CHAPTER XLII.

THE FLORAL SHOOT.

I. The Parts of the Flower.

The portion of the stem on which the flowers are borne is the _flower_ shoot or axis, or taken together with the flowers, it is known as the _Flower Cluster_.

=804. The flower.=—The flower is best understood by an examination, first of one of the types known as a “complete” flower, as in the buttercup, the spring-beauty, the blood-root, the apple, the rose, etc.

There are two sets of organs or members in the complete flower—(1) the floral envelope; (2) the essential or necessary members or organs.

The floral envelope when complete consists of—1st, an outer envelope, the _calyx_, made up of several leaf-like structures (_sepals_), very often possessing chlorophyll, which envelop all the other parts of the flower when in bud; 2d, an inner envelope, the _corolla_, also made up of several leaf-like parts (_petals_), usually bright colored and larger than the sepals. The outer and inner floral envelopes are usually in whorls (though in close spirals in many of the buttercup family, etc.), and for reasons discussed elsewhere (Chapter XXXIV) represent leaves. The essential or necessary members of the flower are also usually in whorls and likewise represent leaves, but only in rare cases is there any suggestion, either in their form or color, of a leaf relationship. These members are in two sets: (1) The outer, or _andrœcium_, consisting of a few or many parts (_stamens_); (2) the inner set, the _gynœcium_, consisting of a few or many parts (_carpels_).

=805. Purpose of the flower.=—While the ultimate purpose of all plants is the production of seed or its equivalent through which the plant gains distribution and perpetuation, the flower is the specialized part of the seed plant which utilizes the food and energies contributed by other members of the plant organization for the production of seed. In addition to this there are definite functions performed by the members of the flower, which come under the general head of plant work, or flower work.

=806. The calyx, or the sepals.=—These are chiefly protective, affording protection to the young stamens and carpels in the flower bud. Where the corolla is absent, sepals are usually present and then assume the function of the petals. In a few instances the calyx may possibly ultimately join in the formation of the fruit (examples: the butternut, walnut, hickory).

=807. The corolla, or petals.=—The petals are partly protective in the bud, but their chief function where well developed seems to be that of attracting insects, which through their visits to the flower aid in “_pollination_,” especially “_cross pollination_.”

=808. The stamens.=—The stamens (= microsporophylls) are flower organs for the production of _pollen_, or _pollen-spores_ (= microspores). The _stalk_ (not always present) is the _filament_, the _anther_ is borne on the filament when the latter is present. The anther consists of the _anther sacs_ or _pollen sacs_ (microsporangium) containing the pollen-spores, and the _connective_, the sterile tissue lying between and supporting the anther sac. The stamens are usually separate, but sometimes they are united by their filaments, or by their anthers. When the pollen is ripe they open by slits or pores and the pollen is scattered; or in rarer cases the pollen mass (_pollinium_) is removed through the agency of insects (see Insect pollination, Chap. XLIII).

=809. The pistil.=—The pistil consists of the “_ovary_,” the _style_ (not always present), and the _stigma_. These are well shown in a _simple pistil_, common examples of which are found in the buttercup, marsh marigold, the pea, bean, etc. The simple pistil is equivalent to a _carpel_ (= macrosporophyll), while the _compound pistil_ consists of two or several carpels joined, as in the toothwort, trillium, lily, etc. The _ovary_ is the enlarged part which below is attached to the receptacle of the flower, and contains within the _ovules_. The _style_, when present, is a slender elongation of the upper end of the ovary. The _stigma_ is supported on the end of the style when the latter is present. It is often on a capitate enlargement of the style or extends down one side, or when the style is absent it is usually seated directly on the upper end of the ovary. The stigmatic surface is glutinous or “sticky,” and serves to hold the pollen-spores when they come in contact with it.

The _ovules_ are within the ovary and are arranged in different ways in different plants. The pollen grain (or better pollen-spore = microspore), after it has been transferred to the stigma, “germinates,” and the pollen tube grows down through the tissue of the stigma and style, or courses down the stylar canal until it reaches the ovule. Here it usually enters the ovule (macrosporangium) at the _micropyle_ (in some of the ament-bearing plants it enters at the _chalaza_), and the sperm cells are emptied into the embryo sac in the interior of the ovule.

=810. Fertilization.=—One of the sperms unites with the egg in the embryo sac. This is _fertilization_, and from the fertilized egg the young embryo is formed still within the ovule. _Double fertilization_,—the other sperm cell sometimes unites with one or both of the “polar” nuclei which have united to form the “definitive” or “endosperm” nucleus. As a result of fertilization, the embryo plant is formed within the ovule, the coats of which enlarge by growth forming the seed coats, and altogether forming the seed. (See Chapters XXXIV, XXXV, XXXVI.)

II. Kinds of Flowers.

=811. Absence of certain flower parts.=—The _complete_ flower contains all the four series of parts. When any one of the series of parts is lacking, the flower is said to be _incomplete_. Where only one series of the floral envelopes is present the flowers are said to be _apetalous_ (the petals are absent), examples: elm, buckwheat, etc. Flowers which lack both floral envelopes are _naked_. When pistils are absent but stamens are present the flowers are _staminate_, whether floral envelopes are present or not; and so when stamens are absent and pistils present the flower is _pistillate_. If both stamens and pistils are absent the flower is said to be _sterile_ or _neutral_ (snowball, marginal or showy flowers in hydrangea). Flowers with both stamens and pistils, whether or not they have floral envelopes, are _perfect_ (or hermaphrodite), so if only one of these sets of _essential organs_ of the flower is present the flower is _imperfect_, or _diclinous_. Sometimes the imperfect, or diclinous, flowers are on the same plant, and the plant is said to be _monœcious_ (of one household). When staminate flowers are on certain individual plants, and the pistillate flowers of the same species are on other individuals, the plant is _diœcious_ (or of two households). When some of the flowers of a plant are diclinous and others are perfect, they are said to be _polygamous_.

Many of these variations relating to the presence or absence of flower parts in one way or another contribute to the well-being of the plant. Some indicate a division of labor; thus in the neutral flowers of certain species of hydrangea or viburnum, the showy petals serve to attract insects which aid in the pollination of the fertile flowers. It must not be understood, however, that all variations in plants which results in new or different forms of flowers is for the good of the species. For example, under cultivation the flowers of viburnum and hydrangea sometimes are all neutral and showy. While such variations sometimes contribute to the happiness of man, the plant has lost the power of developing seed. In diclinous flowers cross pollination is necessitated.

=812. Form of the flower.=—The flower as a whole has _form_. This is so characteristic that in general all flowers of the different individuals of a species are of the same shape, though they may vary in size. In general, flowers of closely related plants of different species are of the same type as to form, so that often in the shape of the flower alone we can see the relationship of kind, though the form of the flower is not the most important nor always the sure index of kinship. Since many flowers resemble certain familiar objects, names are often used which relate to these objects.

Flowers are said to be _regular_, or _irregular_. In a regular flower all of the parts of a set or series are of the same shape and size, while in irregular flowers the parts are of a different shape or size in some of the sets. The flowers of the pea family (_Papilionaceæ_), of the mint family (_Labiatæ_), of the morning glory, larkspur, monkshood, etc., are irregular (fig. 450). The corolla usually gives the characteristic form to the flower, and the name is usually applied to the form of the corolla.

Some of the different forms are wheel-shaped or _rotate_ corolla when the petals spread out at once like the spokes of a wheel, as in the potato, tomato, or bittersweet; _salver-shaped_ when the petals spread out at right angles from the end of a corolla tube, as in the phlox; _bell-shaped_, or _campanulate_, as in the harebell or campanula; _funnel-shaped_, as in the morning glory; _tubular_, when the ends of the petals spread but little or none from the end of the corolla tube, as in the turnip flower or in the disk florets of the composites. The _butterfly_, or _papilionaceous_ corolla is peculiar as in the pea or bean. The upper petal is the “banner,” the two lateral ones the “wings,” and the two lower the “keel.”

The _labiate_ corolla is characteristic of the mint family where the gamosepalous corolla is unequally divided, so that the two upper lobes are sharply separated from the three lower forming two “lips.” The labiate corolla of the toad-flax, or snapdragon is _personate_, or _masked_, because the lower lip arches upward like a palate and closes the entrance to the corolla tube; that of the dead nettle (_Lamium_) is _ringent_ or _gaping_, because the lips are spread wide apart. In some plants the labiate corolla is not very marked and differs but slightly from a regular form.

The _ligulate_ or _strap-shaped_ corolla is characteristic of the flowers of the dandelion or chicory, or of the ray flowers of other composites (fig. 451). The lower part of the gamosepalous corolla is tubular, and the upper part is strap-shaped, as if that part of the tube were split on one side and spread out flat.

These forms of the flower should be studied in appropriate examples.

=813. Union of flower parts.=—In the buttercup flower all the parts of each series are separate from one another and from other series of parts. Each one is attached to the _receptacle_ of the flower, which is a very much shortened portion of the flower axis. The calyx being composed of separate and distinct parts is said to be _polysepalous_, and the corolla is likewise _polypetalous_. The stamens are _distinct_, and the pistils are _simple_. In many flowers, however, there is a greater or lesser _union_ of parts.

=814. Union of parts of the same series or cycle.=—The parts _coalesce_, either slightly or to a great extent. Usually they are not so completely coalesced but what the number of parts of the series can be determined. Where the sepals are united the calyx is _gamosepalous_, when the petals are united the corolla is _gamopetalous_.

Union of the sepals or of the corolla is quite common, but union of the stamens is rare except in a few families where it is quite characteristic. When the stamens are united by their anthers, they are _syngenœsious_. This is the case in most flowers of the composite family. When all the stamens are united into one group by their filaments, they are _monadelphous_ (one brotherhood), as in hollyhock, hibiscus, cotton, marsh-mallow, etc. When they are united by their filaments in two groups, they are _diadelphous_ (two brotherhoods), as in the pea and most members of the pea family. In most species of St. John’s wort (Hypericum), the stamens are united in threes (_triadelphous_).

=815. The carpels are often united.=—The pistil is then said to be _compound_. Where the pistils are consolidated, usually the adjacent walls coalesce and thus separate the cavity of each ovary. Each cavity in the compound pistil is a _locule_. In some cases the adjacent walls disappear so that there is one common cavity for the compound pistil (examples: purslane, chickweeds, pinks, etc.). In a few cases there is a false partition (example, in the toothwort and other crucifers). The compound pistil is very often lobed slightly, so that the different pistils can be discerned. More often the styles or stigmas are distinct, and thus indicate the number of pistils united.

=816. Union of the parts of different series.=—While in the buttercup and many other flowers, all the different parts are inserted on the torus or receptacle, in other flowers one series of parts may be joined to another. This is _adnation_ of parts, or the two or more series are _adnate_. In the morning glory the stamens are inserted on the inner face of the corolla tube; the same is true in the mint family, and there are many other examples. The insertion of parts, whether free or adnate, is usually spoken of in reference to their relation to the pistil. Thus, in the buttercup the floral envelopes and stamens are all free and _hypogynous_, they are _below_ the pistil. The pistil in this case is _superior_. In the cherry, pear, etc., the petals and stamens are borne on the edge of the more or less elevated tube of the calyx, and are said to be _perigynous_, i.e., around the pistil. In the cranberry, huckleberry, etc., the calyx is for the most part united with the wall of the ovary with the short calyx limbs projecting from the upper surface. The petals and stamens are inserted on the edge of the calyx above the ovary; they are, therefore, _epigynous_, and the ovary being under the calyx, as it were, is _inferior_.

III. Arrangement of Flowers, or Mode of Inflorescence.

=817. Flowers are solitary or clustered.=—_Solitary flowers_ are more simple in their arrangement, i.e., it is easier for us to determine and name their relation to each other and to other parts of the plant. They are either _axillary_, i.e., on short lateral shoots in the axils of ordinary foliage leaves, or they are _terminal_, i.e., they are borne on the end of the main axis of an ordinary foliage shoot. In either case they are so far separated, and the foliage leaves are so prominent, they do not form recognizable groups or clusters. The manner of arrangement of flowers on the shoot is called _inflorescence_, while the group of flowers so arranged is the _flower cluster_.

Two different modes of inflorescence are usually recognized in the arrangement of flowers on the stem. (1) The _corymbose_, or _indeterminate inflorescence_ (also indefinite inflorescence), in which the flowers arise from axillary buds, and the terminal bud may continue to grow. (2) The _cymose_ or _determinate inflorescence_ (also _definite inflorescence_) in which the flowers arise from terminal buds. This arrests the growth of the shoot in length.

There are several advantages to the plant in the different modes of inflorescence, chief among which is the massing of the flowers, thus increasing the chances for effective pollination.

A. FLOWER CLUSTERS WITH INDETERMINATE INFLORESCENCE.

=818. The simplest mode of indeterminate inflorescence= is where the flowers arise in the axils of normal foliage leaves, while the terminal bud, as in the florist’s smilax, the bellwort, moneywort, apricot, etc., continues to grow. The flowers are _solitary_ and _axillary_. In other cases which are far more numerous, the flowers are associated into more or less definite clusters in which are a number of recognizable types. The word type used in this sense, it should be understood, does not refer to an original structure which is the source of others. It merely refers to a mode of inflorescence which we attempt to recognize, and about which we group those forms which have a resemblance to one another. There are many forms of flower clusters which do not conform to any one of our recognized types, and are very puzzling. The evolution of the flower clusters has been _natural_, and we cannot make them all conform to an _artificial_ classification. These _types_ are named merely as a matter of convenience in the expression of our ideas. The types usually recognized are as follows:

=819. The raceme.=—The flower-shoot is more or less elongated, and the leaves are reduced to a minute size termed _bracts_, while the flowers on lateral axes are solitary in the axils of the bracts. The reduction in the size of the leaves and the somewhat limited growth of the shoot in length, makes the flowers more prominent, and brings them into closer relation than if they were formed in the axils of the leaves on the ordinary foliage shoot. The choke cherry, currant, pokeweed, sourwood, etc., are examples of a raceme (fig. 569). In most plants with the raceme type, while the inflorescence is indeterminate, and the uppermost flowers (those toward the end of the main shoot) are younger, still the period of flowering is somewhat restricted and the raceme stops growing. In a few plants, however, as in the common “shepherd’s-purse,” the raceme continues to grow throughout the summer, so that the lower flowers may have ripened their seed while the terminal portion of the raceme is still growing and producing new flowers. Compound racemes are formed when by branching of the flower-shoot there are several racemes in a cluster, as in the false Solomon’s seal (Smilacina racemosa).

=820. The panicle.=—The panicle is developed from the raceme type by the branching of the lateral flower-axes forming a loose open flower cluster, as in the _oat_.

=821. The thyrsus= is a compact panicle of pyramidal form, as in the lilac, horse-chestnut, etc.

=822. The corymb.=—The corymb shows likewise an easy transition from the raceme type, by the shortening of the main axis of inflorescence, and the lengthening of the lower, lateral flower peduncles so that the flower cluster is more or less flattened on top. This represents the _simple corymb_. A _compound corymb_ is one in which some of the flower peduncles branch again forming secondary corymbs, as in the mountain-ash. It is like a panicle with the lower flower stalks elongated.

=823. The umbel.=—The umbel is developed from the raceme, or corymb. The main flower-shoot remains very short or undeveloped with several flowers on long peduncles arising close together around this shortened axis, in the form of a whorl or cluster. Examples are found in the milkweed, water pennywort (Hydrocotyle), the oxheart cherry, etc. A _compound_ umbel is one in which the peduncles are branched, forming secondary umbels, as in the caraway, parsnip, carrot, etc.

=824. The spike.=—In the spike the main axis is long, and the solitary flowers in the axils of the bracts are usually sessile, and often very much crowded. The plaintain, mullein (fig. 422), etc., are examples. The spike is a raceme, only the flowers are sessile and crowded. In the grasses the flower cluster is branched, and the branchlets bearing a few flowers are spikelets.

=825. The head.=—When the flower axis is very much shortened and the flowers crowded and sessile or nearly so, forming a globose or compressed cluster, it is a _head_ or _capitulum_. The transition is from a spike by the shortening of the main axis, as in the clover, button bush (_Cephalanthus_), etc., or in the shortening of the peduncles in an umbel, as in the daisy, dandelion, and other composite flowers. In these the head is surrounded by an involucre, which in the young head often envelopes the mass of flowers, thus affording them protection. In some other composites (Lactuca, for example) the involucre affords protection for a longer period, even while the seeds are ripening.

=826. The spadix.=—When the main axis of the flower cluster is fleshy, the spike or head forms a _spadix_, as in the Indian turnip, the skunk-cabbage, the calla, etc. The spadix is usually more or less enclosed in a _spathe_, a somewhat strap-shaped leaf.

=827. The catkin.=—A spike which is usually caducous, i.e., falls away after the maturity of the flower or fruit, is called a catkin, or an _ament_. The flower clusters of the alder, willow, (fig. 555), poplar, and the staminate flower clusters of the oak, hickory, hazel, birch, etc., are _aments_. So characteristic is this mode of inflorescence that the plants are called _amentiferous_, or _amentaceous_.

=828. Anthesis of flowers with indeterminate inflorescence.=—In the anthesis of the raceme as well as in other corymbose forms the lower (or outer) flowers being older, open first. The opening of the flowers then takes place from below, upward; or from the outside, inward toward the center of inflorescence. The _anthesis_, i.e., the opening of the flowers of corymbose forms is said to be _centripetal_, i.e., it progresses from outside, inward. The anthesis of the fuller’s teazel is peculiar, since it shows both types. There are several distinct advantages to the plant where anthesis extends over a period of time, as it favors cross pollination, favors the formation of seed in case conditions should be unfavorable at one period of anthesis, distributes the drain on the plant for food, etc.

B. FLOWER CLUSTERS WITH DETERMINATE INFLORESCENCE.

=829. The simplest mode of determinate inflorescence= is a plant with a solitary terminal flower, as in the hepatica, the tulip, etc. The leaves in these two plants are clustered in the form of a rosette, and the aerial shoot is naked and bears the single flower at its summit. Such a flower-shoot is a _scape_. As in the case of the indeterminate inflorescence, so here the larger number of flower-shoots are more complex and specialized, resulting in the evolution of flower clusters or masses. Accompanying the association of flowers into clusters there has been a reduction in leaf surface on the flower-shoot so that the flowers predominate in mass and are more conspicuous. Among the recognized modes of determinate inflorescence, the following are the chief ones:

=830. The cyme.=—In the cyme the terminal flower on the main axis opens first and the remaining flowers are borne on lateral shoots, which arise from the axils of leaves or bracts, below. These lateral shoots usually branch and elongate so that the terminal flowers on all the branches reach nearly the same height as the terminal flower on the main shoot, forming a somewhat flattened or convex top of the flower cluster. This is illustrated in the basswood flower. The anthesis of the cyme is _centrifugal_, i.e., from the inside outward to the margin. But it is often more or less mixed, since the lateral shoots if they bear more than one flower are diminutive cymes and the terminal flower opens before the lateral ones. Where the flower cluster is quite large and the branching quite extensive, _compound cymes_ are formed, as in the dogwood, hydrangea, etc.

=831. The helicoid cyme.=—Where successive lateral branching takes place, and always continues on the same side a curved flower cluster is formed, as in the forget-me-not and most members of the borage family. This is known as a _helicoid cyme_ (fig. 453, _C_). Each new branch becomes in turn the “false” axis bearing a new branch on the same side.

=832. The scorpioid cyme.=—_A scorpioid cyme_ (fig. 453, _B_) is formed where each new branch arises on alternate sides of the “false” axis.

=833. The forking cyme= is where each “false” axis produces two branches opposite, so that it represents a false dichotomy (example, the flower cluster of chickweed).

=834.= Some of these flower clusters are peculiar and it is difficult to see how the helicoid, or scorpioid, cymes are of any advantage to the plant over a true cyme. The inflorescence of the plant being determinate, if the flowering is to be extended over a considerable period a peculiar form would necessarily result. In the _helicoid cyme_ continued branching takes place on one side, and the result in the forget-me-not is a continued inflorescence in its effect like that of a continued raceme (compare shepherd’s-purse). But we should not expect that all of the complex and specialized structures from simple and generalized ones are beneficial to the plant. In many plants we recognize evolution in the direction of advantageous structures. But since the plant cannot consciously evolve these structures, we must also recognize that there may be phases of retrogression in which the structures evolved are not so beneficial to the plant as the more simple and generalized ones of its ancestors. Variation and change do not result in advancing the plant or plant structures merely along the lines which will be beneficial. The tendency is in all directions. The result in general may be diagramed by a tree with divergent and wide-reaching branches. Some die out; others remain subordinate or dormant; while still others droop downward, showing a retrogression. But in this backward evolution they do not return to the condition of their ancestors, nor is the same course retraced. A new downward course is followed just as the downward-growing branch follows a course of its own, and does not return in the trunk.