Vegetable Teratology. Maxwell T. Masters
expressed by the following symbol:
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Thus, while in both the annular and the spiral modes of development the individual members of each complete series necessarily alternate one with another, in the former case the series themselves alternate, while in the successive arrangement they are placed directly one over the other. There are, of course, exceptions, but the rule is as has been stated, and the effect is to prevent one leaf from interfering with the development and growth of its neighbours.
In the case of the whorled or simultaneous arrangement the conditions of growth must be uniform on all sides, but in the successive or spiral disposition the conditions influencing growth act with unequal force, on different sides of the stem, at the same time. In the whorl there is an illustration of radiating symmetry, while in the spiral arrangement there is a transition to the bilateral symmetry. There are frequent passages from one to the other even under normal circumstances; thus, while the one arrangement obtains in the ordinary leaves, the parts of the flower may be disposed according to the other method. In the annular disposition it generally happens that the rings are separated one from the other by the development of the stem between them, the internodes between the constituent leaves themselves of course being undeveloped; on the other hand, in the spiral or successive arrangement there is no such alternate growth and arrest of growth of the stem between the leaves, or between successive cycles, but the growth is, under favorable conditions, continuous—leaf is separated from leaf, and cycle from cycle, by the continually elongating stem. Thus, the two modes of growth correspond precisely with those observed in the case of definite and indefinite inflorescence respectively.
Fig. 1.—Diagram showing the arrangement of parts in a complete, regular, pentamerous flower: s, sepals; p, petals; st, stamens; o, ovaries.
The same arrangements, that are observed in the disposition of the leaves, apply equally well to the several parts of the flower; thus, in what is for convenience considered the typical flower, there is a calyx of five or more distinct sepals, equal in size, and arranged in a whorl, a corolla of a similar number of petals alternating with the sepals, five stamens placed in the same position with reference to the petals, and five carpels alternating with the stamens. Throughout this book this arrangement is taken as the standard of reference. Nevertheless the spiral order does occur in the floral leaves as well as in those of the stem; it often happens, especially when the organs are numerous, that they form spiral series; and the same holds good very generally, when the parts of the flower are uneven in number, as in the very common quincuncial arrangement of the sepals, &c.
To these general remarks, intended to show the agreement between the disposition of the leaves of the stem and those of the flower, it is merely necessary to add that the arrangement of the placentas, as well as that of the ovules borne on them, is also definite, and takes place according to methods explained in all the text-books, and on which, therefore, it is not necessary to dilate in this place.
The branches of the stem or axis correspond for the most part in disposition with that of the leaves from the axils of which they originate, subject, however, to numerous disturbing causes, and to alterations from the usual or typical order brought about by the development of buds. These latter organs, as it seems, may be found in almost any situation, though their ordinary position is in the axil of a leaf or at the end of a stem or branch.
The points just mentioned are of primary importance in structural botany, and as such are seized on not only by the morphologist, but by the systematic botanist, who finds in them the characters by which he may separate one group from another. Thanks to the labours of those observers who have devoted their attention to that difficult but most important branch of study, organogeny, or the investigation of the development of the various organs, and to the researches of the students of comparative anatomy or morphology, the main principles regulating the arrangement and form of the organs of flowering plants seem to be fairly well established, though in matters of detail much remains to be cleared up, even in such important points as the share which the axis takes in the construction of the flower and fruit, the nature of the placenta, the construction of the ovules, and other points.
The facts already known justify the adoption of a standard or typical arrangement as just mentioned. The intrinsic value of this type is shown by the facility with which all varieties of form or arrangement may be explained by reference to certain modifications of it. It must, however, be considered as an abstraction, and should be looked on in the light rather of a scaffolding, which enables us to see the building and its several parts, than of the edifice itself, but which latter, from our imperfect knowledge and limited powers, we could not see without some such assistance.
The typical form may be, hypothetically at least, considered as the primitive one transmitted by hereditary descent from generation to generation, and modified to suit the requirements of the individual, or in accordance with circumstances. If it be borne in mind that it is but an artificial contrivance, more or less true—a means to an end, and not the end itself—no harm will arise from its employment; and as knowledge increases, or as circumstances demand, the hypothetical type can be replaced by another more in accordance with the actual state of science.
Teratological changes in the arrangement of organs depend upon arrest of growth, as when parts usually spirally arranged remain verticillate, owing to the non-development of the internodes, or to excessive growth, or development; but in many instances it is impossible, without studying the development of the malformed flower, to ascertain whether the altered arrangement is due to an excessive or to a diminished action. Practically, however, it is of comparatively little importance to know whether, say, the isolation of parts, that are usually combined together, is congenital (i.e. the result of an arrest of growth preventing their union), or whether it be due to a separation of parts primitively undivided; the effect remains the same, though the cause may have been very different.
The principal alterations to be mentioned under this head may therefore be conveniently arranged under the following categories:—Union, Independence, Displacement, Prolification, Heterotaxy, and Heterogamy.
PART I.
UNION OF ORGANS.
The union of parts, usually separate in their adult condition, is of very common occurrence as a malformation. The instances of its manifestation admit of being grouped under the heads of Cohesion, where parts of the same whorl, or of the same organ, are united together; and of Adhesion, where the union takes place between members of different whorls, or between two or more ordinarily wholly detached and distinct parts. In either case, the apparent union may be congenital (that is, the result of a primitive integrity or a lack of separation), or it may really consist in a coalition of parts originally distinct and separate. In practice it is not always easy to distinguish between these two different conditions. Indeed, in most cases it cannot be done without tracing the development of the flower throughout all its stages. It is needless to make more than a passing allusion to the frequency with which both congenital integrity or subsequent coalescence of organs exist under ordinary circumstances. Considered as a teratological phenomenon, union admits of being grouped into several subdivisions, such as Cohesion, Adhesion, Synanthy, Syncarpy, Synophty, &c. Each of these subdivisions will be separately treated, but it maybe here said that, in all or any case, the degree of fusion may be very slight, or it may be so perfect that there may be a complete amalgamation of two or more parts, while to all outward appearance the organ may be single. The column of Orchids may be referred to as an illustration under natural circumstances of the complete union of many usually distinct parts.
In the uncertainty that exists in many cases as to the real nature of the occurrence, it would be idle to attempt to explain the causes of fusions. It is clear, however, that an arrest of development will tend towards the maintenance of primordial