Chapter VII: The Flower (1)

I. DISSECTION OF TYPES WITH SUPERIOR OVARY

MATERIAL.—For monocotyls, any flower of the lily family,
such as tulip, dogtooth violet (_Erythronium_), trillium,
star-of-Bethlehem, yucca, bear’s grass, and the like. The large
garden lilies make particularly good examples, but they are for the
most part spring bloomers. For autumn, spiderwort (_Tradescantia_),
arrow grass (_Sagittaria_), or late specimens of colchicum and
tiger lily may be used. Any of these will meet the essential
conditions of the analysis given in the text, but care should be
taken not to select for this exercise lily-like flowers of the iris
and amaryllis families, which have the _ovary inferior_.

For examples of hypogynous dicotyls, flax, linden, pinks,
corn cockle, wood sorrel, poppies, tomato blossoms, and other
common flowers can usually be obtained without difficulty. In
autumn, the geraniums so largely cultivated for ornament will
meet all the conditions of the analysis. Specimens of the cress
family—wallflower, cabbage, mustard, turnip—can generally be found
everywhere and at all seasons, and they possess the advantage of
having their flowers throughout the order put up on so nearly the
same pattern that a description of one species will answer, even in
details, for the rest.

For sympetalous specimens of the hypogynous type, hyacinth, lily of
the valley, bearberry, huckleberry, or other equivalent forms may
be used.

APPLIANCES.—A compound microscope may be needed for examining
minute objects, such as pollen grains and ovules; but for all other
purposes, a good hand lens, with the pupil’s ordinary laboratory
equipment of drawing-materials, notebook, and dissecting needles,
will be sufficient for the studies outlined in this and the four
succeeding sections.

=211. The floral envelopes.=—Make a sketch of your specimen flower from the outside. Is it solitary, or one of a cluster? If the latter, refer to 160-162 and tell the nature of the cluster. Notice the color; is it conspicuous enough to attract attention or not? Can this have anything to do with its clustered or solitary position? Label the head of the peduncle that supports the flower, _receptacle_; the outer greenish leaves, _sepals_; the inner, lighter-colored ones, _petals_. The sepals taken together form the _calyx_, and the petals, the _corolla_. Where the petals and sepals are all separate and distinct, as in the tulip and the star-of-Bethlehem, the corolla is said to be _polypetalous_ and the calyx _polysepalous_, words meaning, respectively, many-petaled and many-sepaled. _Monopetalous_ and _monosepalous_, or _sympetalous_ and _synsepalous_, are terms used to describe a condition in which the petals or sepals are all united into one, as in the morning-glory and lily of the valley. In many flowers, there is little or no difference between the two sets of organs. In such cases the calyx and corolla together are called the _perianth_, but the distinction of parts is always observed, the outer divisions being regarded as sepals, the inner ones as petals. These two sets of organs constitute the _floral envelopes_, and are not essential parts of the flower, as it can fulfill its office of producing fruit and seed without them. Note their number, mode of attachment to the receptacle, and how they alternate with each other. Remove one of the sepals and one of the petals, and notice any differences between them as to size, shape, or color. Which is most like a foliage leaf? Hold each up to the light and try to make out the veining. Is it the same as that of the foliage leaves? If a light-colored flower is used, examine a specimen that has stood in coloring fluid. How many of each set are there?

=212. The essential organs.=—Next sketch the flower on its inner face, labeling the appendages just within the petals, _stamens_, and the central organ within the ring of stamens, _pistil_. These are called essential _organs_ because they are necessary to the production of fruit and seed. Note their mode of insertion, three of the stamens in a flower like the star-of-Bethlehem alternating with the petals, and the other three with these and with the lobes of the base of the pistil.

=213. The stamens.=—Notice whether the stamens are all alike, or whether there are differences as to size, height, shape, color, etc. Do these differences, if there are any, occur indiscriminately and without order, or in regular succession between the alternating stamens? Examine one of the little powdery yellow bodies at the tip of the stamens, and see whether they face toward the pistil or away from it.

Remove one of the stamens and sketch as it appears under the lens, labeling the powdery yellow body at the top, _anther_, and the stalklike body supporting it, _filament_. Usually the filaments are threadlike, whence their name, but sometimes, as in the star-of-Bethlehem, they are flattened and look like altered petals. See if you can find such a one. What would you infer from this fact as to the possible origin of the stamens? (100.)

Notice the two little sacs or pouches that compose the anther, as to their shape and manner of opening, or dehiscing, to discharge the powder contained in them. This powder is called _pollen_, and will be seen under the lens to consist of little yellow grains. These are of different shapes, colors, and sizes, in different plants, and their surface often appears beautifully grooved and striate when sufficiently magnified. Place some of the pollen under the microscope and draw two of the grains, with their markings. In the hibiscus and others of the mallow family, they are large enough to be seen with a hand lens.

=214. The pistil.=—Remove the stamens and sketch the pistil as it stands on the receptacle. Label the round or oval enlargement at the base, _ovary_, the threadlike appendage rising from its center, _style_, and the tip end of the style, _stigma_. In some specimens the style may be very short, or wanting. In this case the stigma is _sessile_, and the pistil consists of stigma and ovary alone. If the stigma is lobed or parted, count the divisions and see if there is any correspondence between them and the number of petals and sepals, or of the lobes of the ovary. Examine the tip with a lens and notice the sticky, mucilaginous exudation that moistens it. Can you think of any use for this? If not, touch one of the powdery anthers to it, and examine it again with a lens. What do you see? Can you blow or dust the pollen from the stigma?

=215. Pollination=, or the transfer of pollen from the anther to the stigma, is a matter of great importance, as the pistil cannot develop seed without it, except in the case of a few plants like the Alpine everlasting, some species of meadow rue (_Thalictrum_), and _Alchemilla_, which have the unusual faculty of perfecting seeds in the absence of pollen. Note the relative position of pistils and stamens and see if it is such that the pollen can reach the stigma without external agency.

=216. The ovary.=—Observe the shape of the ovary, and the number of ridges, or grooves, that divide the surface. Select a flower which has begun to wither, so that the ovary is well developed, cut a cross section near the middle, and try to make out the number of _locules_, or internal divisions. Do you perceive any correspondence in number between these and the ridges or lobes outside (Fig. 280)? Between them and the lobes of the stigma? The walls that inclose the cavities of the ovary are called _carpels_, and the ridges or depressions that mark their point of union on the outside are the _sutures_, or seams. The little round bodies in the locules, as the compartments of the ovary are called, are the _ovules_, which will later be developed into seeds. Their place of attachment is the _placenta_. If they are attached to the walls of the carpels (Fig. 281), the placenta is _parietal_; if to a central axis formed by the edges of the carpels projecting inwards (Fig. 282), it is central and axial; if instead of being attached to the carpels, the ovules are borne on a projection from the receptacle, the placenta is a _free central_ one (Fig. 283). If your cross section shows a central placenta, make a vertical cut down to the receptacle and find out whether it is free, or axial. What appears to be the primary office of the ovary? Make an enlarged sketch of your specimen in both vertical and horizontal section, labeling correctly all the parts observed.

=217. Numerical plan.=—Make a horizontal diagram of the plan of the whole flower, after the model given in Fig. 284, showing the order of attachment of the different cycles,—sepals, petals, stamens, and pistils,—the number of organs in each set, and their mode of alternation with the organs of the other cycles. Notice that the parts of each set are in threes, or multiples of three. This is called the numerical plan of the flower, and is the prevailing number among monocotyls. It is expressed in botanical language by saying that the flower is _trimerous_, a word meaning measured, or divided off, into parts for three.

=218. Vertical order.=—Next make a vertical diagram of your specimen after the manner shown in Fig. 269, and note carefully that the ovary stands _above_ the other organs (this is true of all the lily family), and is entirely separate and distinct from them. In such cases the ovary is said to be _free_, or _superior_, and the other organs _inferior_, or _hypogynous_, a word meaning “inserted under the pistil.” These terms should be remembered, as the distinction is an important one in plant evolution.

=219. Summary of observations.=—In the flower just examined, we found that there were four sets of floral organs present—sepals, petals, stamens, and pistil; that the individual organs in each set were alike in size and shape; that there were the same number, or multiples of the same number of parts in each set, and that all the parts of each set were entirely separate and disconnected, the one from the other, and from those of the other cycles. Such a flower is said to be:—

_Perfect_, that is, provided with both kinds of organs essential to the production of seed—stamens and pistil.

_Complete_, having all the kinds of organs that a flower can have: viz. two sets of essential organs, and two sets of floral envelopes.

_Symmetrical_, having the same number of organs, or multiples of the same number, in each set.

_Regular_, having all the parts of each set of the same size and shape, as in the wild rose and bellflower, or if different, arranged in regular order or pairs, so that there will be a correspondence between the two sides of the flower, as in the violet, sweet pea, sage, and larkspur. For convenience, the two kinds may be distinguished as _complete_ and _bilateral regularity_, respectively.

The opposites of these terms are: _imperfect_, _incomplete_, _asymmetrical_ or _unsymmetrical_, and _irregular_.

Note that regularity refers to form, symmetry to number of parts, and that a flower may be perfect without being complete.

=220. Dissection of a typical dicotyl flower.=—(Poppy, flax, pink, tomato, linden, etc., can be substituted for the specimen used in the text.) Gently remove the sepals and petals from a wallflower, stock, mustard, or other cress flower, lay them on the table before you in exactly the order in which they grew on the stem, and sketch them. How many of each are there, and how do they alternate with one another? Sketch the pistil and stamens as they stand on the receptacle; how many of the latter are there? Notice that two of the six are outside and a little below the others, alternate with the petals, while the other four stand opposite them, as is natural, if they were alternating with another ring of stamens between themselves and the corolla. Put a dot before two of the sepals in your first drawing to indicate the position of the two outer stamens, and a cross before the other two to show where stamens are wanting to complete the symmetry of this set, as in Fig. 287. When parts necessary to complete the plan of a flower are wanting, as in this case, they are said to be _obsolete_, _suppressed_, or _aborted_. Place dots before the petals to represent the other four stamens. Sketch one of the anthers as it appears under a lens, showing the arrow-shaped base, and the mode of attachment to the filament. Is it such that the pollen can reach the stigma without external agency? In what manner do the anthers open to discharge their pollen? Are the anthers and stigma mature at the same time? Remove all the stamens from a flower and sketch the pistil, showing the long, slender ovary, the very short style, and the _capitate_ (that is, round and knoblike) stigma. Make cross and vertical sections of one of the older pistils lower down on the stem. How many ovules does it contain? How are they attached? Represent the position of the pistil by a small circle in the center of your sketch of the separate parts. You have now a complete ground plan of the flower. Diagram a vertical section, as in Fig. 289, showing the position of the ovary with reference to the other parts, and report in your notebook as to the following points:—

Numerical plan Presence or absence of parts
Symmetry Union of parts
Regularity (complete or bilateral) Position of ovary

II. DISSECTION OF TYPES WITH INFERIOR OVARY

MATERIAL.—For monocotyls: in spring and early summer, iris,
snowflake, freesia, crocus, narcissus, daffodil, can be used;
in autumn, gladiolus, blackberry lily, fall crocus, star grass
(_Hypoxys_). For dicotyls: in spring, flowers of apple, pear,
quince, gooseberry, squash, gourd, melon (with both male and female
flowers); in late summer and autumn, fuchsia, evening primrose
(_Œnothera_), willow-herb (_Epilobium_).

=221. Study of a monocotyl flower.=—Compare with the specimens examined in the last section, a narcissus, snowflake, or iris flower. What difference do you notice in the position of the ovary? Would you call it _inferior_ (below the other parts) or _superior_ (above them)? How was it in the lily and the hyacinth? If your specimen is an iris, notice that it is sessile in the axil of a large bract called a _spathe_, which conceals the lower part of the flower. Remove the spathe and observe that the lower part of the perianth is united into a long, narrow tube, from the top of which the sepals and petals extend as long, curving lobes.

=222. Arrangement of parts.=—Sketch the outside of the flower, labeling the oblong, three-lobed enlargement at the base, _ovary_; the prolongation above it, _tube of the perianth_; the three outer lobes with the broad sessile bases, _sepals_; the others, with their bases narrowed and bent inward, _petals_. Now turn the flower over and sketch the inside, labeling the three large, petal-like expansions in the center, _stigmas_. Do you see any stamens? Remove one of the sepals and look under the stigma; what do you find there? Notice the little honey pockets at the foot of the stamen. Run the head of your pencil into them and see what would happen to the head of an insect probing for honey.

Remove the perianth and sketch the remaining organs in profile, showing the position of the stamens. Do you see any advantage in their position? Can you determine the use of the crest of hairlike filaments on the upper side of the sepals? Remove a stamen and sketch it.

=223. The pistil.=—Remove as much of the upper part of the perianth tube as you can without injuring the pistil, and with a sharp knife cut a vertical section down through the ovary so as to show the long style and its connection with the placenta. Make a sketch of this longitudinal section (see Fig. 291), labeling the parts observed. Notice whether the placenta is central or parietal. Draw a cross section of the ovary; how many locules has it? How many ovules in each? Where are they attached? Is the placenta free central or axial (Fig. 293)? Examine with a lens the little flap at the base of the two-cleft apex of one of the stigmas, and look for a moist spot to which the pollen will adhere. Label this in your sketch, _stigmatic surface_. No seeds can be matured unless some of the pollen reaches this surface; can you think by what agency it is carried there? What insects have you seen hovering about the iris? Notice that in drawing his head _out_ of the flower, an insect would not touch the stigmatic surface, since it is on the _upper_ side of the flap and he would be probing _under_ it. But in entering the next flower that he visits, he is likely to strike his head against the flap and turn it under, thus dusting it with pollen brought from another flower.

=224. Diagrams.=—Draw diagrams showing the horizontal and vertical arrangement of parts in the iris or other specimen examined, and compare with those made of the monocotyl studied in the preceding section. In what respect does it differ from them? How do you account for the difference in the number of stamens, if there is any? (220.)

=225. The vertical order.=—The difference in vertical arrangement is an important one. Bear in mind that flowers of this type have the ovary _inferior_, that is, inserted _under_ the other organs (Figs. 296, 304), which are then said to be superior, or _epigynous_, a word which, as you know from the prefix _epi_ (47), means over or above the pistil. To make the matter clear, the two sets of terms employed for describing the position of the ovary are given below in parallel columns:

Hypogynous Epigynous
Ovary superior Ovary inferior
Calyx or perianth inferior Calyx or perianth superior

The epigynous arrangement is considered as marking a higher stage of floral development than the hypogynous, which is characteristic of a more simple and primitive structure.

=226. Dissection of a dicotyl flower.=—Sketch a blossom of quince or apple, fuchsia, evening primrose, etc., first from the outside, then from the inside, and then in vertical section, labeling the parts as in your other sketches. Notice in the pear or apple how the ovary is sunk in the hollowed-out receptacle. Where are the other parts attached? Are they inferior or superior? Hold up a petal to the light and examine its venation through a lens. (Use for this purpose a petal from a flower that has stood in red ink for two or three hours.) Is it parallel-veined or net-veined? If the flowers are clustered, what is the order of inflorescence? Does the position of the flowers on their branch correspond to that of the leaf axils on the same kind of plant?

=227. The stamens.=—Remove the petals from a flower and examine the stamens with a lens. Notice the attachment and shape of the anthers. Are they all of the same color? How do you account for the difference, if there is any? Is the position of the pistil and stamens such that the pollen from the anthers can readily reach the stigmas without external aid? Examine the pistil in flowers of different ages, and see if the stigma is mature (that is, moist and sticky) at the same time that the anthers are discharging their pollen. Make an enlarged sketch of a stamen showing the shape of the anther and the method of opening to discharge pollen.

=228. The pistils.=—How many pistils do you find in the apple blossom (or other flower under examination)? Are they distinct, or united? Find where the styles originate; what do you see there? Make a cross section of the ovary and count the locules; how does their number compare with that of the styles? Can you make out the number of ovules in each? If not, use a young fruit; as it is only an enlarged ovary, it will show the parts correctly. Compare it with a ripe fruit and see if all the ovules matured. Can you think of any reasons why some of them might fail? Do you see any signs of nourishment stored in the ovary? Name all the ways you can think of in which the ovary can benefit the ovules and seeds. Draw the ovary in cross and vertical sections, labeling correctly all the parts.

=229. The numerical plan of dicotyls.=—Diagram the plan of the flower in cross and vertical section. How many parts are there in each set? Can you tell readily the number of stamens? When the individuals of any set or cycle of organs are too numerous to be easily counted, like the stamens of the apple, pear, and peach, or the petals of the water lily, they are said to be _indefinite_. It is very seldom that perfect symmetry is found in all parts of the flower. The stamens and pistil, in particular, show a great tendency to variation, so that the numerical plan is generally determined by the calyx and corolla. Where the parts are in fives, as in the pear, quince, and wild rose, the flower is said to be _pentamerous_, or in sets of five. This is the prevailing number among dicotyls, though other orders are not uncommon. In the mustard family (220) and other well-known species, the fourfold order prevails, while some of the saxifrages have their parts in twos, and the magnolia and the pawpaw have a threefold arrangement.

=230. Intermediate types.=—Flowers like the peach and rose represent an intermediate type in which the calyx, petals, and stamens are attached to a prolongation of the receptacle that extends above the ovary, but is not united with it (Fig. 301). In general, a flower is not considered as belonging to the epigynous kind unless the ovary is more or less consolidated with the parts around it (Fig. 304).

III. STUDY OF A COMPOSITE FLOWER

MATERIAL.—The largest heads attainable should be selected, as the
florets are small at best, and difficult to handle. The large
cultivated sunflower (_Helianthus annuus_) makes an ideal specimen,
if accessible. Oxeye daisy and dandelion can be obtained throughout
the season almost everywhere, but the former has no pappus, and
the latter does not show the tubular disk flowers. Other common
specimens are: for spring, mayweed, Jerusalem artichoke, coreopsis,
arnica; for late summer and autumn, China aster, golden aster
(_Chrysopsis_), sneezeweed, elecampane—and, in fact, the great
majority of flowers to be found at this season are of the composite
family. Oxeye daisy is used as a model in the text on account of
its general accessibility, but almost any specimen of the radiate
kind will meet all essential conditions of the analysis.

=231. The ray flowers.=—Examine the upper side of an oxeye daisy through a lens. Of what is the yellow button in the center composed? Count the narrow, petal-like rays disposed around the center. To decide what they are, look for a small two-cleft body at the base of the ray; this is the pistil. Do you see any stamens in the ray? An examination will show that all rays contain pistils, but no stamens; they are, therefore, not petals, but the corollas of imperfect flowers. Look at the upper edge of a ray of sneezeweed, coreopsis, arnica, chicory, etc., for small teeth or notches; these represent the lobes of a sympetalous corolla. Split one of the tubular corollas of the disk down one side and open it out flat; does it throw any light on the morphology of the ray? In many composite plants, as the sunflower, coneflower, coreopsis, the rays are all _neutral_; that is, they have neither pistil nor stamens. Are they of any use in such cases? If you are in doubt, remove all the rays from a head; would the disk be noticeable enough to attract attention without them? What is the principal office of the rays?

=232. The involucre.=—Look at the cluster of green, leafy scales on the under side of the head. It is not a calyx, but a collection of bracts, called an _involucre_. Have you ever noticed the bracts under the separate flowers on a raceme? (161.) What would be the position of the bracts if all the flowers of the raceme were compacted into a head like the daisy or sunflower? Is the involucre of any use? Cut it away gently so as not to disturb the other organs and see what happens to the rays.

=233. The disk flowers.=—Cut a vertical section through the head of a flower and notice the broad, flat receptacle (in some cases round or columnar) on which the tiny florets are seated. Observe whether it is naked, or whether it bears chaffy scales inclosing the florets. Make an enlarged drawing of this section, showing the insertion of the different parts and labeling them all correctly. What differences do you observe between the disk and the ray flowers?

=234. The pappus.=—Open one of the disk flowers with a dissecting needle and observe the small striate (in some specimens, hairy) body to which the base of the style is attached. This is the ovary, inclosed in the lower part of the calyx, which has become incorporated with it. When mature, it will form a small, one-seeded fruit called an _akene_. Can you see the ovule? Where is it attached? (Use a mature akene for this purpose.) In most plants of this family, the akene is surmounted by delicate hairy bristles, as in the dandelion, wild lettuce, and groundsel; or by small chaffy scales, as in the sneezeweed and sunflower, and sometimes by hooks and barbed hairs, like those of the tickseed, bur marigold, and cocklebur. These appendages constitute the _pappus_. They are modifications of the sepals, and serve an important purpose in aiding the distribution of the seed. Can you suggest some of the ways in which they may aid in accomplishing this object?

=235. The stamens and pistil.=—Remove the corolla of a disk flower carefully so as not to disturb the inclosed organs, and notice how the stamens are united into a tube by their anthers. Flatten out the tube and make an enlarged sketch of it, showing the long, narrow shape of the anthers and their mode of attachment. Can you make out how they open to discharge their pollen? Examine one of the younger florets near the center of the disk, and observe that the tip of the style is inclosed in the anther tube with the lobes of the stigma pressed tightly together by their inner faces (Fig. 315), so that it is impossible for any of the pollen to reach the stigmatic surface. It remains in this position till the anthers have shed their pollen, then, as may be seen by examining an older flower, the style begins to elongate, pushing up the pollen that has fallen on the hairy outside of the closed stigma, and forcing it out of the corolla tube, where it can be scattered by insects among the other flowers of the cluster. When the pollen of its own floret has been thus disposed of, the stigma lobes open and curl outward, ready to receive the pollen from other flowers. This arrangement is practically universal among plants of the composite family; can you divine its object? It will be shown later, that much larger and stronger seeds are produced when the pistil is pollinated from a different flower, or, better still, from a different plant of the same species; hence, you see what a useful adaptation this is.

=236. Nature of a composite flower.=—It will be evident, from the examination just made, that the daisy, dandelion, sunflower, etc., are not single flowers, but compact heads of small blossoms so closely united as to appear like a single individual; hence they are said to be _composite_, or compound. They are the most numerous and widely disseminated of all plants, comprising one seventh of the entire flowering vegetation of the globe, and are regarded by botanists as representing the most advanced stage of floral evolution. Can you point out some of the adaptations to which their success in solving the problems of plant life is due? (164.)

IV. SPECIALIZED FLOWERS

MATERIAL.—For spring and early summer: sweet pea, black locust,
wistaria, lupine, or any of the characteristic butterfly-shaped
flowers of the pea family. For autumn or late summer: tropæolum,
monkshood, or a bilabiate flower—snapdragon, digitalis, dead
nettle, salvia, catalpa, etc.—of the mint or figwort family.

=237. Irregularity and specialization.=—Irregularity and bilateral regularity are, as a rule, indicative of specialization, or adaptation to a particular purpose, such as the ready distribution of pollen, or its protection against injury. These adaptations are more noticeable in the corolla than in other parts, and hence flowers of this kind are usually classed according to the shape of their corollas. The most highly specialized flowers in this respect are the orchids, but they are too rare and difficult of access to be available objects for study. The most familiar and widely distributed kinds of specialized corollas are the _bilabiate_, or two-lipped, and the _papilionaceous_, or butterfly, forms. The first is characteristic of the mint and figwort families, of which the toadflax, sage, and catalpa are familiar examples. The second comprises the well-known papilionaceous flowers of the pea family, named from the Latin word _papilio_, a butterfly, on account of their general resemblance to that insect.

=238. Dissection of a papilionaceous flower.=—Sketch a blossom of any kind of pea or vetch as it appears on the outside. Are the sepals all of the same length and shape? If not, which are the shorter, the upper or the lower ones?

Turn the flower over and examine its inner face. Notice the large, round, and usually upright petal at the back, the two smaller ones on each side, and the boat-shaped body between them, formed of two small petals more or less united at the apex. Press the side petals gently down with the thumb and forefinger and notice how the essential organs are forced out from the little boat in which they are concealed. Observe how the end of the style is bent over so as to bring the stigma uppermost when the petals are depressed. Imagine the legs of a bee or a butterfly resting there as he probed for honey; with what organ would his body first come in contact when he alighted? If his thorax and abdomen had previously become dusted with pollen when visiting another flower, where would the pollen be deposited? Do you notice anything in the color, shape, or odor of this flower that would be likely to attract insects? Have you ever observed insects hovering around flowers of this kind; for example, in clover and pea fields, and about locust trees and wistaria vines? What kind of insects, chiefly, have you seen about them?

Remove the sepals and petals from one side, and sketch the flower in longitudinal section, showing the position of the pistil and stamens. Then remove all the petals, and spread in their natural order on the table before you, and sketch as they lie (Fig. 319). Label the large, round upper one, _standard_ or _vexillum_; the smaller pair on each side, _wings_, and the two more or less coherent ones in which the pistil and stamens are contained, _keel_.

=239. The stamens.=—Count the stamens, and notice how they are united into two sets of nine and one. Stamens united in this way, no matter what the number in each set, are said to be _diadelphous_, that is, in two brotherhoods. Notice the position of the lone brother, whether below the pistil—next to the keel—or above, facing the _vexillum_. Would the projection of the pistil, when the wings are depressed, be facilitated to the same extent if the opening in the stamen tube were on the other side, or if the filaments were _monadelphous_—all united into one set? Flatten out the stamen tube, or sheath, formed by the united filaments, and sketch it.

=240. The pistil.=—Remove all the parts from around the pistil, and sketch it as it stands upon the receptacle. Look through your lens for the stigmatic surface (223). See if there are any hairs on the style, and if so, whether they are on the front, the back, or all around. Can you think of a use for these hairs? Notice how the long, narrow ovary is attached to the receptacle; is it sessile, or raised on a short footstalk? If the latter, label the footstalk, _stipe_. Select a well-developed pistil from one of the lower flowers, open the ovary parallel with its flattened sides, and sketch the two halves as they appear under the lens. Notice to which side the ovules are attached, the upper (toward the vexillum) or the lower, and label it, placenta. How many locules has the ovary? How many carpels? How can you tell (216)?

=241. Plan of the flower.=—Diagram the flower in horizontal and vertical section, and decide upon the following points:—

Numerical plan
Symmetry
Regularity
Union of parts
Position of the ovary

=242. Significance of these distinctions.=—These distinctions are important to remember, not only because they are very useful in grouping and classifying plants, but because they mark successive stages in the evolution of the flower. In general, flowers of a primitive type and less advanced organization are characterized by having their organs free and hypogynous, while the more highly developed forms show a tendency to consolidation and union of parts, and the epigynous mode of insertion. Irregularity also, since it indicates specialization and adaptation to a particular purpose, may be regarded as a mark of advanced evolution.

=243. Dissection of a bilabiate flower.=—Make a similar study of the flower of a salvia, dead nettle, catalpa, or other specimen of the bilabiate kind. Make diagrams and report as to (1) numerical plan; (2) presence or absence of parts; (3) regularity; (4) union of parts; (5) position of ovary. Observe especially the relative position of stigma and anthers; is it such that the pollen can reach the stigma without external aid? Does the peculiar shape of the corolla serve any other purpose than to attract the attention of insect visitors by its conspicuous appearance? What is the use of the projecting underlip? Is it any convenience to a bee, for instance, to have a platform to rest on while gathering pollen or honey? What is the use of the arched upper lip? Cut it away and notice the exposed condition of the stamens and pistil. Turn a flower upside down; what would be the effect on a visiting bee or butterfly? (Exps. 83, 84.)

=244. Morphology of the flower.=—We have seen that the venation of petals and sepals corresponds in a general way with that of foliage leaves of the class to which they belong, and that their arrangement around their axis is analogous to the arrangement of foliage leaves on the branch. In our study of inflorescence, it was observed that flowers and flower buds occur in the same positions where leaf buds occur, and that they are subject to the same laws of arrangement and growth. We learned, also, in our study of leaves, something about the wonderful modifications that these organs are capable of undergoing; and finally, an examination of a number of different flowers has shown them capable of undergoing modifications to an equal or even greater extent, and examples of the transition of almost any floral organ into another may be observed by one who will take the trouble to look for it. Stamens and petals are found in all stages of transformation, from the slightly flattened filament of the star-of-Bethlehem, or the yellow pollen speck on the petal of a rose, to the brilliant staminodia, or transformed stamens of the canna (Fig. 327), which simulate petals so perfectly that their real nature is never suspected by the ordinary observer. The transition from spines and bracts to the brilliant corolla of the cactus (Fig. 328) is so gradual that we are hardly aware of it till we examine a specimen and see it actually going on before our eyes.

It must not be supposed, however, that an organ is ever developed as one thing and then deliberately changed into something else. When we speak loosely of one organ being modified into another, the meaning is merely that it has developed into one thing instead of into something else that it was equally capable of developing into.

=245. The course of floral evolution.=—For the reasons mentioned, the flower is regarded as merely a branch with modified leaves and the internodes indefinitely shortened so as to bring the successive cycles into close contact, the whole being greatly altered and specialized to serve a particular purpose. With this conception of the nature of the flower, we can readily see that the less specialized its organs are and the more nearly they approach in structure and arrangement to the condition of an undifferentiated branch, the more primitive and undeveloped is the type to which it belongs. On the other hand, if the parts are highly specialized and widely differentiated from the crude branch, a proportionately high stage of floral evolution is indicated.

V. FUNCTION AND WORK OF THE FLOWER

MATERIAL.—For this exercise, flowers of the mallow
family—hollyhock, abutilon, mallow, hibiscus, cotton, okra,
etc.—are particularly recommended because they have pollen grains
so large that they can be studied fairly well with a hand lens.
Lily, tulip, iris, etc., will also meet all essential conditions of
the study outlined in the text. A strand of silk from a pollinated
ear of corn is an excellent example for showing the growth of the
pollen tube, under the microscope.

APPLIANCES.—A compound microscope; a watch crystal; sugar solution
of 5 to 15 per cent.

EXPERIMENT 77. TO SHOW THE GERMINATION OF POLLEN GRAINS.—Put a drop
of 5 per cent sugar solution into a watch crystal or a concave
slide, seal by smearing the edges with vaseline, and cover with a
glass to keep out the dust. Examine at intervals of five minutes
under the microscope (a hand lens will show the result with the
specimens recommended, though not so well), and the pollen grains
will be observed to send out long filaments or tubes into the
sirup, as a germinating seedling sends its radicle into the soil.

=246. Office of the flower.=—The one object of the flower is the production of fruit and seed, and all its wonderful specializations and variations of form and color tend either directly or indirectly to this end.

=247. Pollination and fertilization.=—It was stated in 215 that only in very exceptional cases can seed be developed unless some of the pollen reaches the stigma. This act, called _pollination_, is an essential step in seed production, but is not sufficient to secure that end unless it leads to the process known as _fertilization_. Successful pollination is a necessary preliminary to fertilization, and the one begins where the other ends.

=248. The next step toward fertilization.=—Examine with a lens the pollinated pistil of a mallow, lily, or other large flower, and notice the flabby, withered appearance of grains that have stood for some time on the stigma, as compared with those of a newly opened anther. Can you account for the difference? Touch the tip of your tongue to the stigma, or apply the proper chemical test, and it will be seen that the sticky fluid which it exudes, contains sugar. Refer to Exp. 77 and say what effect this substance has on the pollen.

=249. The pollen tube.=—The same thing happens when a pollen grain falls on the moist surface of the stigma. It begins to germinate by sending a little tube down into the substance of the pistil, and the withered appearance of the grains on the stigma results from the nourishment in them having been exhausted, just as the endosperm of the seed is exhausted when the embryo begins to germinate. Here, however, the analogy ends, for the pollen tube is not adapted, like the radicle of the seedling, to absorb and convey nourishment up to the other parts, but to feed and carry down to the ovary two small bodies called _generative cells_, which it discharges there, and then its work is done and it disappears. So it must be borne in mind that when we speak of the germination of the pollen grains, we mean something really very different from the germination of a seed.

=250. The course of the pollen tube.=—Cut the thinnest possible section through a freshly pollinated pistil and place under the microscope. Watch the pollen tubes from the grains on the stigma as they descend through the style toward the ovary. A pollinated strand of corn silk—which is only a very much elongated style—is excellent for this purpose. It is so thin and transparent that no section need be made, and the tube can be traced as it works its way down through the entire length of the threadlike style to the young grain, or ovary, on the cob. The time required for the tube to penetrate to the ovary varies in different flowers according to the distance traversed and the rate of growth. In the crocus it takes from one to three days; in the spotted calla, about five days; and in orchids, from ten to thirty days. As a rule, it occupies only a few hours. Sometimes the pistil is hollow, affording a free passage to the pollen tube; in other cases, it is solid, and the growing tube eats its way down, as it were, feeding on the substance of the pistil as it grows. How is it in the flower you are examining? It takes a grain of pollen to fertilize each ovule, and where more than one seed is produced to a carpel, as is commonly the case, at least as many pollen tubes must find their way to each locule of the ovary as there are ovules—provided all are fertilized.

=251. Fertilization.=—When a pollen tube has penetrated to the ovary, it next enters one of the ovules, usually through the micropyle (Fig. 330, _m_). There it penetrates the wall of a baglike inclosure called the _embryo sac_ (Fig. 330, _u_, _t_, _z_), where one of the generative cells emitted by the pollen tube fuses with a large cell contained in the embryo sac, known as the _germ cell_, or _egg cell_ (Fig. 330, _z_). The fusion of these two bodies is what constitutes fertilization. The cell formed by their union finally develops into the embryo, and the other contents of the sac into the endosperm, and the ripened ovules become seeds.

=252. Stability of the process of fertilization.=—The phenomena that characterize the functions of fertilization and reproduction are the most uniform and stable of all the life processes, varying little not only in different species and orders, but throughout the whole vegetable kingdom. And since these functions furnish a more reliable standard for judging of the real affinities of the different groups than do mere external resemblances, which are more liable to variation and may often be accidental, they have been chosen by botanists as the ultimate basis for the classification of plants.

=253. Embryology.=—The study of the developing plantlet, known as _embryology_, is a comparatively recent branch of science, and has greatly enlarged our knowledge of the life history of both plants and animals, by bringing to light resemblances that exist between the most widely divergent species in their earlier stages of development and thus showing traces of a common origin. It has shown further, that every individual plant or animal, in its development from the embryo to the mature state, passes briefly through stages apparently similar to those which the species has traversed in the course of its evolution. This summary repetition, by the individual, of the evolutionary progress of its kind is known as the _biogenetic law_, and through its intelligent application some of the most intricate problems in both physiology and psychology have been solved.

Practical Questions

1. Does the biogenetic law throw any light on the resemblances
sometimes observed between leaves of different ages in unlike
species; for example, the fig and the mulberry? (170; Field Work,
p. 195.)

2. Can you name any other examples of plants or parts of plants
which show mutual resemblances in their early stages that do not
exist at maturity?

3. Are there other causes than those acting under the biogenetic
law to which some of these resemblances may be referred; for
instance, the down and waxy coating on young leaves and bud scales?
(148, 207.)

VI. HYBRIDIZATION

MATERIAL.—Several potted plants of tulip, lily, or any attainable
large flowered kind; or preferably a small plot in a garden or
nursery.

APPLIANCES.—A pair of dissecting scissors, a camel’s-hair brush,
and some paper bags.

EXPERIMENT 78. DOES IT MAKE ANY DIFFERENCE WHETHER A FLOWER HAS
ITS OVULES FERTILIZED WITH ITS OWN POLLEN OR WITH THAT OF ANOTHER
FLOWER OF THE SAME KIND?—Carefully remove the _unopened_ anthers
from a bud of a tulip, or other large flower just ready to unfold
(Fig. 331), inclose the mutilated bud in a small paper bag until
the stigma is mature, as shown by stickiness, then transfer to it
with a camel’s-hair brush some pollen from another flower. On the
stigma of a second flower of the same kind place some of its own
pollen, and cover with a paper bag until the stigma withers, to
keep foreign pollen from reaching it by means of wind or insects.
Watch until seeds are matured. Which flower produces the more
seeds or the better ones? Plant the seeds; which produce the more
vigorous progeny?

EXPERIMENT 79. CAN A FLOWER BE FERTILIZED WITH POLLEN OF A
DIFFERENT KIND?—Dust the stigma of a tulip or a lily, from which
the stamens have been removed, with pollen from a narcissus, iris,
or amaryllis. Cover to protect from wind and insects. Are any seeds
produced?

Experiments of this kind, to be conclusive, ought to be performed
on a sufficient number of plants and through at least three
generations. This is hardly practicable for class work, but
students who are specially interested in the subject may carry
on experiments at home, or supply their place, to some extent,
by observations out of doors, if there are any farms or gardens
accessible.

=254. Self-fertilization= takes place when a stigma is pollinated from the same flower. Horticulturists have long known that continued self-fertilization, or “in-breeding” as it is called by nurserymen, tends to deteriorate a stock; but Charles Darwin was the first to explain, by a series of pains-taking experiments, the meaning of those careful adjustments which the more highly organized plants, as a rule, have developed to guard against it.

=255. Cross-fertilization= is effected by the pollination of a stigma from another flower of the same variety or species. As used by practical horticulturists, the expression means that the two factors, pollen and ovule, belong to different plants. Since pollination is the necessary antecedent to fertilization, and the only means by which we can control it, the breeder’s part in crossing is concerned with this act only and nature does the rest. Darwin’s experiments—and they are confirmed by the experience of plant growers everywhere—prove that the offspring from crossing different plants of the same kind is usually stronger and more productive than that from self-fertilized ones; and if the parent stocks are grown in different places and under different conditions, the offspring is more vigorous than that from the same kind of plants grown under like conditions. For instance, plants from crossed seeds of morning-glory vines growing near each other exceeded in height those from self-fertilized seeds as 100:76; while the offspring of plants growing under different conditions exceeded those of the other cross, in height, as 100:78; in number of pods, as 100:57, and in weight of pods, as 100:51. Knowledge of this kind, when applied to the raising of fruits and grains for market, is of incalculable value to gardeners and farmers, and also to the amateur who raises fruits or flowers for pleasure.

=256. Hybridization= is the crossing of two plants of different species or of widely separated varieties of the same species. The resulting offspring is a _hybrid_. Hybridization can take place only within certain limits. If the species are too unlike, the pollen will either not take effect at all, or the resulting offspring will be too weak and spindling to live; or if they survive, will not be able to set seed (Exp. 79).

=257. Effects of hybridization.=—The most important practical uses of hybridizing are: (1) it “breaks the type” by causing plants to vary, and thus gives the breeder a fresh starting point for a new strain; and (2) when the parent species are not too unlike, it accentuates the good effects of crossing, and sometimes gives rise to offspring greatly surpassing either parent in size and vigor. In regard to variability it may act in three ways: (1) the hybrid may wholly resemble one parent or the other, in which case there is, of course, no variation; (2) it may resemble one parent more than the other; or (3) it may show a blending of the characters of the two, as when a cross between a red poppy and a white gives rise to a light pink, or a mixed red and white variety. In the first two cases, the characters of the parent that manifest themselves are said to be _dominant_; those which do not, _recessive_.

=258. Mendel’s Law.=—So long ago as the middle of the last century it was discovered by Gregor Mendel, an Austrian investigator, that hybrids vary in certain cases according to a fixed law, by means of which the proportionate share of the characteristics of the two parent forms inherited by the offspring can be foretold with almost mathematical precision. The controversy over Darwin’s “Origin of Species,” which was raging at the time, caused Mendel’s discoveries to be overlooked for a generation, and it is only within the last few years that their importance has been realized. The principle of variation demonstrated by him in a series of experiments, and confirmed by later investigators is, briefly, this: If two parents differing in some fixed characteristic be crossed, the entire offspring, in the first generation, will be like the parent possessing the dominant quality. If all the seed of this generation is planted and carefully protected from foreign pollen, its offspring composing the second generation from the parents will vary in the proportion of ¾ dominants (_D_, _D′_, line 2 of the diagram) to ¼ recessives (_R_). Planting _all_ the seeds of the second generation and carefully shielding their progeny from foreign pollen, we get from _D_, line 2, all pure dominants (_D_, line 3)—that is, plants producing only their own type, and from _R_, line 2, all pure recessives (_R_, line 3). But from each of the two sets of dominants, _D′D′_, line 2, marked “impure” in the diagram, and so called because their seeds may produce both dominants and recessives, we get the same result as in the second generation, namely: pure dominants (_D′D′_, line 3), pure recessives (_R′R′_, line 3), and impure dominants (_D″D″_, _D″D″_, line 3). If it were possible to distinguish the seeds of these impure dominants before germination and plant them only, for no matter how many generations, the result would always be approximately the same,—¼ pure dominants, ¼ pure recessives, and ²⁄₄ impure dominants capable of producing both dominants and recessives in the proportion of 3:1.

=259. Practical applications.=—Four principles of great importance to plant breeders follow from this law in cases to which it applies: (1) the absence of variation in the first generation of hybrids is no sign that it may not occur later; (2) pure recessives always breed true; hence, if they show the desired character, no further selection is necessary for that character; (3) pure dominants always breed true, but the distinction between pure and impure is usually not apparent in one generation; (4) the descendants of “impure” parents cannot be depended upon to come true to either type, but impure dominants may breed recessives, and _vice versa_, with the presumption, however, of 3:1 in favor of dominants.

Practical Questions

1. Would hybridization account for some of the diversities
mentioned in 170? (See 257.)

2. To what cases would it not apply? (256; Exp. 79.)

3. Would it be worth while to try to hybridize the potato and
squash? The squash and pumpkin? The lily and rose? Sweetbrier and
wild rose? Apple and peach? Wild crab and sweet apple? Blackberry
and strawberry? Blackberry and raspberry? Lemon and watermelon?
Lemon and orange? Why, or why not, in each case? (256; Exps. 78,
79.)

VII. PLANT BREEDING

MATERIAL.—If practicable, visit a market garden, a florist’s
establishment, or, lacking these, the fruit and vegetable stalls of
a city market.