Fig. 119.Fig. 119.—Anisocarpous sympetalæ(Tubifloræ).A, inflorescence of hound’s-tongue,Cynoglossum(Borragineæ), × ½.B, section of a flower, × 2.C, nearly ripe fruit, × 1.D, flowering branch of nightshade,Solanum(Solaneæ), × ½.E, a single flower, × 1.F, section of the flower, × 2.G, young fruit, × 1.H, flower ofPetunia(Solaneæ), × ½.I, diagram of the flower.
Fig. 119.—Anisocarpous sympetalæ(Tubifloræ).A, inflorescence of hound’s-tongue,Cynoglossum(Borragineæ), × ½.B, section of a flower, × 2.C, nearly ripe fruit, × 1.D, flowering branch of nightshade,Solanum(Solaneæ), × ½.E, a single flower, × 1.F, section of the flower, × 2.G, young fruit, × 1.H, flower ofPetunia(Solaneæ), × ½.I, diagram of the flower.
The third family (Hydrophyllaceæ) includes several species of water-leaf (Hydrophyllum) (Fig. 118,E) andPhacelia, among our wild flowers, and species ofNemophila,Whitlaviaand others from the western states, but now common in gardens.
The Borage family (Borragineæ) includes the forget-me-not (Myosotis) and a few pretty wild flowers,e.g.the orange-flowered puccoons (Lithospermum); but it also embraces a number of the most troublesome weeds, among which are the hound’s-tongue (Cynoglossum) (Fig. 119,A), and the “beggar’s-ticks” (Echinospermum), whose prickly fruits (Fig. 119,C) become detached on the slightest provocation, and adhere to whatever they touch with great tenacity. The flowers in this family are arranged in one-sided inflorescences which are coiled up at first and straighten as the flowers expand.
The last family (Solaneæ) includes the nightshades (Solanum) (Fig. 119,D), to which genus the potato (S. tuberosum) and the egg-plant (S. Melongena) also belong. Many of the family contain a poisonous principle,e.g.the deadly nightshade (Atropa), tobacco (Nicotiana), stramonium (Datura), and others. Of the cultivated plants, besides those already mentioned, the tomato (Lycopersicum), and various species ofPetunia(Fig. 119,H),Solanum, andDaturaare the commonest.
The second order of theAnisocarpæconsists of plants whose flowers usually exhibit very marked, bilateral symmetry (Zygomorphism). From the flower often being two-lipped (seeFig. 120), the name of the order (Labiatifloræ) is derived.
Of the nine families constituting the order, all but one are represented within our limits, but the great majority belong to two families, the mints (Labiatæ) and the figworts (Scrophularineæ). The mints are very common and easily recognizable on account of their square stems, opposite leaves, strongly bilabiate flowers, and the ovary splitting into four seed-like fruits (Fig. 120,D,F).
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 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.
Fig. 120.Fig. 120.—Anisocarpous sympetalæ(Labiatifloræ).A, dead nettle,Lamium, (Labiatæ), × ½.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(Labiatæ), × 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(Scrophularineæ), × 1.I, cross-section of the young fruit, × 2.J, flower of speedwell,Veronica(Scrophularineæ), × 2.K, fruit ofVeronica, × 2.L, cross-section ofK.M, flower of moth-mullein,Verbascum(Scrophularineæ), × ½.N, flower of toad-flax,Linaria(Scrophularineæ), × 1.O, leaf of bladder-weed,Utricularia(Lentibulariaceæ), × 1.x, one of the “traps.”P, a single trap, × 5.
Fig. 120.—Anisocarpous sympetalæ(Labiatifloræ).A, dead nettle,Lamium, (Labiatæ), × ½.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(Labiatæ), × 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(Scrophularineæ), × 1.I, cross-section of the young fruit, × 2.J, flower of speedwell,Veronica(Scrophularineæ), × 2.K, fruit ofVeronica, × 2.L, cross-section ofK.M, flower of moth-mullein,Verbascum(Scrophularineæ), × ½.N, flower of toad-flax,Linaria(Scrophularineæ), × 1.O, leaf of bladder-weed,Utricularia(Lentibulariaceæ), × 1.x, one of the “traps.”P, a single trap, × 5.
TheScrophularineædiffer mainly from theLabiatæin 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 labiates, but in the mullein (Verbascum) (Fig. 120,M), where the flower is only slightly zygomorphic, there is a fifth rudimentarystamen, 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 ofGerardia, mullein,Mimulus, etc., while a few like the figwort,Scrophularia(Fig. 120,H), and speedwells (Veronica) have duller-colored or smaller flowers.
Fig. 121.Fig. 121.—Anisocarpous sympetalæ(Labiatifloræ).A, flowering branch of trumpet-creeper,Tecoma(Bignoniaceæ), × ¼.B, a single flower, divided lengthwise, × ½.C, cross-section of the ovary, × 2.D, diagram of the flower.E, flower of vervain,Verbena(Verbenæ), × 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(Plantagineæ), × 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.
Fig. 121.—Anisocarpous sympetalæ(Labiatifloræ).A, flowering branch of trumpet-creeper,Tecoma(Bignoniaceæ), × ¼.B, a single flower, divided lengthwise, × ½.C, cross-section of the ovary, × 2.D, diagram of the flower.E, flower of vervain,Verbena(Verbenæ), × 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(Plantagineæ), × 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 familyLentibulariaceæ, 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 familyGesneraceæis mainly a tropical one, represented in the greenhouses by the magnificentGloxiniaandAchimenes, but of native plants there are only a few parasitic forms destitute of chlorophyll and with small, inconspicuous flowers. The commonest of these isEpiphegus, 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 (Bignoniaceæ) is mainly tropical, but in our southern states is represented by the showy trumpet-creeper (Tecoma) (Fig. 121,A), the catalpa, andMartynia.
The other plants likely to be met with by the student belong either to theVerbenaceæ, represented by the showy verbenas of the gardens, and our much less showy wild vervains, also belonging to the genusVerbena(Fig. 121,E); or to the plantain family (Plantagineæ), 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 (Contortæ) of theAnisocarpæincludes five families, all represented by familiar forms. The first, the olive family (Oleaceæ), 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 theGentianaceæincluding the true gentians (Gentiana) (Fig. 122,F), the buck-bean (Menyanthes), thecentauries (ErythræaandSabbatia), and several other less familiar genera;Loganiaceæ, with the pink-root (Spigelia) (Fig. 122,D), as the best-known example;Apocynaceæincluding the dog-bane (Apocynum) (Fig. 122,H), and in the gardens the oleander and periwinkle (Vinca).
Fig. 122.Fig. 122.—Anisocarpous sympetalæ(Contortæ).A, flower of fringe-tree,Chionanthus(Oleaceæ), × 1.B, base of the flower, with part of the calyx and corolla removed, × 2.C, fruit of white ash,Fraxinus(Oleaceæ), × 1.D, flower of pink-root,Spigelia(Loganiaceæ), × ½.E, cross-section of the ovary, × 3.F, flower of fringed gentian,Gentiana(Gentianaceæ), × ½.G, diagram of the flower.H, flowering branch of dog-bane,Apocynum(Apocynaceæ), × ½.I, vertical section of a flower, × 2.J, bud.K, flower of milk-weed,Asclepias(Asclepiadaceæ), × 1.L, vertical section through the upper part of the flower, × 2.gy.pistil.p, pollen masses.an.stamen.M, a pair of pollen masses, × 6.N, a nearly ripe seed, × 1.
Fig. 122.—Anisocarpous sympetalæ(Contortæ).A, flower of fringe-tree,Chionanthus(Oleaceæ), × 1.B, base of the flower, with part of the calyx and corolla removed, × 2.C, fruit of white ash,Fraxinus(Oleaceæ), × 1.D, flower of pink-root,Spigelia(Loganiaceæ), × ½.E, cross-section of the ovary, × 3.F, flower of fringed gentian,Gentiana(Gentianaceæ), × ½.G, diagram of the flower.H, flowering branch of dog-bane,Apocynum(Apocynaceæ), × ½.I, vertical section of a flower, × 2.J, bud.K, flower of milk-weed,Asclepias(Asclepiadaceæ), × 1.L, vertical section through the upper part of the flower, × 2.gy.pistil.p, pollen masses.an.stamen.M, a pair of pollen masses, × 6.N, a nearly ripe seed, × 1.
The last family is the milk-weeds (Asclepiadaceæ), 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), andPhysianthusare the commonest.
Fig. 123.Fig. 123.—Anisocarpous sympetalæ(Campanulinæ).A, vertical section of the bud of American bell-flower,Campanula(Campanulaceæ), × 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(Lobeliaceæ), × 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 cucumber,Cucurbita(Cucurbitaceæ), with an expanded female flower (♀).J, andrœcium of a male flower, showing the peculiar convoluted anthers (an.), × 2.K, cross-section of the ovary, × 2.
Fig. 123.—Anisocarpous sympetalæ(Campanulinæ).A, vertical section of the bud of American bell-flower,Campanula(Campanulaceæ), × 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(Lobeliaceæ), × 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 cucumber,Cucurbita(Cucurbitaceæ), with an expanded female flower (♀).J, andrœcium of a male flower, showing the peculiar convoluted anthers (an.), × 2.K, cross-section of the ovary, × 2.
The fourth order (Campanulinæ) also embraces five families, but of these only three are represented among our wild plants. The bell-flowers (Campanula) (Fig. 123,A,D) are examplesof the familyCampanulaceæ, and numerous species are common, both wild and cultivated.
Fig. 124.Fig. 124.—Anisocarpous sympetalæ(Aggregatæ).A, flowering branch ofHoustonia purpurea, × 1 (Rubiaceæ).B, vertical section of a flower, × 2.C, fruit of bluets (Houstonia cœrulea), × 1.D, cross-section of the same.E, bedstraw,Galium(Rubiaceæ), × ½.F, a single flower, × 2.G, flower of arrow-wood,Viburnum(Caprifoliaceæ), × 2.H, the same, divided vertically.I, flowering branch of trumpet honeysuckle,Lonicera(Caprifoliaceæ), × ½.J, a single flower, the upper part laid open, × 1.K, diagram of the flower.L, part of the inflorescence of valerian,Valeriana, (Valerianeæ), × 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(Dipsaceæ), × ¼.fl.flowers.Q, a single flower, × 1.R, the same, with the corolla laid open.
Fig. 124.—Anisocarpous sympetalæ(Aggregatæ).A, flowering branch ofHoustonia purpurea, × 1 (Rubiaceæ).B, vertical section of a flower, × 2.C, fruit of bluets (Houstonia cœrulea), × 1.D, cross-section of the same.E, bedstraw,Galium(Rubiaceæ), × ½.F, a single flower, × 2.G, flower of arrow-wood,Viburnum(Caprifoliaceæ), × 2.H, the same, divided vertically.I, flowering branch of trumpet honeysuckle,Lonicera(Caprifoliaceæ), × ½.J, a single flower, the upper part laid open, × 1.K, diagram of the flower.L, part of the inflorescence of valerian,Valeriana, (Valerianeæ), × 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(Dipsaceæ), × ¼.fl.flowers.Q, a single flower, × 1.R, the same, with the corolla laid open.
The various species ofLobelia, of which the splendid cardinal-flower (L. Cardinalis) (Fig. 123,E) is one of the most beautiful, represent the very characteristic familyLobeliaceæ. Their milky juice contains more or less marked poisonous properties. The last family of the order is the gourd family (Cucurbitaceæ), represented by a few wild species, but best known by the many cultivated varieties of melons, cucumbers,squashes, etc. They are climbing or running plants, and provided with tendrils. The flowers are usually unisexual, sometimes diœcious, but oftener monœcious (Fig. 123,I).
Fig. 125.Fig. 125.—Anisocarpous sympetalæ(Aggregatæ). Types ofCompositæ.A, inflorescence of Canada thistle (Cirsium), × 1.B, vertical section ofA.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 (Taraxacum), 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.
Fig. 125.—Anisocarpous sympetalæ(Aggregatæ). Types ofCompositæ.A, inflorescence of Canada thistle (Cirsium), × 1.B, vertical section ofA.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 (Taraxacum), 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 theSympetalæ, and hence of the dicotyledons, is known asAggregatæ, 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 (Calycereæ) has no members within our limits.
The lower members of the order,e.g.variousRubiaceæ(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 formsDipsaceæ(Fig. 124,P) andCompositæ(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 nativeCompositæthere 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 theAggregatæare the following: I.Rubiaceæof whichHoustonia(Fig. 124,A),Galium(E),Cephalanthus(button-bush), andMitchella(partridge-berry) are examples; II.Caprifoliaceæ, containing the honeysuckles (Lonicera) (Fig. 124,I),Viburnum(G), snowberry (Symphoricarpus),and elder (Sambucus); III.Valerianeæ, represented by the common valerian (Valeriana) (Fig. 124,L); IV.Dipsaceæ, of which the teasel (Dipsacus) (Fig. 124,P), is the type, and also species of scabious (Scabiosa); V.Compositæto which the innumerable, so-called compound flowers, asters, golden-rods, daisies, sunflowers, etc. belong; VI.Calycereæ.
Fig. 126.Fig. 126.—Aristolochiaceæ.A, plant of wild ginger (Asarum), × ⅓.B, vertical section of the flower, × 1.C, diagram of the flower.
Fig. 126.—Aristolochiaceæ.A, plant of wild ginger (Asarum), × ⅓.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, theAristolochiaceæ, 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.
Ifwe 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 grass, 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 quantities, and the stigmas are long and often feathery, so as to catch the pollen readily. This is very beautifully shown in many grasses.
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 destitute 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 visitsfrom 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]
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 theSympetalæare especially adapted for pollination by insects with long tongues, like the bees and butterflies, 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 theHabenarias(Fig. 90,G). In the first, the two pollen masses 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 thedisc hardening like cement on exposure to the air. As the insect withdraws its tongue, one or both of the pollen masses are dragged out and carried away. The action of the insect may be imitated by thrusting a small grass-stalk or some similar body into the spur of the flower, when on withdrawing it, the two pollen masses 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 masses 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 (Mitchella), 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 andvice 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 butterflies, bees, and similar day-flying insects. Flowers opening at night are usually white or paleyellow, 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 illustrations of this.
Inthe more exact investigations of the tissues, it is often necessary to have recourse to other reagents than those we have used hitherto, in order to bring out plainly the more obscure points of structure. This is especially the case in studies in cell division in the higher plants, where the changes in the dividing nucleus are very complicated.
For studying these the most favorable examples for ready demonstration are found in the final division of the pollen spores, especially of some monocotyledons. An extremely good subject is offered by the common wild onion (Allium Canadense), which flowers about the last of May. The buds, which are generally partially replaced by small bulbs, are enclosed in a spathe or sheath which entirely conceals them. Buds two to three millimetres in length should be selected, and these opened so as to expose the anthers. The latter should now be removed to a slide, and carefully crushed in a drop of dilute acetic acid (one-half acid to one-half distilled water). This at once fixes the nuclei, and by examining with a low power, we can determine at once whether or not we have the right stages. The spore mother cells are recognizable by their thick transparent walls, and if the desired dividing stages are present, a drop of staining fluid should be added and allowed to act for about a minute, the preparation being covered with a cover glass. After the stain is sufficiently deep, it should be carefully withdrawn with blotting paper, and pure water run under the cover glass.The best stain for acetic acid preparations is, perhaps, gentian violet. This is an aniline dye readily soluble in water. For our purpose, however, it is best to make a concentrated, alcoholic solution from the dry powder, and dilute this as it is wanted. A drop of the alcoholic solution is diluted with several times its volume of weak acetic acid (about two parts of distilled water to one of the acid), and a drop of this mixture added to the preparation. In this way the nucleus alone is stained and is rendered very distinct, appearing of a beautiful violet-blue color.If the preparation is to be kept permanently, the acid must all be washed out, and dilute glycerine run under the cover glass. The preparation should then be sealed with Canada balsam or some other cement, but previously all trace of glycerine must be removed from the slide and upper surface of the cover glass. It is generally best to gently wipe the edge of the cover glass with a small brush moistened with alcohol before applying the cement.
For studying these the most favorable examples for ready demonstration are found in the final division of the pollen spores, especially of some monocotyledons. An extremely good subject is offered by the common wild onion (Allium Canadense), which flowers about the last of May. The buds, which are generally partially replaced by small bulbs, are enclosed in a spathe or sheath which entirely conceals them. Buds two to three millimetres in length should be selected, and these opened so as to expose the anthers. The latter should now be removed to a slide, and carefully crushed in a drop of dilute acetic acid (one-half acid to one-half distilled water). This at once fixes the nuclei, and by examining with a low power, we can determine at once whether or not we have the right stages. The spore mother cells are recognizable by their thick transparent walls, and if the desired dividing stages are present, a drop of staining fluid should be added and allowed to act for about a minute, the preparation being covered with a cover glass. After the stain is sufficiently deep, it should be carefully withdrawn with blotting paper, and pure water run under the cover glass.
The best stain for acetic acid preparations is, perhaps, gentian violet. This is an aniline dye readily soluble in water. For our purpose, however, it is best to make a concentrated, alcoholic solution from the dry powder, and dilute this as it is wanted. A drop of the alcoholic solution is diluted with several times its volume of weak acetic acid (about two parts of distilled water to one of the acid), and a drop of this mixture added to the preparation. In this way the nucleus alone is stained and is rendered very distinct, appearing of a beautiful violet-blue color.
If the preparation is to be kept permanently, the acid must all be washed out, and dilute glycerine run under the cover glass. The preparation should then be sealed with Canada balsam or some other cement, but previously all trace of glycerine must be removed from the slide and upper surface of the cover glass. It is generally best to gently wipe the edge of the cover glass with a small brush moistened with alcohol before applying the cement.
Fig. 127.Fig. 127.—A, pollen mother cell of the wild onion.n, nucleus.B–F, early stages in the division of the nucleus.par.nucleolus; acetic acid, gentian violet, × 350.
Fig. 127.—A, pollen mother cell of the wild onion.n, nucleus.B–F, early stages in the division of the nucleus.par.nucleolus; acetic acid, gentian violet, × 350.
If the spore mother cells are still quite young, we shall find the nucleus (Fig. 127,A,n) comparatively small, and presenting a granular appearance when strongly magnified. These granules, which appear isolated, are really parts of filaments or segments, which are closely twisted together, but scarcely visible in the resting nucleus. On one side of the nucleus may usually be seen a large nucleolus (called here, from its lateral position, paranucleus), and the whole nucleus is sharply separated from the surrounding protoplasm by a thin but evident membrane.The first indication of the approaching division of the nucleus is an evident increase in size (B), and at the same time the colored granules become larger, and show more clearly that they are in lines indicating the form of the segments. These granules next become more or less confluent, and the segments become very evident, appearing as deeply stained, much-twisted threads filling the nuclear cavity (Fig. 127,C), and about this time the nucleolus disappears.The next step is the disappearance of the nuclear membrane so that the segments lie apparently free in the protoplasm of the cell. They arrange themselves in a flat plate in the middle of the cell, this plate appearing, when seen from the side, as a band running across the middle of the cell. (Fig. 127,D, shows this plate as seen from the side,Eseen from above.)About the time the nuclear plate is complete, delicate lines may be detected in the protoplasm converging at two points on opposite sides of the cell, and forming a spindle-shaped figure with the nuclear plate occupying its equator. This stage (D), is known as the “nuclear spindle.” The segments of the nuclear plate next divide lengthwise into two similar daughter segments (F), and these then separate, one going to each of the new nuclei. This stage is not always to be met with, as it seems to be rapidly passed over, but patient search will generally reveal some nuclei in this condition.
If the spore mother cells are still quite young, we shall find the nucleus (Fig. 127,A,n) comparatively small, and presenting a granular appearance when strongly magnified. These granules, which appear isolated, are really parts of filaments or segments, which are closely twisted together, but scarcely visible in the resting nucleus. On one side of the nucleus may usually be seen a large nucleolus (called here, from its lateral position, paranucleus), and the whole nucleus is sharply separated from the surrounding protoplasm by a thin but evident membrane.
The first indication of the approaching division of the nucleus is an evident increase in size (B), and at the same time the colored granules become larger, and show more clearly that they are in lines indicating the form of the segments. These granules next become more or less confluent, and the segments become very evident, appearing as deeply stained, much-twisted threads filling the nuclear cavity (Fig. 127,C), and about this time the nucleolus disappears.
The next step is the disappearance of the nuclear membrane so that the segments lie apparently free in the protoplasm of the cell. They arrange themselves in a flat plate in the middle of the cell, this plate appearing, when seen from the side, as a band running across the middle of the cell. (Fig. 127,D, shows this plate as seen from the side,Eseen from above.)
About the time the nuclear plate is complete, delicate lines may be detected in the protoplasm converging at two points on opposite sides of the cell, and forming a spindle-shaped figure with the nuclear plate occupying its equator. This stage (D), is known as the “nuclear spindle.” The segments of the nuclear plate next divide lengthwise into two similar daughter segments (F), and these then separate, one going to each of the new nuclei. This stage is not always to be met with, as it seems to be rapidly passed over, but patient search will generally reveal some nuclei in this condition.
Fig. 128.Fig. 128.—Later stages of nuclear divisions in the pollen mother cell of wild onion, × 350. All the figures are seen from the side, exceptBii, which is viewed from the pole.
Fig. 128.—Later stages of nuclear divisions in the pollen mother cell of wild onion, × 350. All the figures are seen from the side, exceptBii, which is viewed from the pole.
Although this is almost impossible to demonstrate, there are probably as many filaments in the nuclear spindle as there are segments (in this case about sixteen), and along these the nuclear segments travel slowly toward the two poles of the spindle (Fig. 128,A,B). As the two sets of segments separate, they are seen to be connected by very numerous, delicate threads, and about the time the young nuclei reach the poles of the nuclear spindle, the first trace of the division wall appears in the form of isolated particles (microsomes), which arise first as thickenings of these threads in the middle of the cell, and appear in profile as a line of small granules not at first extending across the cell, but later, reaching completely across it (Fig. 128,C,E). These granules constitute the young cell wall or “cell plate,” and finally coalesce to form a continuous membrane (Fig. 128,F).The two daughter nuclei pass through the same changes, but in reverse order that we saw in the mother nucleus previous to the formation of the nuclear plate, and by the time the partition wall is complete the nuclei have practically the same structure as the first stages we examined (Fig. 128,F).[15]This complicated process of nuclear division is known technically as “karyokinesis,” and is found throughout the higher animals as well as plants.
Although this is almost impossible to demonstrate, there are probably as many filaments in the nuclear spindle as there are segments (in this case about sixteen), and along these the nuclear segments travel slowly toward the two poles of the spindle (Fig. 128,A,B). As the two sets of segments separate, they are seen to be connected by very numerous, delicate threads, and about the time the young nuclei reach the poles of the nuclear spindle, the first trace of the division wall appears in the form of isolated particles (microsomes), which arise first as thickenings of these threads in the middle of the cell, and appear in profile as a line of small granules not at first extending across the cell, but later, reaching completely across it (Fig. 128,C,E). These granules constitute the young cell wall or “cell plate,” and finally coalesce to form a continuous membrane (Fig. 128,F).
The two daughter nuclei pass through the same changes, but in reverse order that we saw in the mother nucleus previous to the formation of the nuclear plate, and by the time the partition wall is complete the nuclei have practically the same structure as the first stages we examined (Fig. 128,F).[15]
This complicated process of nuclear division is known technically as “karyokinesis,” and is found throughout the higher animals as well as plants.
The simple method of fixing and staining, just described, while giving excellent results in many cases, is not always applicable, nor as a rule are the permanent preparations so made satisfactory. For permanent preparations, strong alcohol (for very delicate tissues, absolute alcohol, when procurable, is best) is the most convenient fixing agent, and generally very satisfactory. Specimens may be put directly into the alcohol, and allowed to stay two or three days, or indefinitely if not wanted immediately. When alcohol does not give good results, specimens fixed with chromic or picric acid may generally be used, and there are other fixing agents which will not be described here, as they will hardly be used by any except the professional botanist. Chromic acid is best used in a watery solution (five per cent chromic acid, ninety-five per cent distilled water). For most purposes a one per cent solution is best; in this the objects remain from three or four to twenty-four hours, depending on size, but are not injured by remaining longer. Picric acid is used as a saturated solution in distilled water, and the specimen may remain for about the same length of time as in the chromic acid. After the specimen is properly fixed it must be thoroughly washed in several waters, allowing it to remain in the last for twenty-four hours or more until all trace of the acid has been removed, otherwise there is usually difficulty in staining.
As staining agents many colors are used. The most useful are hæmatoxylin, carmine, and various aniline colors, among which may be mentioned, besides gentian violet, safranine, Bismarck brown, methyl violet. Hæmatoxylin and carmine are prepared in various ways, but are best purchased ready for use, all dealers in microscopic supplies having them in stock. The aniline colors may be used either dissolved in alcohol or water, and with all, the best stain, especially of the nucleus,is obtained by using a very dilute, watery solution, and allowing the sections to remain for twenty-four hours or so in the staining mixture.
Hæmatoxylin and carmine preparations may be mounted either in glycerine or balsam. (Canada balsam dissolved in chloroform is the ordinary mounting medium.) In using glycerine it is sometimes necessary to add the glycerine gradually, allowing the water to slowly evaporate, as otherwise the specimens will sometimes collapse owing to the too rapid extraction of the water from the cells. Aniline colors, as a rule, will not keep in glycerine, the color spreading and finally fading entirely, so that with most of them the specimens must be mounted in balsam.
Glycerine mounts must be closed, which may be done with Canada balsam as already described. The balsam is best kept in a wide-mouthed bottle, specially made for the purpose, which has a glass cap covering the neck, and contains a glass rod for applying the balsam.
Before mounting in balsam, the specimen must be completely freed from water by means of absolute alcohol. (Sometimes care must be taken to bring it gradually into the alcohol to avoid collapsing.[16]) If an aniline stain has been used, it will not do to let it stay more than a minute or so in the alcohol, as the latter quickly extracts the stain. After dehydrating, the specimen should be placed on a clean slide in a drop of clove oil (bergamot or origanum oil is equally good), which renders it perfectly transparent, when a drop of balsam should be dropped upon it, and a perfectly clean cover glass placed over the preparation. The chloroform in which the balsam is dissolved will soon evaporate, leaving the object embedded in a transparent film of balsam between the slide and cover glass. No further treatment is necessary. For the finer details ofnuclear division or similar studies, balsam mounts are usually preferable.
It is sometimes found necessary in sectioning very small and delicate organs to embed them in some firm substance which will permit sectioning, but these processes are too difficult and complicated to be described here.
The following books of reference may be recommended. This list is, of course, not exhaustive, but includes those works which will probably be of most value to the general student.
These four works are translations from the German, and take the place of Sachs’s Text-book of Botany, a very admirable work published first about twenty years ago, and now somewhat antiquated. Together they constitute a fairly exhaustive treatise on general botany.—New York, McMillan & Co.
These four works are translations from the German, and take the place of Sachs’s Text-book of Botany, a very admirable work published first about twenty years ago, and now somewhat antiquated. Together they constitute a fairly exhaustive treatise on general botany.—New York, McMillan & Co.
These two books cover somewhat the same ground as 1 and 2, but are much less exhaustive.
These two books cover somewhat the same ground as 1 and 2, but are much less exhaustive.
Where the student reads German, the original is to be preferred, as it is much more complete than the translations, which are made from an abridgment of the original work. This book and the next (7 and 8) are laboratory manuals, and are largely devoted to methods of work.
Where the student reads German, the original is to be preferred, as it is much more complete than the translations, which are made from an abridgment of the original work. This book and the next (7 and 8) are laboratory manuals, and are largely devoted to methods of work.
For identifying plants the following books may be mentioned:—