=Heterophylly.=--As a general rule, the leaves or leaf-organs in each portion of a plant, from the rhizome or underground axis, where it exists, to the carpellary leaf, have their own special configuration, subject only to slight variations, dependent upon age, conditions of growth, &c. The cotyledons are very uniform in shape in each plant, and are scarcely ever subject to variation. The leaves near the base of the stem, the root-leaves as they are not unfrequently called, sometimes differ in form from the stem-leaves; these again differ from the bracts or leaves in proximity to the flower. The floral envelopes themselves, as well as the bud-scales, all have their own allotted form in particular plants, a form by which they may, in most cases, be readily recognised. Hence, then, in the majority of plants there is naturally very considerable difference in the form of the leaf-organs, according to the place they occupy and the functions they have to fulfil; but, in addition to this, it not unfrequently happens that the leaf-organs in the same portion of the stem are subject to great variation in form.

This is the condition to which the term heterophylly properly applies.

The variation in form is usually dependent on a greater or less degree of lobing of the margin of the leaf; thus, in the yellow jasmine, almost every intermediate stage may be traced from an ovate entire leaf to one very deeply and irregularly stalked. _Broussonettia papyrifera_, and _Laurus Sa.s.safras_, and the species of _Panax_, may be mentioned as presenting this condition. Sometimes in the last-named genus, as also in _Pteridophyllum_, every gradation between simple and compound leaves may be traced. The horse-radish (_Cochlearia Armoracia_) may also be instanced as a common ill.u.s.tration of polymorphism in the leaves. In ferns it is likewise of frequent occurrence, markedly so in _Scolopendrium D'Urvillei_, in which plant every gradation from a simple oblong frond to an exceedingly divided one may be found springing from the same rhizome at the same time.

[Ill.u.s.tration: FIG. 177.--_Syringa persica laciniata_, showing polymorphous leaves.]

A similar protean state, but little less remarkable, occurs in many of our British ferns, notably in _Scolopendrium vulgare_, of which Mr.

Moore enumerates no fewer than 155 varieties,[366] many of the forms occurring on the same plant at the same time. Cultivators have availed themselves of this tendency to produce multiform foliage, not only for the purposes of decoration or curiosity, as in the many cut-leaved or crisped-leaved varieties, but also for more material uses, as, for instance, the many varieties of cabbages, of lettuces, &c. Most of these variations are mentioned under the head of the particular morphological change of which they are ill.u.s.trations.

The effect of a change in the conditions of growth in producing diversity in the form of the leaf may be here alluded to. _Ficus stipulata_, a plant used to cover the walls of plant-stoves in this country, and growing naturally on walls in India, like ivy, produces leaves of very different form, size, and texture, when grown as a standard, from what it does when adhering to a wall. _Marcgraavia umbellata_ furnishes another example of a similar nature, as indeed, to a less extent, does the common ivy.

Allusion has been already made to the occasional persistence of forms in adult life, which are commonly confined to a young state, as in the case of some conifers which present on the same plant, at the same time, two different forms of leaves. Mention has also been made of the presence of advent.i.tious buds on leaves and in other situations. The leaves that spring from these buds are usually of the same form as the other leaves of the plant, but now and then they differ. Of this a remarkable ill.u.s.tration is afforded by a fern, _Pteris quadriaurita_, in which the fronds emerging from an advent.i.tious bud are very different from the ordinary fronds.

[Ill.u.s.tration: FIG. 178.--Portion of a frond of _Pteris quadriaurita_, with an advent.i.tious bud, the form of the const.i.tuent foliage of which is very different from that of the parent frond.]

=Dimorphism.=--This term, applied specially to the varied form which the flowers or some of their const.i.tuent elements a.s.sume on the same plant, is an a.n.a.logous phenomenon to what has been above spoken of as heterophylly, and, like it, it cannot, except under special circ.u.mstances, be considered as of teratological importance. A few ill.u.s.trative cases, however, may here be cited.

Sir George Mackenzie describes a variety of the potato[367] (_Solanum tuberosum_), which produces first double and sterile flowers, and subsequently single fertile ones; the other portions of the plant do not differ much.

_Stackhousia juncea_, according to Clarke, has mixed with its perfect flowers a number of apetalous blossoms dest.i.tute of anthers.[368]

This peculiarity is well exemplified in the tribe _Gaudichaudieae_ of the order _Malpighiaceae_. A. de Jussieu, in his monograph, speaks of these flowers as being very small, green, dest.i.tute of petals, or nearly so, with a single, generally imperfect anther; the carpels also are more or less imperfect, but not sufficiently so to prevent some seeds from being formed. A similar production of imperfect flowers has been noticed in many other orders, _e.g._ _Violaceae_, _Campanulaceae_, &c. In some cases these supplementary blossoms are more fertile and prolific in good seeds than are the normally constructed flowers. M. Durieu de Maisonneuve alludes to a case where flowers of this description are produced below the surface of the ground. The plant in question is _Scrophularia arguta_, and it appears that towards the end of the summer the lowest branches springing from the stem bend downwards, and penetrate the soil; the branches immediately above the lowest ones also bend downwards, but do not always enter the earth. These branches bear fertile flowers: those which are completely below the soil are completely dest.i.tute of petals; those which are on the surface have a four-lobed corolla whose divisions are nearly equal, like those of _Veronica_.[369]

To Sprengel, and specially to Darwin, physiologists are indebted for the demonstration of the relation of di- and trimorphic flowers to fertilisation. In certain genera of orchids, such as _Catasetum_, &c., flowers of such different form are produced that botanists, without hesitation, considered them as belonging to different genera, until the fact of their occasional production on the same plant showed that they were not of even specific importance. It was reserved for Mr. Darwin to show experimentally that these very different flowers are really s.e.xual forms of one and the same species, ordinarily occurring on different plants, i.e. dioecious, but occasionally formed on the same spike. The same excellent observer has demonstrated that the di- and trimorphic forms of _Primula_, of _Linum_, _Lythrum_, and other plants--forms differing mainly in the relative length of the stamens and styles, are also connected with striking differences in the number of perfect seeds produced. The most perfect degree of fertility is obtained when the stigma of one form is fertilised by the pollen taken from stamens of a corresponding height. On the other hand, when the union is, as Mr.

Darwin states, illegitimate, that is, when the pollen is taken from stamens not corresponding in length to the style, more or less complete sterility ensues in the progeny, sometimes even utter infertility, such as happens when two distinct species are crossed, so that, in point of fact, the offspring of these illegitimate unions correspond almost precisely to hybrids.[370]

Mere variations of form arising from hybridisation or other causes hardly fall within the limits of this work, though it is quite impossible to say where variations end and malformations begin. There are, however, two or three cases cited by Mr. Darwin[371] from Gallesio and Risso to which it is desirable to allude. Gallesio impregnated an orange with pollen from a lemon, and the fruit borne on the mother tree had a raised stripe of peel like that of a lemon both in colour and taste, but the pulp was like that of an orange, and included only imperfect seeds. Risso describes a variety of the common orange which produces "rounded-oval leaves, spotted with yellow, borne on petioles, with heart-shaped wings; when these leaves fall off they are succeeded by longer and narrower leaves, with undulated margins, of a pale green colour, embroidered with yellow, borne on foot-stalks without wings.

The fruit whilst young is pear-shaped, yellow, longitudinally striated and sweet; but, as it ripens, it becomes spherical, of a reddish-yellow, and bitter."

=Sports or bud variations.=--These curious departures from the normal form can only be mentioned incidentally in this place, as they pertain more to variation than to malformation.

The occasional production of shoots bearing leaves, flowers, or fruits of a different character from those found on the normal plant, is a fact of which gardeners have largely availed themselves in the cultivation of new varieties. The productions in question have been attributed to various causes, such as cross-breeding, grafting, budding, dissociation of hybrid characters, or reversion to some ancestral form, all of which explanations may be true in certain cases, but none of them supply the clue to the reason why one particular branch should be so affected, and the rest not; or why the same plant, at the same time, as often happens in Pelargoniums, should produce two, three, or more "sports" of a different character.

These bud variations may be perpetuated by grafts or by cuttings, sometimes even by seed. With reference to cuttings a curious circ.u.mstance has been observed, viz., that if taken from the lower part of the stem, near the root, the peculiarity is not transmitted, but the young plant reverts to the characters of the typical form (Carriere).

This circ.u.mstance, however, is not of universal occurrence.

For further particulars on this interesting subject the reader is referred to Darwin's 'Variation of Animals and Plants,' i, p. 373, where numerous references are given, and wherein certain well-known and highly remarkable instances, such as the _Cytisus Adami_, the trifacial orange, &c., are discussed.

FOOTNOTES:

[366] 'Nature-printed Ferns,' 8vo edition, vol. ii, p. 197.

[367] 'Gard. Chron.,' 1845. p. 790.

[368] 'A New Arrangement of Phaenog. Plants,' p. 36.

[369] 'Bull. Soc. Bot. France,' 1856, t. iii, p. 569.

[370] The reader will find an abstract of Mr. Darwin's views in his work on the 'Variation of Animals and Plants,' vol. ii, p. 181.

[371] Loc. cit., i, 336.

CHAPTER III.

ALTERATIONS OF COLOUR.[372]

Changes in the colour of the several organs of plants are more often either pathological or the result of variation than of malformation properly so called.

Alterations in colour arise from a diminished or an increased amount of colouring matter, or from an unusual distribution of the solid or fluid matters on which the colour depends. The superposition of cells containing colouring material of different tints produces naturally a very different set of hues from those which are manifested when the colours are not blended. Referring the reader to the ordinary text-books on vegetable physiology and chemistry for details as to the nature and disposition of colouring materials in plants under natural circ.u.mstances, it will only be necessary to cite a few instances of deviation from the general colour of plants or their organs.

=Albinism.=--This change is due to the deficient formation of green colouring matter or chlorophyll, and is more a pathological condition than a deformity.

It seems necessary to draw a distinction between this state and ordinary blanching or etiolation. In the former case chlorophyll seems never to be formed in the affected parts, even if they be exposed to light, while an etiolated organ, when placed under favorable circ.u.mstances, speedily a.s.sumes a green colour. In _Richardia aethiopica_ one or more leaves become occasionally as white as the spathe is usually.

=Virescence.=--Engelmann[373] pointed out that, so far as flowers were concerned, there are two ways in which they a.s.sume a green colour, either by a simple development of chlorophyll in place of the colouring matter proper to the flower, or by an actual development of leaf-like organs in the room of the petals--frondescence. Morren[374] judiciously proposed to keep these two conditions separate, calling the one virescence, the other frondescence (see p. 241).

Many of the cases recorded as reversions of the parts of the flower to leaves are simply instances of virescence; indeed, it is not in all cases easy to distinguish between the two states. The examination of the arrangement of the veins is often of a.s.sistance in determining this point; for instance, if, under ordinary circ.u.mstances, the venation of the petal be such as is characteristic of the sheath of the leaf, while in the green-coloured flower of the same species the venation is more like that which belongs to the blade of the leaf, the inference would, of course, be that the green colour was due to frondescence or phyllody.

The persistence or duration of petals is often increased when they are subject to this change; instead of falling off speedily they become persistent when so affected.

Some flowers are more liable to virescence than others. The common honeysuckle, _Lonicera Periclymenum_, is one of these, and it is noticeable in this plant that the calyx remains unaffected--a circ.u.mstance which Morren says shows the distinctness of virescence from frondescence; for, in this instance, we have the most foliaceous portion of the flower remaining unchanged, while the corolla and other organs, usually less leaf-like in their nature, a.s.sume a green colour; but this may rather be attributed to the axial nature of the so-called adherent calyx. The stamens in these green-flowered honeysuckles are usually green also, but with abortive anthers, and the pistil also is in a rudimentary condition. _Umbelliferae_ are not unfrequently subject to this change, _e.g._, _Torilis Anthriscus_, _Daucus Carota_, _Heracleum Sphondylium_, _Carum carui_, &c. _Primulaceae_, again, are frequently subject to virescence. Among _Compositae_ the following species are recorded as having had green flowers--_Cirsium tricephalodes_, _Senecio vulgaris_, _Calendula officinalis_, _Pyrethrum Parthenium_, _Carduus crispus_, _Hypochaeris radicata_, _Hieracium prealtum_, _Cirsium arvense_, _Coreopsis Drummondi_.[375] In _Ranunculaceae_ virescence has been observed in _Delphinium elatum_, _cra.s.sicaule_ and _Ajacis_, _Anemone hortensis_ and _nemorosa_, _Aquilegia vulgaris_, _Ranunculus Philonotis_.

Many of these cases, and others that might be cited, are probably instances of frondescence or phyllody (see p. 241).

=Chromatism.=--This term is here intended to apply specially to those cases in which any organ of a plant a.s.sumes a colour approximating to that of the petals, or in which the normal green is replaced by tints of some other colour. To a certain extent the change in question is the same as that spoken of under the head of petalody (see p. 283), but there are cases in which, while the ordinary situation and form are those of leaves, the coloration is that of the petals. Such was the case in the _Gesnera_ mentioned by Morren (see p. 88), and in which a leaf occupied the position of an inflorescence, and became brightly coloured.

In tulips the presence of a highly coloured leaf on the flower-stalk, below the flower, is not uncommon. So also the bracts or leaves below the perianth in _Anemone coronaria_ and _hortensis_ not unfrequently a.s.sume the coloration usually confined to the parts of the perianth. A similar ill.u.s.tration has presented itself, as this sheet is pa.s.sing through the press, in which two of the leaflets of the compound leaf of a rose were brightly coloured like the petals, the others being of their ordinary green colour.

The occurrence of coloured bracts, as in _Poinsettia_, _Bougainvillea_, &c., is very common under natural conditions, and need not here be further alluded to.

Increased intensity of colour often accompanies teratological changes; an instance has just been alluded to in the _Gesnera_; the feather hyacinth, _Muscari comosum_, furnishes another ill.u.s.tration, the advent.i.tious pedicels being brightly coloured.

In fasciated stems, also, of herbaceous plants, it not unfrequently happens that the upper portions of the stem are brightly coloured.

The occurrence of flowers or fruits of different colours on the same plant, or even in the same cl.u.s.ter, is a phenomenon which does not come within the scope of the present book; the reader may, however, be referred to the excellent summary on this subject published by Mr.

Darwin in his work on the 'Variation of Animals and Plants under Domestication.'

FOOTNOTES:

[372] These deviations are treated of under the head of alterations of form, because they are not, in a teratological point of view, of sufficient importance to demand a specific heading, while they appeal to the sight in the same way as the deviations from the customary forms of organs.

[373] 'De Antholys,' p. 32, -- 38.

[374] 'Bull. Acad. Belg.,' xvii, part 2, p. 131, c. tab.

[375] See Cramer, 'Bildungsabweich,' pp. 17, 55, 82, 65. See also Lucas, 'Verhandl. des Bot. Vereins. Brandenb.,' heft 1, 2, _Anchusa_. Christ, 'Flora,' 1867. pp. 376, tab. 5, 6, _Stachys_.