outer segments of the perianth and the tubular petals. To Dr. Sankey, of Sandywell Park, I am indebted for the flower of a Pelargonium, in which one of the petals had the form of a cup supported on a long stalk. This cup-shaped organ was placed at the back of the flower, and had the dark colour proper to the petals in that situation. I have seen a petal of Clarkia similarly tubular, while some of the cultivated varieties of Primula sinensis exhibit tubular petals so perfect in shape as closely to resemble perfect corollas.