Part 22

Forms of inflorescence occur, in which both the definite and indefinite types are represented--_mixed_ inflorescences. Thus in Composite plants, such as hawk weeds (_Hieracia_) and ragworts (_Senecio_, fig. 21), the _heads_ of flowers, taken as a whole, are developed centrifugally, the terminal head first, while the _florets_, or small flowers on the receptacle, open centripetally, those at the circumference first. So also in Labiatae, such as dead-nettle (_Lamium_), the different whorls of inflorescence are developed centripetally, while the florets of the verticillaster are centrifugal. This mixed character presents difficulties in such cases as Labiatae, where the leaves, in place of retaining their ordinary form, become bracts, and thus might lead to the supposition of the whole series of flowers being one inflorescence. In such cases the cymes are described as spiked, racemose, or panicled, according to circumstances. In _Saxifraga umbrosa_ (London-pride) and in the horse-chestnut we meet with a raceme of scorpioid cymes; in sea-pink, a capitulum of contracted scorpioid cymes (often called a glomerulus); in laurustinus, a compound umbel of dichasial cymes; a scorpioid cyme of capitula in _Vernonia scorpioides_. The so-called catkins of the birch are, in reality, spikes of contracted dichasial cymes. In the bell-flower (_Campanula_) there is a racemose uniparous cyme. In the privet (_Ligustrum vulgare_) there are numerous racemes of dichasia arranged in a racemose manner along an axis; the whole inflorescence thus has an appearance not unlike a bunch of grapes, and has been called a _thyrsus_.

TABULAR VIEW OF INFLORESCENCES

A. Indefinite Centripetal Inflorescence. I. Flowers solitary, axillary. _Vinca_, _Veronica hederifolia_. II. Flowers in groups, pedicellate. 1. Elongated form (Raceme), _Hyacinth_, _Laburnum_, _Currant_. (Corymb), _Ornithogalum_. 2. Contracted or shortened form (Umbel), _Cowslip_, _Astrantia_. III. Flowers in groups, sessile. 1. Elongated form (Spike), _Plantago_. (Spikelet), _Grasses_. (Amentum, Catkin), _Willow_, _Hazel_. (Spadix) _Arum_, some _Palms_. (Strobilus), _Hop_. 2. Contracted or shortened form (Capitulum), _Daisy_, _Dandelion_, _Scabious_. IV. Compound Indefinite Inflorescence. a. Compound Spike, _Rye-grass_. b. Compound Spadix, _Palms_. c. Compound Raceme, _Astilbe_. d. Compound Umbel, _Hemlock_ and most _Umbelliferae_. e. Raceme of Capitula, _Petasites_. f. Raceme of Umbels, _Ivy_. B. Definite Centrifugal Inflorescence. I. Flowers solitary, terminal. _Gentianella_, _Tulip_. II. Flowers in Cymes. 1. Uniparous Cyme. a. Helicoid Cyme (axes forming a spiral). Elongated form, _Alstromeria_. Contracted form, _Witsenia corymbosa_.

b. Scorpioid Cyme (axes unilateral, two rows). Elongated form, _Forget-me-not_, _Symphytum_, _Henbane_. Contracted form, _Erodium_, _Alchemilla arvensis_. 2. Biparous Cyme (Dichotomous), including 3-5 chotomous Cymes (Dichasium, Cymose Umbel, Anthela). a. Elongated form, _Cerastium_, _Stellaria_. b. Contracted form (Verticillaster), _Dead-nettle_, _Pelargonium_. 3. Compound Definite Inflorescence. _Streptocarpus polyanthus_, many _Calceolarias_. C. Mixed Inflorescence. Raceme of Scorpioid Cymes, _Horse-chestnut_. Scorpioid Cyme of Capitula, _Vernonia scorpioides_. Compound Umbel of Dichotomous Cymes, _Laurustinus_. Capitulum of contracted Scorpioid Cymes (Glomerulus), _Sea-pink_.

The flower.

The flower consists of the floral axis bearing the sporophylls (stamens and carpels), usually with certain protective envelopes. The axis is usually very much contracted, no internodes being developed, and the portion bearing the floral leaves, termed the _thalamus_ or _torus_, frequently expands into a conical, flattened or hollowed expansion; at other times, though rarely, the internodes are developed and it is elongated. Upon this torus the parts of the flower are arranged in a crowded manner, usually forming a series of verticils, the parts of which alternate; but they are sometimes arranged spirally especially if the floral axis be elongated. In a typical flower, as in fig. 22, we recognize four distinct whorls of leaves: an outer whorl, the _calyx_ of _sepals_; within it, another whorl, the parts alternating with those of the outer whorl, the _corolla_ of _petals_; next a whorl of parts alternating with the parts of the corolla, the _androecium_ of _stamens_; and in the centre the _gynoecium_ of _carpels_. Fig. 23 is a diagrammatic representation of the arrangement of the parts of such a flower; it is known as a _floral diagram_. The flower is supposed to be cut transversely, and the parts of each whorl are distinguished by a different symbol. Of these whorls the two internal, forming the sporophylls, constitute the _essential_ organs of reproduction; the two outer whorls are the protective coverings or floral _envelopes_. The sepals are generally of a greenish colour; their function is mainly protective, shielding the more delicate internal organs before the flower opens. The petals are usually showy, and normally alternate with the sepals. Sometimes, as usually in monocotyledons, the calyx and corolla are similar; in such cases the term _perianth_, or _perigone_, is applied. Thus, in the tulip, crocus, lily, hyacinth, we speak of the parts of the perianth, in place of calyx and corolla, although in these plants there is an outer whorl (calyx), of three parts, and an inner (corolla), of a similar number, alternating with them. When the parts of the calyx are in appearance like petals they are said to be _petaloid_, as in Liliaceae. In some cases the petals have the appearance of sepals, then they are _sepaloid_, as in Juncaceae. In plants, as _Nymphaea alba_, where a spiral arrangement of the floral leaves occurs, it is not easy to say where the calyx ends and the corolla begins, as these two whorls pass insensibly into each other. When both calyx and corolla are present, the plants are _dichlamydeous_; when one only is present, the flower is termed _monochlamydeous_ or _apetalous_, having no petals (fig. 24). Sometimes both are absent, when the flower is _achlamydeous_, or naked, as in willow. The outermost series of the essential organs, collectively termed the _androecium_, is composed of the microsporophylls known as the staminal leaves or _stamens_. In their most differentiated form each consists of a stalk, the _filament_ (fig. 25, f), supporting at its summit the anther (a), consisting of the pollen-sacs which contain the powdery _pollen_ (p), the microspores, which is ultimately discharged therefrom. The _gynoecium_ or _pistil_ is the central portion of the flower, terminating the floral axis. It consists of one or more _carpels_ (megasporophylls), either separate (fig. 22, c) or combined (fig. 24). The parts distinguished in the pistil are the _ovary_ (fig. 26, o), which is the lower portion enclosing the _ovules_ destined to become seeds, and the _stigma_ (g), a portion of loose cellular tissue, the receptive surface on which the pollen is deposited, which is either sessile on the apex of the ovary, as in the poppy, or is separated from it by a prolonged portion called the _style_ (s). The androecium and gynoecium are not present in all flowers. When both are present the flower is hermaphrodite; and in descriptive botany such a flower is indicated by the symbol [female/male]. When only one of those organs is present the flower is _unisexual_ or _diclinous_, and is either male (_staminate_), [male] or female (_pistillate_), [female]. A flower then normally consists of the four series of leaves--calyx, corolla, androecium and gynoecium--and when these are all present the flower is _complete_. These are usually densely crowded upon the thalamus, but in some instances, after apical growth has ceased in the axis, an elongation of portions of the receptacle by intercalary growth occurs, by which changes in the position of the parts may be brought about. Thus in _Lychnis_ an elongation of the axis betwixt the calyx and the corolla takes place, and in this way they are separated by an interval. Again, in the passion-flower (_Passiflora_) the stamens are separated from the corolla by an elongated portion of the axis, which has consequently been termed the _androphore_, and in _Passiflora_ also, fraxinella (fig. 27), Capparidaceae, and some other plants, the ovary is raised upon a distinct stalk termed the _gynophore_; it is thus separated from the stamens, and is said to be _stipitate_. Usually the successive whorls of the flower, disposed from below upwards or from without inwards upon the floral axis, are of the same number of parts, or are a multiple of the same number of parts, those of one whorl alternating with those of the whorls next it.

In the more primitive types of flowers the torus is more or less convex, and the series of organs follow in regular succession, culminating in the carpels, in the formation of which the growth of the axis is closed (fig. 28). This arrangement is known as hypogynous, the other series (calyx, corolla and stamens) being beneath (_hypo-_) the gynoecium. In other cases, the apex of the growing point ceases to develop, and the parts below form a cup around it, from the rim of which the outer members of the flower are developed around (_peri-_) the carpels, which are formed from the apex of the growing-point at the bottom of the cup. This arrangement is known as _perigynous_ (fig. 29). In many cases this is carried farther and a cavity is formed which is roofed over by the carpels, so that the outer members of the flower spring from the edge of the receptacle which is immediately above the ovary (epigynous), hence the term epigyny (fig. 30).

Symmetry of the flower.

When a flower consists of parts arranged in whorls it is said to be _cyclic_, and if all the whorls have an equal number of parts and are alternate it is _eucyclic_ (figs. 22, 23). In contrast to the cyclic flowers are those, as in Magnoliaceae, where the parts are in spirals (_acyclic_). Flowers which are cyclic at one portion and spiral at another, as in many Ranunculaceae, are termed _hemicyclic_. In spiral flowers the distinction into series is by no means easy, and usually there is a gradual passage from sepaloid through petaloid to staminal parts, as in the water-lily family, Nymphaeaceae (figs. 31, 32), although in some plants there is no such distinction, the parts being all petaloid, as in _Trollius_. Normally, the parts of successive whorls alternate; but in some cases we find the parts of one whorl opposite or _superposed_ to those of the next whorl. In some cases, as in the vine-family Ampelidaceae, this seems to be the ordinary mode of development, but the superposition of the stamens on the sepals in many plants, as in the pink family, Caryophyllaceae, is due to the suppression or abortion of the whorl of petals, and this idea is borne out by the development, in some plants of the order, of the suppressed whorl. As a rule, whenever we find the parts of one whorl superposed on those of another we may suspect some abnormality.

A flower is said to be _symmetrical_ when each of its whorls consists of an equal number of parts, or when the parts of any one whorl are multiples of that preceding it. Thus, a symmetrical flower may have five sepals, five petals, five stamens and five carpels, or the number of any of these parts may be ten, twenty or some multiple of five. Fig. 23 is a diagram of a symmetrical flower, with five parts in each whorl, alternating with each other. Fig. 33 is a diagram of a symmetrical flower of stone-crop, with five sepals, five alternating petals, ten stamens and five carpels. Here the number of parts in the staminal whorl is double that in the others, and in such a case the additional five parts form a second row alternating with the others. In the staminal whorl especially it is common to find additional rows. Fig. 34 shows a symmetrical flower, with five parts in the three outer rows, and ten divisions in the inner. In this case it is the gynoecium which has an additional number of parts. Fig. 35 shows a flower of heath, with four divisions of the calyx and corolla, eight stamens in two rows, and four divisions of the pistil. In fig. 36 there are three parts in each whorl; and in fig. 37 there are three divisions of the calyx, corolla and pistil, and six stamens in two rows. In all these cases the flower is symmetrical. In Monocotyledons it is usual for the staminal whorl to be double, it rarely having more than two rows, whilst amongst dicotyledons there are often very numerous rows of stamens. The floral envelopes are rarely multiplied. Flowers in which the number of parts in each whorl is the same, are _isomerous_ (of equal number); when the number in some of the whorls is different, the flower is _anisomerous_ (of unequal number). The pistillate whorl is very liable to changes. It frequently happens that when it is fully formed, the number of its parts is not in conformity with that of the other whorls. In such circumstances, however, a flower has been called symmetrical, provided the parts of the other whorls are normal,--the permanent state of the pistil not being taken into account in determining symmetry. Thus fig. 38 shows a pentamerous symmetrical flower, with dimerous pistil. Symmetry, then, in botanical language, has reference to a certain definite numerical relation of parts. A flower in which the parts are arranged in twos is called _dimerous_; when the parts of the whorls are three, four or five, the flower is _trimerous_, _tetramerous_ or _pentamerous_, respectively. The symmetry which is most commonly met with is trimerous and pentamerous--the former occurring generally among monocotyledons, the latter among dicotyledons. Dimerous and tetramerous symmetry occur also among dicotyledons.

The various parts of the flower have a certain definite relation to the axis. Thus, in axillary tetramerous flowers (fig. 35), one sepal is next the axis, and is called _superior_ or _posterior_; another is next the bract, and is _inferior_ or _anterior_, and the other two are _lateral_; and certain terms are used to indicate that position. A plane passing through the anterior and posterior sepal and through the floral axis is termed the _median plane_ of the flower; a plane cutting it at right angles, and passing through the lateral sepals, is the _lateral plane_; whilst the planes which bisect the angles formed by the lateral and median planes are the _diagonal planes_, and in these flowers the petals which alternate with the sepals are cut by the diagonal planes.

In a pentamerous flower one sepal may be superior, as in the calyx of Rosaceae and Labiatae; or it may be inferior, as in the calyx of Leguminosae (fig. 39)--the reverse, by the law of alternation, being the case with the petals. Thus, in the blossom of the pea (figs. 39, 40), the odd petal (vexillum) st is superior, while the odd sepal is inferior. In the order Scrophulariaceae one of the two carpels is posterior and the other anterior, whilst in Convolvulaceae the carpels are arranged laterally. Sometimes the twisting of a part makes a change in the position of other parts, as in Orchids, where the twisting of the ovary changes the position of the labellum.

When the different members of each whorl are like in size and shape, the flower is said to be _regular_; while differences in the size and shape of the parts of a whorl make the flower _irregular_, as in the papilionaceous flower, represented in fig. 39. When a flower can be divided by a single plane into two exactly similar parts; then it is said to be _zygomorphic_. Such flowers as Papilionaceae, Labiatae, are examples. In contrast with this are _polysymmetrical_ or actinomorphic flowers, which have a radial symmetry and can be divided by several planes into several exactly similar portions; such are all regular, symmetrical flowers. When the parts of any whorl are not equal to or some multiple of the others, then the flower is _asymmetrical_. This want of symmetry may be brought about in various ways. Alteration in the symmetrical arrangement as well as in the completeness and regularity of flowers has been traced to _suppression_ or the _non-development_ of parts, _degeneration_ or imperfect formation, _cohesion_ or union of parts of the same whorl, _adhesion_ or union of the parts of different whorls, _multiplication_ of parts, and _deduplication_ (sometimes called _chorisis_) or splitting of parts.

By _suppression_ or non-appearance of a part at the place where it ought to appear if the structure was normal, the symmetry or completeness of the flower is disturbed. This suppression when confined to the parts of certain verticils makes the flower asymmetrical. Thus, in many Caryophyllaceae, as _Polycarpon_ and _Holosteum_, while the calyx and corolla are pentamerous, there are only three or four stamens and three carpels; in _Impatiens Noli-me-tangere_ the calyx is composed of three parts, while the other verticils have five; in labiate flowers there are five parts of the calyx and corolla, and only four stamens; and in _Tropaeolum pentaphyllum_ there are five sepals, two petals, eight stamens and three carpels. In all these cases the want of symmetry is traced to the suppression of certain parts. In the last-mentioned plant the normal number is five, hence it is said that there are three petals suppressed, as shown by the position of the two remaining ones; there are two rows of stamens, in each of which one is wanting; and there are two carpels suppressed. In many instances the parts which are afterwards suppressed can be seen in the early stages of growth, and occasionally some vestiges of them remain in the fully developed flower. By the suppression of the verticil of the stamens, or of the carpels, flowers become _unisexual_ or _diclinous_, and by the suppression of one or both of the floral envelopes, monochlamydeous and achlamydeous flowers are produced. The suppression of parts of the flower may be carried so far that at last a flower consists of only one part of one whorl. In the Euphorbiaceae we have an excellent example of the gradual suppression of parts, where from an apetalous, trimerous, staminal flower we pass to one where one of the stamens is suppressed, and then to forms where two of them are wanting. We next have flowers in which the calyx is suppressed, and its place occupied by one, two or three bracts (so that the flower is, properly speaking, achlamydeous), and only one or two stamens are produced. And finally, we find flowers consisting of a single stamen with a bract. There is thus traced a _degradation_, as it is called, from a flower with three stamens and three divisions of the calyx, to one with a single bract and a single stamen.

_Degeneration_, or the transformation of parts, often gives rise either to an apparent want of symmetry or to irregularity in form. In unisexual flowers it is not uncommon to find vestiges of the undeveloped stamens in the form of filiform bodies or scales. In double flowers transformations of the stamens and pistils take place, so that they appear as petals. In _Canna_, what are called petals are in reality metamorphosed stamens. In the capitula of Compositae we sometimes find the florets converted into green leaves. The limb of the calyx may appear as a rim, as in some Umbelliferae; or as pappus, in Compositae and _Valeriana_. In _Scrophularia_ the fifth stamen appears as a scale-like body; in other Scrophulariaceae, as in _Pentstemon_, it assumes the form of a filament, with hairs at its apex in place of an anther.

_Cohesion_, or the union of parts of the same whorl, and _adhesion_, or the growing together of parts of different whorls, are causes of change both as regards form and symmetry. Thus in _Cucurbita_ the stamens are originally five in number, but subsequently some cohere, so that three stamens only are seen in the mature flower. Adhesion is well seen in the _gynostemium_ of orchids, where the stamens and stigmas adhere. In Capparidaceae the calyx and petals occupy their usual position, but the axis is prolonged in the form of a gynophore, to which the stamens are united.