Elements of Structural and Systematic Botany Part 20

The great majority of them, too, have the surface covered with glandular hairs secreting a strong-scented volatile oil, giving the peculiar odor to these plants. The dead nettle (_Lamium_) (Fig. 120, _A_) is a thoroughly typical example. The sage, mints, catnip, thyme, lavender, etc., will recall the peculiarities of the family.

The stamens are usually four in number through the abortion of one of them, but sometimes only two perfect stamens are present.

[Ill.u.s.tration: FIG. 120.--_Anisocarpous sympetalae_ (_l.a.b.i.atiflorae_).

_A_, dead nettle, _Lamium_, (_l.a.b.i.atae_), . _B_, a single flower, 1. _C_, the stamens and pistil, 1. _D_, cross-section of the ovary, 2. _E_, diagram of the flower; the position of the absent stamen is indicated by the small circle. _F_, fruit of the common sage, _Salvia_ (_l.a.b.i.atae_), 1. Part of the persistent calyx has been removed to show the four seed-like fruits, or nutlets. _G_, section of a nutlet, 3. The embryo fills the seed completely. _H_, part of an inflorescence of figwort, _Scrophularia_ (_Scrophularineae_), 1. _I_, cross-section of the young fruit, 2. _J_, flower of speedwell, _Veronica_ (_Scrophularineae_), 2. _K_, fruit of _Veronica_, 2.

_L_, cross-section of _K_. _M_, flower of moth-mullein, _Verbasc.u.m_ (_Scrophularineae_), . _N_, flower of toad-flax, _Linaria_ (_Scrophularineae_), 1. _O_, leaf of bladder-weed, _Utricularia_ (_Lentibulariaceae_), 1. _x_, one of the "traps." _P_, a single trap, 5.]

The _Scrophularineae_ differ mainly from the _l.a.b.i.atae_ in having round stems, and the ovary not splitting into separate one-seeded fruits.

The leaves are also sometimes alternate. There are generally four stamens, two long and two short, as in the l.a.b.i.ates, but in the mullein (_Verbasc.u.m_) (Fig. 120, _M_), where the flower is only slightly zygomorphic, there is a fifth rudimentary stamen, while in others (_e.g._ _Veronica_) (Fig. 120, _J_) there are but two stamens.

Many have large, showy flowers, as in the cultivated foxglove (_Digitalis_), and the native species of _Gerardia_, mullein, _Mimulus_, etc., while a few like the figwort, _Scrophularia_ (Fig. 120, _H_), and speedwells (_Veronica_) have duller-colored or smaller flowers.

[Ill.u.s.tration: FIG. 121.--_Anisocarpous sympetalae_ (_l.a.b.i.atiflorae_).

_A_, flowering branch of trumpet-creeper, _Tecoma_ (_Bignoniaceae_), . _B_, a single flower, divided lengthwise, . _C_, cross-section of the ovary, 2. _D_, diagram of the flower. _E_, flower of vervain, _Verbena_ (_Verbenae_), 2: i, from the side; ii, from in front; iii, the corolla laid open. _F_, nearly ripe fruit of the same, 2. _G_, part of a spike of flowers of the common plantain, _Plantago_ (_Plantagineae_), 1; The upper flowers have the pistils mature, but the stamens are not yet ripe. _H_, a flower from the upper (younger) part of the spike. _I_, an older expanded flower, with ripe stamens, 3.]

The curious bladder-weed (_Utricularia_) is the type of the family _Lentibulariaceae_, aquatic or semi-aquatic plants which possess special contrivances for capturing insects or small water animals.

These in the bladder-weed are little sacs (Fig. 120, _P_) which act as traps from which the animals cannot escape after being captured. There does not appear to be here any actual digestion, but simply an absorption of the products of decomposition, as in the pitcher-plant.

In the nearly related land form, _Pinguicula_, however, there is much the same arrangement as in the sundew.

The family _Gesneraceae_ is mainly a tropical one, represented in the greenhouses by the magnificent _Gloxinia_ and _Achimenes_, but of native plants there are only a few parasitic forms dest.i.tute of chlorophyll and with small, inconspicuous flowers. The commonest of these is _Epiphegus_, a much-branched, brownish plant, common in autumn about the roots of beech-trees upon which it is parasitic, and whence it derives its common name, "beech-drops."

The bignonia family (_Bignoniaceae_) is mainly tropical, but in our southern states is represented by the showy trumpet-creeper (_Tecoma_) (Fig. 121, _A_), the catalpa, and _Martynia_.

The other plants likely to be met with by the student belong either to the _Verbenaceae_, represented by the showy verbenas of the gardens, and our much less showy wild vervains, also belonging to the genus _Verbena_ (Fig. 121, _E_); or to the plantain family (_Plantagineae_), of which the various species of plantain (_Plantago_) are familiar to every one (Fig. 121, _G_, _I_). The latter seem to be forms in which the flowers have become inconspicuous, and are wind fertilized, while probably all of its showy-flowered relatives are dependent on insects for fertilization.

The third order (_Contortae_) of the _Anisocarpae_ includes five families, all represented by familiar forms. The first, the olive family (_Oleaceae_), besides the olive, contains the lilac and jasmine among cultivated plants, and the various species of ash (_Fraxinus_), and the pretty fringe-tree (_Chionanthus_) (Fig. 122, _A_), often cultivated for its abundant white flowers. The other families are the _Gentianaceae_ including the true gentians (_Gentiana_) (Fig. 122, _F_), the buck-bean (_Menyanthes_), the centauries (_Erythraea_ and _Sabbatia_), and several other less familiar genera; _Loganiaceae_, with the pink-root (_Spigelia_) (Fig. 122, _D_), as the best-known example; _Apocynaceae_ including the dog-bane (_Apocynum_) (Fig. 122, _H_), and in the gardens the oleander and periwinkle (_Vinca_).

[Ill.u.s.tration: FIG. 122.--_Anisocarpous sympetalae_ (_Contortae_). _A_, flower of fringe-tree, _Chionanthus_ (_Oleaceae_), 1. _B_, base of the flower, with part of the calyx and corolla removed, 2. _C_, fruit of white ash, _Fraxinus_ (_Oleaceae_), 1. _D_, flower of pink-root, _Spigelia_ (_Loganiaceae_), . _E_, cross-section of the ovary, 3. _F_, flower of fringed gentian, _Gentiana_ (_Gentianaceae_), . _G_, diagram of the flower. _H_, flowering branch of dog-bane, _Apocynum_ (_Apocynaceae_), . _I_, vertical section of a flower, 2. _J_, bud. _K_, flower of milk-weed, _Asclepias_ (_Asclepiadaceae_), 1. _L_, vertical section through the upper part of the flower, 2.

_gy._ pistil. _p_, pollen ma.s.ses. _an._ stamen. _M_, a pair of pollen ma.s.ses, 6. _N_, a nearly ripe seed, 1.]

The last family is the milk-weeds (_Asclepiadaceae_), which have extremely complicated flowers. Our numerous milk-weeds (Fig. 122, _K_) are familiar representatives, and exhibit perfectly the peculiarities of the family. Like the dog-banes, the plants contain a milky juice which is often poisonous. Besides the true milk-weeds (_Asclepias_), there are several other genera within the United States, but mostly southern in their distribution. Many of them are twining plants and occasionally cultivated for their showy flowers. Of the cultivated forms, the wax-plant (_Hoya_), and _Physianthus_ are the commonest.

[Ill.u.s.tration: FIG. 123.--_Anisocarpous sympetalae_ (_Campanulinae_).

_A_, vertical section of the bud of American bell-flower, _Campanula_ (_Campanulaceae_), 2. _B_, an expanded flower, 1. The stamens have discharged their pollen, and the stigma has opened. _C_, cross-section of the ovary, 3. _D_, flower of the Carpathian bell-flower (_Campanula Carpatica_), 1. _E_, flower of cardinal-flower, _Lobelia_ (_Lobeliaceae_), 1. _F_, the same, with the corolla and sepals removed. _an._ the united anthers. _gy._ the tip of the pistil.

_G_, the tip of the pistil, 2, showing the circle of hairs surrounding the stigma. _H_, cross-section of the ovary, 3. _I_, tip of a branch of cuc.u.mber, _Cucurbita_ (_Cucurbitaceae_), with an expanded female flower (?). _J_, andrcium of a male flower, showing the peculiar convoluted anthers (_an._), 2. _K_, cross-section of the ovary, 2.]

The fourth order (_Campanulinae_) also embraces five families, but of these only three are represented among our wild plants. The bell-flowers (_Campanula_) (Fig. 123, _A_, _D_) are examples of the family _Campanulaceae_, and numerous species are common, both wild and cultivated.

[Ill.u.s.tration: FIG. 124.--_Anisocarpous sympetalae_ (_Aggregatae_). _A_, flowering branch of _Houstonia purpurea_, 1 (_Rubiaceae_). _B_, vertical section of a flower, 2. _C_, fruit of bluets (_Houstonia crulea_), 1. _D_, cross-section of the same. _E_, bedstraw, _Galium_ (_Rubiaceae_), . _F_, a single flower, 2. _G_, flower of arrow-wood, _Viburnum_ (_Caprifoliaceae_), 2. _H_, the same, divided vertically. _I_, flowering branch of trumpet honeysuckle, _Lonicera_ (_Caprifoliaceae_), . _J_, a single flower, the upper part laid open, 1. _K_, diagram of the flower. _L_, part of the inflorescence of valerian, _Valeriana_, (_Valerianeae_), 1. _M_, young; _N_, older flower, 2. _O_, cross-section of the young fruit; one division of the three contains a perfect seed, the others are crowded to one side by its growth. _P_, inflorescence of teasel, _Dipsacus_ (_Dipsaceae_), . _fl._ flowers. _Q_, a single flower, 1. _R_, the same, with the corolla laid open.]

The various species of _Lobelia_, of which the splendid cardinal-flower (_L. Cardinalis_) (Fig. 123, _E_) is one of the most beautiful, represent the very characteristic family _Lobeliaceae_.

Their milky juice contains more or less marked poisonous properties.

The last family of the order is the gourd family (_Cucurbitaceae_), represented by a few wild species, but best known by the many cultivated varieties of melons, cuc.u.mbers, squashes, etc. They are climbing or running plants, and provided with tendrils. The flowers are usually unis.e.xual, sometimes dicious, but oftener moncious (Fig. 123, _I_).

[Ill.u.s.tration: FIG. 125.--_Anisocarpous sympetalae_ (_Aggregatae_).

Types of _Compositae_. _A_, inflorescence of Canada thistle (_Cirsium_), 1. _B_, vertical section of _A_. _r_, the receptacle or enlarged end of the stem, to which the separate flowers are attached.

_C_, a single flower, 2. _o_, the ovary. _p_, the "pappus" (calyx lobes). _an._ the united anthers. _D_, the upper part of the stamens and pistil, 3: i, from a young flower; ii, from an older one. _an._ anthers. _gy._ pistil. _E_, ripe fruit, 1. _F_, inflorescence of may-weed (_Maruta_). The central part (disc) is occupied by perfect tubular flowers (_G_), the flowers about the edge (rays) are sterile, with the corolla much enlarged and white, 2. _G_, a single flower from the disc, 3. _H_, inflorescence of dandelion (_Taraxac.u.m_), the flowers all alike, with strap-shaped corollas, 1. _I_, a single flower, 2. _c_, the split, strap-shaped corolla. _J_, two ripe fruits, still attached to the receptacle (_r_). The pappus is raised on a long stalk, 1. _K_, a single fruit, 2.]

The last and highest order of the _Sympetalae_, and hence of the dicotyledons, is known as _Aggregatae_, from the tendency to have the flowers densely crowded into a head, which not infrequently is closely surrounded by bracts so that the whole inflorescence resembles a single flower. There are six families, five of which have common representatives, but the last family (_Calycereae_) has no members within our limits.

The lower members of the order, _e.g._ various _Rubiaceae_ (Fig. 124, _A_, _E_), have the flowers in loose inflorescences, but as we examine the higher families, the tendency for the flowers to become crowded becomes more and more evident, and in the highest of our native forms _Dipsaceae_ (Fig. 124, _P_) and _Compositae_ (Fig. 125) this is very marked indeed. In the latter family, which is by far the largest of all the angiosperms, including about ten thousand species, the differentiation is carried still further. Among our native _Compositae_ there are three well-marked types. The first of these may be represented by the thistles (Fig. 125, _A_). The so-called flower of the thistle is in reality a close head of small, tubular flowers (Fig. 125, _C_), each perfect in all respects, having an inferior one-celled ovary, five stamens with the anthers united, and a five-parted corolla. The sepals (here called the "pappus") (_p_) have the form of fine hairs. These little flowers are attached to the enlarged upper end of the flower stalk (receptacle, _r_), and are surrounded by closely overlapping bracts or scale leaves which look like a calyx; the flowers, on superficial examination, appear as single petals. In other forms like the daisy and may-weed (Fig. 125, _F_), only the central flowers are perfect, and the edge of the inflorescence is composed of flowers whose corollas are split and flattened out, but the stamens and sometimes the pistils are wanting in these so-called "ray-flowers." In the third group, of which the dandelion (Fig. 125, _H_), chicory, lettuce, etc., are examples, all of the flowers have strap-shaped, split corollas, and contain both stamens and pistils.

The families of the _Aggregatae_ are the following: I. _Rubiaceae_ of which _Houstonia_ (Fig. 124, _A_), _Galium_ (_E_), _Cephalanthus_ (b.u.t.ton-bush), and _Mitch.e.l.la_ (partridge-berry) are examples; II. _Caprifoliaceae_, containing the honeysuckles (_Lonicera_) (Fig. 124, _I_), _Viburnum_ (_G_), s...o...b..rry (_Symphoricarpus_), and elder (_Sambucus_); III. _Valerianeae_, represented by the common valerian (_Valeriana_) (Fig. 124, _L_); IV. _Dipsaceae_, of which the teasel (_Dipsacus_) (Fig. 124, _P_), is the type, and also species of scabious (_Scabiosa_); V. _Compositae_ to which the innumerable, so-called compound flowers, asters, golden-rods, daisies, sunflowers, etc. belong; VI. _Calycereae_.

[Ill.u.s.tration: FIG. 126.--_Aristolochiaceae_. _A_, plant of wild ginger (_Asarum_), ?. _B_, vertical section of the flower, 1. _C_, diagram of the flower.]

Besides the groups already mentioned, there are several families of dicotyledons whose affinities are very doubtful. They are largely parasitic, _e.g._ mistletoe; or water plants, as the horned pond-weed (_Ceratophyllum_). One family, the _Aristolochiaceae_, represented by the curious "Dutchman's pipe" (_Aristolochia sipho_), a woody twiner with very large leaves, and the common wild ginger (_Asarum_) (Fig. 126), do not appear to be in any wise parasitic, but the structure of their curious flowers differs widely from any other group of plants.

CHAPTER XX.

FERTILIZATION OF FLOWERS.

If we compare the flowers of different plants, we shall find almost infinite variety in structure, and this variation at first appears to follow no fixed laws; but as we study the matter more thoroughly, we find that these variations have a deep significance, and almost without exception have to do with the fertilization of the flower.

In the simpler flowers, such as those of a gra.s.s, sedge, or rush among the monocotyledons, or an oak, hazel, or plantain, among dicotyledons, the flowers are extremely inconspicuous and often reduced to the simplest form. In such plants, the pollen is conveyed from the male flowers to the female by the wind, and to this end the former are usually placed above the latter so that these are dusted with the pollen whenever the plant is shaken by the wind. In these plants, the male flowers often outnumber the female enormously, and the pollen is produced in great quant.i.ties, and the stigmas are long and often feathery, so as to catch the pollen readily. This is very beautifully shown in many gra.s.ses.

If, however, we examine the higher groups of flowering plants, we see that the outer leaves of the flower become more conspicuous, and that this is often correlated with the development of a sweet fluid (nectar) in certain parts of the flower, while the wind-fertilized flowers are dest.i.tute of this as well as of odor.

If we watch any bright-colored or sweet-scented flower for any length of time, we shall hardly fail to observe the visits of insects to it, in search of pollen or honey, and attracted to the flower by its bright color or sweet perfume. In its visits from flower to flower, the insect is almost certain to transfer part of the pollen carried off from one flower to the stigma of another of the same kind, thus effecting pollination.

That the fertilization of a flower by pollen from another is beneficial has been shown by many careful experiments which show that nearly always--at least in flowers where there are special contrivances for cross-fertilization--the number of seeds is greater and the quality better where cross-fertilization has taken place, than where the flower is fertilized by its own pollen. From these experiments, as well as from very numerous studies on the structure of the flower with reference to insect aid in fertilization, we are justified in the conclusion that all bright-colored flowers are, to a great extent, dependent upon insect aid for transferring the pollen from one flower to another, and that many, especially those with tubular or zygomorphic (bilateral) flowers are perfectly incapable of self-fertilization. In a few cases snails have been known to be the conveyers of pollen, and the humming-birds are known in some cases, as for instance the trumpet-creeper (Fig. 121, _A_), to take the place of insects.[14]

[14] In a number of plants with showy flowers, _e.g._ violets, jewel-weed, small, inconspicuous flowers are also formed, which are self-fertilizing. These inconspicuous flowers are called "cleistogamous."

At first sight it would appear that most flowers are especially adapted for self-fertilization; but in fact, although stamens and pistils are in the same flower, there are usually effective preventives for avoiding self-fertilization. In a few cases investigated, it has been found that the pollen from the flower will not germinate upon its own stigma, and in others it seems to act injuriously. One of the commonest means of avoiding self-fertilization is the maturing of stamens and pistils at different times. Usually the stamens ripen first, discharging the pollen and withering before the stigma is ready to receive it, _e.g._ willow-herb (Fig. 113, _D_), campanula (Fig. 123, _A_, _D_), and pea; in the two latter, the pollen is often shed before the flower opens. Not so frequently the stigmas mature first, as in the plantain (Fig. 121, _G_).

In many flowers, the stamens, as they ripen, move so as to place themselves directly before the entrance to the nectary, where they are necessarily struck by any insect searching for honey; after the pollen is shed, they move aside or bend downward, and their place is taken by the pistil, so that an insect which has come from a younger flower will strike the part of the body previously dusted with pollen against the stigma, and deposit the pollen upon it. This arrangement is very beautifully seen in the nasturtium and larkspur (Fig. 99, _J_).

The tubular flowers of the _Sympetalae_ are especially adapted for pollination by insects with long tongues, like the bees and b.u.t.terflies, and in most of these flowers the relative position of the stamens and pistil is such as to ensure cross-fertilization, which in the majority of them appears to be absolutely dependent upon insect aid.

The great orchid family is well known on account of the singular form and brilliant colors of the flowers which have no equals in these respects in the whole vegetable kingdom. As might be expected, there are numerous contrivances for cross-fertilization among them, some of which are so extraordinary as to be scarcely credible. With few exceptions the pollen is so placed as to render its removal by insects necessary. One of the simpler contrivances is readily studied in the little spring-orchis (Fig. 89) or one of the _Habenarias_ (Fig. 90, _G_). In the first, the two pollen ma.s.ses taper below where each is attached to a viscid disc which is covered by a delicate membrane.

These discs are so placed that when an insect enters the flower and thrusts its tongue into the spur of the flower, its head is brought against the membrane covering the discs, rupturing it so as to expose the disc which adheres firmly to the head or tongue of the insect, the substance composing the disc hardening like cement on exposure to the air. As the insect withdraws its tongue, one or both of the pollen ma.s.ses are dragged out and carried away. The action of the insect may be imitated by thrusting a small gra.s.s-stalk or some similar body into the spur of the flower, when on withdrawing it, the two pollen ma.s.ses will be removed from the flower. If we now examine these carefully, we shall see that they change position, being nearly upright at first, but quickly bending downward and forward (Fig. 89, _D_, ii, iii), so that on thrusting the stem into another flower the pollen ma.s.ses strike against the sticky stigmatic surfaces, and a part of the pollen is left adhering to them.

The last arrangement that will be mentioned here is one discovered by Darwin in a number of very widely separated plants, and to which he gave the name "heterostylism." Examples of this are the primroses (_Primula_), loosestrife (_Lythrum_), partridge-berry (_Mitch.e.l.la_), pickerel-weed (_Pontederia_), (Fig. 84, _I_), and others. In these there are two, sometimes three, sets of flowers differing very much in the relative lengths of stamens and pistil, those with long pistils having short stamens and _vice versa_. When an insect visits a flower with short stamens, that part is covered with pollen which in the short-styled (but long-stamened) flower will strike the stigma, as the pistil in one flower is almost exactly of the length of the stamens in the other form. In such flowers as have three forms, _e.g._ _Pontederia_, each flower has two different lengths of stamens, both differing from the style of the same flower. Microscopic examination has shown that there is great variation in the size of the pollen spores in these plants, the large pollen from the long stamens being adapted to the long style of the proper flower.

It will be found that the character of the color of the flower is related to the insects visiting it. Brilliantly colored flowers are usually visited by b.u.t.terflies, bees, and similar day-flying insects.

Flowers opening at night are usually white or pale yellow, colors best seen at night, and in addition usually are very strongly scented so as to attract the night-flying moths which usually fertilize them.

Sometimes dull-colored flowers, which frequently have a very offensive odor, are visited by flies and other carrion-loving insects, which serve to convey pollen to them.

Occasionally, flowers in themselves inconspicuous are surrounded by showy leaves or bracts which take the place of the petals of the showier flowers in attracting insect visitors. The large dogwood (Fig. 110, _J_), the calla, and Jack-in-the-pulpit (Fig. 86, _A_) are ill.u.s.trations of this.

CHAPTER XXI.