Chapter 66

ABORTION.

The sense in which this term is here understood has been explained in the preceding paragraph. It is only necessary to say further, that cases of abortion are to be distinguished from those of suppression, on the one hand, and those of degeneration on the other. In suppression there is from the first an absolute deficiency of a particular organ. In degeneration the part is present, but in a diminished and perverted condition. In abortion it exists, but in a stunted and dwarfed, but not otherwise permuted state.

=Abortion of axile organs.=--When the main stem is arrested in its growth, the habit and general appearance of the plant are materially altered, as in the so-called stemless plants, _plantæ acaules_. In these the internodes are so slightly developed that the leaves are closely crowded in tufts or rosettes. When this shortening of the stem (acaulosia) occurs, without other considerable change in other organs, the deviation is classed under the head of variation rather than of monstrosity; and, indeed, in very many plants, this arrested growth of the axis is the rule rather than the exception. When occurring in an abnormal manner, atrophy of the stem is most frequently attended by other more or less grave alterations in other structures; thus Moquin-Tandon[522] cites an instance of _Camphorosma monspeliaca_, wherein the stems presented the form of very short, hard, woody tubercles, thickly clothed with deformed leaves, and invested by a vast number of hairs, longer and more dense than usual. A similar deformity sometimes occurs in an Indian species of _Artabotrys_; in these specimens the branchlets are contracted in length, and bear numerous closely packed scaly leaves, densely hairy, and much smaller than ordinary.

Spines and thorns may he looked on as atrophied branches, and seem to result from poorness of soil, as the same plants, which, in hungry land, produce spines, develop their branches to the full extent when grown under more favorable conditions.[523]

In the birch an arrest of development in some of the branches is of common occurrence. The branch suddenly ceases to grow in length; at the same time it thickens at the end into a large bulbous knob, from which are developed a profusion of small twigs, whose direction is sometimes exactly the reverse of that of the main branch. (See p. 347.)

The branches of the common spruce fir, especially the lateral ones, when attacked by a particular species of aphis, are very apt to be developed into a cone-like excrescence.[524]

A shortened condition of the flower-stalks occurs occasionally, greatly altering the general character of the inflorescence. This has been observed in pelargoniums and in the Chinese primrose, in both of which the effect was to replace the umbellate form of inflorescence by a capitate one.

=Abortion of the receptacle.=--Here may be mentioned those cases of flowers with habitually inferior ovary (real or apparent), in which the receptacle fails, from some cause or other, to dilate as usual. This has already been alluded to under the head of Prolification, Displacements, &c. (pp. 78, 130, &c., figs. 35-37, 64, &c.), and hence requires only incidental comment in this place. There are, however, certain other cases of a similar nature which may here be referred to; such as the abortive condition of the inferior ovary, or rather of the receptacle, that usually encircles the ovary in _Compositæ_ and _Umbelliferæ_. In the former natural order the following plants have been met with in this condition:--_*Tragopogon pratense!_, *_Cirsium arvense_, _Hypochæris radicata_, _Senecio vulgaris!_, _Coreopsis Drummondi_. In the latter order, _Daucus Carota!_ _OEnanthe crocata!_ and _Thysselinum palustre_, seem most frequently to have been observed in this state.[525] In some gourds the receptacle may be seen partially developed only, and forming a kind of cup, from which the true carpels protrude.

=Abortion of the leaves.=--Arrest of growth in the leaves occurs in different ways; sometimes the whole leaf is smaller than usual; at other times certain parts only are reduced in size; while, in a third class of cases, portions of the leaf are entirely suppressed.

Moquin[526] mentions having seen the leaves of _Chenopodium vulvaria_, and of _Diplotaxis muralis_ reduced to a fourth of their natural size; and he alludes to other cases of the same nature, seen by other observers, in _Hypericum perforatum_ and _Blitum polymorphum_.

_Nicandra physaloides_[527] has also been met with in a similar condition, which, indeed, is a common result of insect-puncture, and of fungous growth in plants. Those instances in which the leaf is diminished in size, without any attendant malformation in other organs, may be regarded rather as variations than as monstrosities, as in the case of the entire-leaved varieties of those plants which ordinarily have cut or divided leaves, _e.g._ _Plantago Coronopus_, var. _integrifolia_, _Papaver Rhoeas integrifolia_, &c. &c. The same remark may be made of those specimens in which one part of the leaf is developed to a less extent than another, as happens in the submerged leaves of such plants as _Ranunculus aquatilis_, _Cabomba aquatica_, the spiney leaves of _Berberis_, the fenestrated leaves of _Ouvirandra_, &c. In the illustrations last cited the relative deficiency of one portion, as contrasted with another, takes place as a constant occurrence, and is uniform and regular throughout the whole leaf. When, on the other hand, the deficiency in question happens accidentally and irregularly, the change may be considered as a malformation. One side of the blade of the leaf is frequently affected in this manner, the other portions remaining unaffected. It would appear as if any plant might be thus altered, but the following species appear to be particularly subject to this change: _Æesculus Hippocastanum_, _Digitalis purpurea_, _Morus alba_, _Fagus silvatica contracta_ (hort.), _Codiæum variegatum_ var. _erosum_ (hort.), _Broussonettia papyrifera_, _Scolopendrium vulgare_, &c.

Frequently this irregular diminution in proportion is coexistent with an unusual degree of cleavage or laciniation of the margin, as in _Acer platanoides laciniatum_, _Tilia asplenifolia_, _Alnus imperialis_ (hort.), _Fagus silvatica_ var. (hort.), &c.

In the case of what are sometimes termed interrupted leaves, the laminar portions of the leaf are here and there deficient on both sides of the midrib, leaving small portions of the latter, as it were, denuded and connecting the segments of the laminæ one with the other. This has been observed amongst other plants in _Veronica latifolia_, _Broussonettia papyrifer_, _Codiæum variegatum_ var. _interruptum_ (hort.), _Scolopendrium vulgare_, &c.[528] (See p. 328.)

In some of the leaves which have been already referred to in illustration of the inordinate growth of the cellular portions, the increased development of parenchyma is associated with a contracted state of the midrib and its branches, producing a puckered appearance of the leaf, an exaggerated degree of that change which produces what are termed "folia bullata." In illustration may be cited various species of _Mentha_, _Perilla_, _Coleus_, _Fagus silvatica crispa_, _Cytisus_, _Laburnum_ var., and other forms, cultivated in gardens for their singularity.

Entire absence of the stalk of the leaf occurs normally in sessile leaves; on the other hand the blade of the leaf is only occasionally developed in the phyllodineous Acacias, in some species of _Oxalis_, _Indigofera_, _Lebeckia_, _Ranunculus_, _Bupleurum_, &c.

De Candolle,[529] from a consideration of _Strelitzia juncea_, in which the petiole alone is developed, was led to the inference that in many monocotyledonous plants the blade of the leaf was never developed, the portion present being the sheath or stalk, unprovided with limb. The correctness of this inference is shown, amongst other things, by the occasional presence of a leaf-blade in _Strelitzia juncea_ itself.

Occasionally the laminar portions of the leaf are completely wanting, leaving only the main ribs, as in the case of _Berberis_, while the adjoining figure (fig. 215) represents an instance of a cabbage wherein the innermost leaves are represented by thick fleshy cylindrical bodies corresponding to the midribs of the ordinary leaves. There is in cultivation a variety of the cabbage which constantly presents this peculiarity.

The suppression of one or more leaflets of a compound leaf has already been referred to at p. 396.

=Abortion of the perianth, calyx, and corolla.=--Illustrations of partial development in these organs are not rare, under ordinary circumstances, as for instance the "obsolete" calyx of Umbellifers. In the cauliflower the branches of the inflorescence are contracted in length, while their succulence is much increased; at their extremities they bear crowds of imperfect flowers, in which the calyx only is visible, and that only in a rudimentary and partially developed condition. Imperfect development of the whole or of some of the constituent parts is more common in the case of the corolla than in that of the calyx. In _Arenaria serpyllifolia_ the petals, especially in autumn, are only one fourth the length of the sepals. _Anagallis phoenicea_, _Honckenya peploides_, _Arabis alpina_, _Ranunculus auricomus_, _Rubus fruticosus_, and _Geranium columbinum_, also frequently afford illustrations of this circumstance.

At fig. 216 is represented a pansy in which four of the five petals were very small and colourless, while the lower spurred petal was of the usual size and colour. In this flower the stamens and pistils were wholly suppressed, and the flower-stalk, instead of being bent near the flower, retained its primary straight direction. Similar atrophic conditions of the corolla occur habitually among _Violaceæ_.

The diminished size of the petals sometimes coexists with an increase in their number, as in a flower of _Streptocarpus Rexii_, mentioned by Bureau.[530]

Among monocotyledons this partial development seems to be even more frequent than in dicotyledons. In addition to the well-known cases of certain species of _Bellevalia_ and _Muscari_, wherein the uppermost flowers of the raceme are more or less atrophied (see p. 347, fig. 179), a few less common illustrations may be cited. In crocuses it is not a very uncommon circumstance to find the three inner segments of the perianth smaller than natural, and generally unequal in size. This occurs without any other perceptible change in the flower.

Schlechtendal[531] mentions a flower of _Fritillaria imperialis_ in which the perianthial leaves were relatively very small, and destitute of the usual nectary, while the stamens, on the other hand, were of their natural size and appearance. Fresenius[532] records a similar occurrence in the same plant.

Morren[533] gives details of like appearances in _Hymenocallis americana_, and Delavaud[534] in _Tigridia pavonia_.

In certain orchids an arrested development of the perianth is habitual, as in _Oncidium abortivum_ (fig. 217), where, on a large branching panicle, numerous abortive, but few perfect, flowers are produced. In a similar way the petals and labellum of _Odontoglossum Uro-Skinneri_ have been found reduced to filamentous processes.

=Abortion of the stamens.=--Atrophy of one or more stamens is of very common occurrence, as a general rule, in many genera of plants, _e.g._ _Scrophularia_, _Erodium_, many _Restiaceæ_, &c. &c. As a strictly teratological condition atrophy of the stamens is more rare than complete suppression. It has been noticed in _Arabis alpina_, _Cerastium glomeratum_, _C. tetrandrum_, _Rhamnus catharticus_, _Anemone_, _Hepatica_, &c. It happens frequently among Orchids both wild and cultivated. In the _Hymenocallis_ flowers described by the elder Morren, four out of five stamens were atrophied. In other flowers, otherwise perfectly formed, one abortive stamen was found bearing a spherical indehiscent anther. All these atrophied anthers of _Hymenocallis_ were found to contain pollen, differing at first sight but little from what is usual, but presenting this important peculiarity, that while the normal pollen does not burst until it comes into contact with the stigma, in the abnormal flowers the outer coat of the pollen-grains split while still within the anther, from which latter, indeed, they could not escape, owing to the indehiscent nature of the latter. Again, the pollen-tube of the abnormal grains cracked, in its turn, on mere exposure to the air, and liberated the fovilla, so that the pollen of these atrophied anthers was necessarily impotent, because it opened before it could be applied to the stigma, even had that been rendered possible by the opening of the anther.

An abortive condition of the stamens and of the pollen, is of very common occurrence among hybridised plants. Gaertner and other writers have spoken of this defective condition as contabescence.[535] It forms one reason for the sterility so frequently observed in the case of true hybrids. In some hybrid passion-flowers, while all other parts of the flower were apparently perfect, even to the ovules, the stamens were atrophied, and distorted, and contained little or no pollen; the few grains of the latter being smaller than usual. (See under Heterogamy, pp. 193-196, and p. 398.)

=Abortion of the pistil, fruit, &c.=--Traces of the carpels occur in many male flowers of unisexual plants, _e.g._ _Sterculiaceæ_, _Euphorbiaceæ_, _Restiaceæ_, &c. &c., and in some natural orders there appears to be a tendency towards a dioecious condition, _e.g._ _Caryophylleæ_, as in _Lychnis dioica_, _Silene otites_, _Arenaria tetraquetra_, &c. The last-named plant is stated to have, in some cases, imperfect pistils; in others, rudimentary stamens; while a third set of flowers are hermaphrodite.[536] The ovary of aconites, according to Moquin, is very subject to atrophy.

During the maturation of the pistil, and its passage to the fruit, great changes of consistence frequently take place, owing to the development of cellular tissue, or of woody matter, according as the fruit is succulent or woody. It sometimes happens that, owing to some disturbing causes, the changes that usually occur fail to do so; thus, the stone of plums is occasionally deficient, as in what are termed bladder-plums (fig. 218); some of these, consisting merely of a thin bladder, are curiously like the pods of _Colutea_.[537]

MM. Fournier and Bonnet[538] describe a fruit of a _Rubus_, with perfectly dry fruits, like those of a _Geum_, and this form was considered by Steudel to form a distinct species. It is, however, merely a variety in which the fruits have not become succulent.[539]

Schlechtendal describes[540] the ordinarily baccate fruit of a vine as becoming dry, and even dehiscing by valves like a capsule.

In maize it occasionally happens that one or two of the longitudinal series of fruits become abortive, leaving a smooth furrow, at first of a greenish colour, but ultimately of a reddish yellow. Often a second row of fruits, opposite to the first, is also atrophied, so that the whole spike changes its cylindrical form for a flattened one.[541] See also under Heterogamy, Meiophylly, &c.

=Abortion of the ovules.=--In the case of a pluri-ovulate ovary it rarely happens that all the ovules attain to maturity, some never get fertilised, others, pressed on by their neighbours on either side, become impeded in their development, and finally disappear, or remain as rudiments.[542] This is the case, under ordinary circumstances, and still more so in the case of hybrid plants, or of monsters. Where the outer coats of the ovule become more or less leafy in appearance (see p. 262), the inner investments become more or less atrophied, or are even more frequently entirely suppressed, as is also the nucleus.

In other cases, a simple arrest of development takes place; the ovule, for instance, which should be anatropal, remains straight, while the integuments, checked in their development, form imperfect sheaths from which the shrivelled nucleus protrudes.

=Depauperated Ferns.=--The preceding illustrations have been taken from flowering plants chiefly, but a similar defective development is manifested in cryptogamous plants. The contraction and imperfect development of the fronds of some varieties of ferns, hence called depauperated, may receive passing notice, as also the cases in which the sori or clusters of spore cases are denuded of their usual covering, owing to the abortion or imperfect development of the indusium, as in what are termed exindusiate varieties.[543]

=General remarks on abortion, coincident changes, &c.=--Reference has already been made, while treating of hypertrophy, suppression, &c., to certain other changes affecting the flower at the same time. Atrophy of one organ or set of organs, for instance, is frequently accompanied by a compensating hypertrophy or by an increased number of other parts. In the feather-hyacinth, _Muscari comosum_, var., _monstrosum_, the absence of flowers is compensated for by the inordinate formation of brightly coloured threads which appear to be modified pedicels (see pp. 347, 348); so also in the wig plant, _Rhus Cotinus_. So the atrophy of the stamens, in some flowers, is coincident with the hypertrophy of the pistil. Thus, Unger, 'Denkschr. d. Kais. Acad. der Wissensch. Math. Nat. Classe,' Mai 25, 1848, p. 103, tab. ix, describes a case wherein the corolla and stamens of _Desmodium marylandicum_ were atrophied, while the calyx and legume, on the other hand, were hypertrophied.

Fusion of the members of one whorl with one another, or with the components of an adjacent series, often entails atrophy or suppression, either in the united organs themselves, or in adjacent ones. A foliaceous condition of the outer portions of a flower is very generally attended by atrophy or complete suppression of the inner portions.

From this point of view the observations of Morren[544] on the different degrees of atrophy up to complete suppression, observable in the flowers of _Bellevalia comosa_, are of importance. According to this observer, the most highly differentiated parts, such as the stigmas, the ovules, and the anthers, are the first to disappear, the filaments often being developed without anthers. Ultimately a deformed and empty perianth alone remains. In the ordinary course of things the mouth of the perianth is open, but in some of these malformations it is closed, and when that happens, the effects of atrophy are the more observable in the stamens and pistils.

The impotence of the pollen in certain atrophied flowers, as noticed by the same observer, is of much interest, especially in reference to the sexual relationship between the different forms in polymorphic flowers as studied by Mr. Darwin.

A change in direction may also be noted as a common accompaniment of atrophy or suppression; thus, in a capsule of _Veronica Beccabunga_, which was one-celled by the abortion of one carpel, the style was lateral instead of terminal.

As to the causes of these structural deviations but little is known; certain of them have been already alluded to. In some cases atrophy and suppression maybe regarded as permanent states of a condition usually transitory, but this is clearly not always the case. Among external causes anything bringing about an enfeebled condition might be supposed to lead to atrophy, or suppression of some parts.

Gaertner[545] attributes the arrested development and fall of flowers to some among the following causes:--1. non-application of the pollen of the same variety, and consequent imperfect fertilisation; 2. any considerable injury to the calyx, &c.; 3. destruction of the style or stigma before the fertilisation of the ovary; 4. application to the stigma of imperfect or heterogeneous pollen or indifferent pulverulent matter; 5. defective conceptive power in the ovary.

Abortion of the ovules is considered by the same authority to be due to--1. deficiency of heat; 2. excess of moisture; 3. peculiar formation of the ovary; 4. over-luxuriant development of roots or buds; 5. peculiar conditions of cultivation; thus, cuttings and layers produce sterile and abortive seeds much more frequently than plants of the same species raised from seed; 6. abortion of the seed is often combined with luxuriant development of the walls of the fruit.

Temperature and climatal changes in general seem not to be without effect, as has been already mentioned in the case of _Arenaria tetraquetra_, which is polygamous when growing in mountain districts. Other illustrations of a similar character are mentioned under the head of Heterogamy (p. 196).

Pressure has been already alluded to as one of the most obvious of the inducing causes of atrophy and suppression.

In the case of _Ranunculus auricomus_ before cited, in which the petals are rarely perfect, M. de Rochebrune considers that the deficiencies in question depend, in great measure, on the amount of moisture in the localities where the plant grows. In most places the flowers and carpels are apt to become more or less abortive, while the leaves are luxuriant; while, in dry places, the foliage is small, but the flowers are more perfect. This is quite consonant with other facts relating to the development of flowers or of leaves in general.

But while external agencies undoubtedly play some part in bringing about these changes, it is almost certain that internal causes inherent to the organization of the plant are more important. Mr. Darwin[546] accounts for the existence of rudimentary organs by the operation of the general rule of inheritance, and explains their stunted condition as the effect of disuse, not so much, of course, in the particular flower as in its predecessors. This disuse may be the result of the superior efficacy of foreign pollen as contrasted with that formed in the individual flower itself. In this way many hermaphrodite flowers tend to become dioecious, as in _Caryophyllaceæ_, _Orchidaceæ_, _Plantaginaceæ_, _Primulaceæ_ and other orders.

Although many of the circumstances above mentioned apply to plants whose structure is habitually rudimentary, there is no reason why they may not, under due restrictions, be applied to plants whose organs are only occasionally defective.

For further remarks on the subject of Abortion, the reader is referred to the sections relating to suppression, etc., also to Moquin-Tandon, 'El. Terat. Veget.,' p. 120; C. Morren, "De l'atrophie en général," in 'Bull. Acad. Belg.,' t. xviii, 1851,