CHAPTER I.

FISSION.

When an organ becomes divided it receives at the hands of descriptive botanists the appellations cleft, partite, or sect, according to the depth of the division; hence in considering the teratological instances of this nature, the term fission has suggested itself as an appropriate one to be applied to the subdivision of an habitually entire or undivided organ. It thus corresponds pretty nearly in its application with the term Chorisis or "dédoublement," or with the "disjonctions qui divisent les organes" of Moquin-Tandon.[67] It is usually, but not always, a concomitant with hypertrophy, and dependent on luxuriance of growth.

It must be understood therefore that the term, as generally applied, does not so much indicate the cleavage of a persistent organ, as it does the formation and development of two or more growing points instead of one, whence results a branching or forking (di-tri-chotomy) of the affected organ. In some instances it seems rather to be due to the relative deficiency of cellular, as contrasted with fibro-vascular tissue.

=Fission of axile organs.=--This condition is scarcely to be distinguished from multiplication of the axile organs (which see). A little attention, however, will generally show whether the unusual number of branches is a consequence of the development of a large number of distinct shoots, as happens, for instance, when a tree is pollarded, or of a division of one. M. Fournier[68] gives as an illustration the case of a specimen of _Ruscus aculeatus_ in which there occurred a division of the foliaceous branches into two segments, reaching as far as the insertion of the flower, but no further. He also mentions lateral cleavage effected by a notching of the margin, the notch being anterior to the flowers and always directed towards their insertion. In the allied genus _Danaë_, Webb, 'Phyt. Canar.,' p. 320, describes the fascicles of flowers as in "crenulis brevibus ad marginem ramulorum dispositis." Sometimes, on the other hand, _Danaë_ has a fascicle of flowers inserted on the middle of the upper surface, as in _Ruscus_. Wigand mentions an instance in _Digitalis lutea_, where the upper part of the stem was divided into six or seven racemes; possibly this was a case of fasciation, but such a division of the inflorescence is by no means uncommon in the spicate species of _Veronica_. I have also seen it in _Plantago lanceolata_, _Reseda luteola_, _Campanula medium_, _Epacris impressa_, and a bifurcation of the axis of the spikelet within the outer glumes in _Lolium perenne_[69] and _Anthoxanthum odoratum_. In the Kew Museum is preserved a cone of _Abies excelsa_,[70] dividing into two divisions, each bearing bracts and scales. A similar thing frequently occurs in the male catkins of _Cedrus Libani_ (fig. 25).

This subdivision of axial organs is not unfrequently the result of some injury or mutilation, thus Duval Jouve alludes to the frequency with which branched stems are produced in the various species of _Equisetum_, as a consequence of injuries to the main stem, but this is rather to be considered as a multiplication of parts than as a subdivision of one.

=Fission of foliar organs.=--Many leaves exhibit constantly the process of fission, such as the _Salisburia adiantifolia_, and which is due perhaps as much to the absence or relatively small proportion of cellular as compared with vascular tissue, as to absolute fission. In the same way we have laciniated leaves of the Persian lilac, _Syringa persica_, and Moquin mentions instances in a species of _Mercurialis_ in which the leaves were deeply slashed. In _Chenopodium Quinoa_ the leaves were so numerous and the clefts so deep, that the species was hardly recognisable, while on a branch of _Rhus Cotinus_ observed by De Candolle the lobes were so narrow and so fine as to give the plant the aspect of an _Umbellifer_. Wigand ('Flora,' 1856, p. 706) speaks of the leaves of _Dipsacus fullonum_ with bi-partite leaves; Moquin mentions the occurrence of a leaf of an oleander bi-lobed at the summit, so as to give the appearance of a fusion of two leaves. Steinheil has recorded an instance in _Scabiosa atropurpurea_ in which one of the stem leaves presented the following peculiarities. It was simple below, but divided above into two equal lobes, provided each with a median nerve.[71] Steinheil has also recorded a _Cerastium_ in which one of the leaves was provided with two midribs; above this leaf was a group of ternate leaves. I have seen similar instances in the common Elm, _Ulmus campestris_, and also in the common nettle, _Urtica dioica_, the leaves of which latter thus resembled those of _Urtica biloba_, which are habitually bilobed at the summit. M. Clos[72] mentions an instance where the terminal leaf and first bract of _Orchis sambucina_ were divided into two segments. The same author also mentions the leaves of _Anemiopsis californica_, which were divided in their upper halves each into two lobes--also leaves of a lentil springing from a fasciated stem and completely divided into two segments, but with only a single bud in the axil. The axillary branches in like manner showed traces of cleavage. Fig. 26 represents a case of this kind in _Lamium album_, conjoined with suppression of the flowers on one side of the stem. I have also in my herbarium a leaf of _Arum maculatum_, with a stalk single at the base, but dividing into two separate stalks, each bearing a hastate lamina, the form of which is so perfect that were it not from the venation of the sheath it would be considered that there was here a union of two leaves rather than a bifurcation of one. A garden Pelargonium presented the same appearance.

Fern fronds are particularly liable to this kind of subdivision, and they exhibit it in almost every degree, from a simple bifurcation of the frond to the formation of large tufts of small lobes all formed on the same plan by the repeated forking of the pinnules. These may be considered as cases of hypertrophy.

Moquin-Tandon, at a meeting of the Botanical Society of France (April 3rd, 1858) exhibited a leaf of _Cerasus Lauro-Cerasus_ divided in such a manner as to resemble a leaf of _Citrus_ or of _Phyllarthron_. In this case, therefore, the disunion must have taken place laterally, and not from apex towards base, as is most common. The leaves of the common horse-radish, _Cochlearia Armoracia_, are very subject to this pinnated subdivision of the margin, and numerous other illustrations might be given.

A. Braun describes a singular case in a leaf of _Irina glabra_ wherein the blade of the leaf on one side was deeply and irregularly laciniated, the other side remaining entire. (Verhandl., d. 35, Naturforscherversammlung, tab. 3.) Laciniate varieties of plants are of frequent occurrence in gardens where they are often cultivated for their beauty or singularity; thus, there are laciniated alders, fern-leaved beeches and limes, oak-leaved laburnums, &c. A list of several of these is subjoined. A similar fission takes place constantly in the cotyledons of some plants, sometimes, as in _Coniferæ_, to such an extent as to give an appearance as if there were several cotyledons.[73]

It is not always easy to recognise, at a first glance, whether the division be the result of disunion or of an incomplete union of two leaves, but we may be guided by the number of leaves in the cycle or the whorl. The number is complete in cases of partial disjunction, while in cases of fusion it is incomplete. Again, in instances of disjunction, there is only one point of origin, but, when two leaves are grafted together, two such points may generally be detected at the base of the leaf, or a transverse section of the leaf-stalk will show indications of fusion. The number and position of the midribs will also serve as a guide, as in cases of fusion there are generally two or more midribs, according to the number of fused leaves; but as Moquin well remarks, this latter character cannot be always depended upon, for the median nerve may divide without any corresponding separation of the cellular portions of the leaf. The author just quoted cites examples of this kind in _Cardamine pratensis_, _Hedera Helix_, _Plantago major_, _Geranium nodosum_.

The following list of plants commonly producing leaves that are cleft or divided, to a greater extent than is usual in the species, is mainly taken from one given by Schlechtendal, 'Bot. Zeit.,' 1844, p. 441, with additions from other sources. The ! indicates that the author has himself met with the deviation in question. Many are cultivated as garden varieties under the names here given.

Trollius europæus dissectus. Chelidonium majus laciniatum! Glaucium luteum. Brassica oleracea! Tilia parvifolia laciniata. asplenifolia! Acer platanoides laciniatum. crispum. Æsculus Hippocastanum incisum! asplenifolium. Vitis vinifera apiifolia! laciniosa. Ilex Aquifolium! Rhus Toxicodendron quercifolium. Cotinus. Ervum Lens. Cytisus Laburnum quercifolium! incisum. Rubus fraticosus laciniatus! Pyrcis communis. Cerasus Lauro-cerasus. Apium graveolens! Pimpinella magna. Saxifraga. Cratægus Oxyacantha laciniata. quercifolia! Ribes nigrum. Sambucus nigra laciniata! racemosa laciniata. Dipsacus fullonum. Scabiosa atropurpurea! Symphoricarpus racemosus. Helianthus sp.! Lonicera Periclymenum quercifolia! Syringa persica laciniata! Syringa vulgaris! Nerium Oleander! Lamium purpureum. album! Salvia officinalis. Solanum Dulcamara! Fraxinus excelsior crispa. Veronica austriaca. Polemonium cæruleum. Juglans regia laciniata! heterophylla. filicifolia. Anemiopsis californica. Chenopodium Quinoa. Ulmus americana incisa. Fagus sylvatica heterophylla! laciniata! aspleniifolia! incisa. salicifolia! Mercurialis perennis. Urtica dioica. Quercus Cerris laciniata! pubescens filicina. Betula populifolia laciniata. alba dalecarlica. Alnus incana laciniata! glutinosa laciniata! quercifolia. oxyacanthifolia. Corylus Avellana heterophylla! laciniata! urticifolia. Carpinus Betulus incisa! quercifolia. heterophylla. Castanea vesca heterophylla. quercifolia. incisa. Populus alba acerifolia. palmata. quercifolia. balsamifera. Orchis sambucina. Arum maculatum. Filices sp. pl.

See also Schlechtendal, 'Bot. Zeit.,' tom. xiii, p. 823. A. Braun, loc. supra citat. For Ferns too numerous for insertion, see Moore, 'Nature-Printed Ferns,' 8vo ed., 2 vols. Clos, 'Mém. Acad. Toulouse,' 1862, p. 51.

=Fission of the petals, &c.=--The floral leaves are subject to a similar process of cleavage to that which has just been mentioned as taking place in the leaves. This, indeed, occurs very often as a normal occurrence as in the petals of mignonette (_Reseda_), or those of _Alsine media_ and many other plants. Here, however, we have only to allude to those instances in which the cleavage occurs in flowers whose sepals or petals are usually entire. Under this category Moquin mentions a petal of _Brassica oleracea_ completely split into two. Linné in his 'Flora Lapponica' (pp. 145 and 164) mentions quadrifid petals of _Lychnis dioica_, and much divided petals of _Rubus arcticus_. Among other plants subject to this division of sepals or petals may be mentioned as having come within the writer's personal observation, _Ranunculus Lingua_, _R. acris_, _Papaver somniferum_, and others of this genus, _Saponaria sp._, _Dianthus_, _Narcissus_, &c.

In some of the garden varieties of _Cyclamen_ the corolla looks at first sight as if double, and the plan of the flower is oblong or elliptical, instead of circular. In these flowers each lobe of the corolla is divided almost to the base into two lobes, so that there appear to be ten lobes to the corolla instead of five, as usual. The stamens are normal in form and number in these flowers.

In the paroquet tulips of gardeners the segments of the perianth are deeply and irregularly gashed, the segments occasionally becoming rolled up and their margins coherent so as to form little tubular spurs. I have also noticed the segments of the perianth in _Crocus_ and _Colchicum_ deeply cleft, so much so sometimes, as to equal in this particular the stigmas. In the flowers of a species of _Oncidium_, communicated to me by Mr. Currey, the lip was divided into three segments perfectly distinct one from the other, but confluent with the column; the two side pieces had callosities at the upper edge close to the base, the central piece had a similar wartlike process in its centre. In these flowers the ovary, the stigma, and the anther were all in a rudimentary condition. Some verbenas raised by Mr. Wills offer a curious illustration of this condition. It will be remembered that some of the lobes or petals of a verbena are normally divided at the base to a slight degree, but in the flowers in question this is carried to such an extent that the enlarged lobes are pushed into the centre of the flower and simulate, at a first glance, a distinct and separate organ, though in reality it is but an enlargement of what occurs normally.[74]

Moquin mentions having seen the stamens of _Matthiola incana_ and _Silene conica_ completely divided, each section bearing half an anther, exactly as happens in _Polygalaceæ_. In tulips and lilies the same author mentions division of the anther only, the filament remaining entire, as happens naturally in many species of _Vaccinium_.

A division of the individual carpels occurs very frequently when those organs become more or less leafy, as in _Trifolium repens_, and other plants to be hereafter mentioned.

The instances given in this chapter have all been cases wherein the division or the accessory growth has taken place in one plane only and that plane the same as that of the affected organ, but there are other examples, probably equally due to fissiparous division, where the new growth is either parallel to, or even at angle with the primary organ. Of such nature are some of those instances wherein two leaves appear to be placed back to back. These partake of the nature of excrescences or of exaggerated developments, and hence will be more fully treated of under the head of hypertrophy. It must be remembered that in some of these cases the fission may be a resumption of characters proper to the species under natural conditions, but lost by cultivation or otherwise. Thus, Mr. Buckman accounts for "finger-and-toe" in root-crops on the principle of reversion to the wild form.

FOOTNOTES:

[67] Loc. cit., p. 295.

[68] 'Bull. Soc. Bot. France,' 1857, p. 758.

[69] Masters, 'Jourl. Linn. Soc.,' vol. vii, p. 121.

[70] Cramer, 'Bildungsabweichungen,' p. 4, tab. vi, fig. 4, figures a case of the same kind in _Pinus Cembra_.

[71] 'Ann. des Science Nat.,' 2nd series, t. iv, p. 147, tab. v, figs. 3 and 4.

[72] 'Mém. Acad. Scien. Toulouse,' 5th series, vol. iii.

[73] Duchartre, 'Ann. Sc. Nat.,' 3rd series, 1848, vol. x, p. 207.

[74] Masters, 'Rep. Bot. Congress,' London, 1866, p. 136, tab. 7, f. 15, 16.