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.
[Illustration: FIGS. 265-267.—Flower of a monocotyl
(star-of-Bethlehem), with superior ovary dissected: 265, entire
flower, showing the different sets of organs: _pet_, petals; _sep_,
sepals; _sta_, stamens; _pist_, pistil; _ped_, peduncle; 266, side
view with all the petals and sepals but two removed to show order
of the parts: _r_, receptacle; _o_, ovary; _sty_, style; _stig_,
stigma—parts composing the pistil; _f_, filament; _a_, anther—parts
composing the stamen; 267, cross section of the ovary: _c_, _c_,
carpels; _ov_, ovules; _pl_, placenta.]
=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?
[Illustration: FIGS. 268-269.—Yucca blossom: 268, external view:
_br_, bract; _pd_, peduncle; _r_, receptacle; _s_, sepal; _pet_,
petal; 269, vertical section: _ped_, peduncle; _br_, bract; _r_,
receptacle; _per_, perianth; _sta_, stamen; _o_, ovary; _sty_, style;
_stg_, stigma. The last three parts named compose the pistil.]
[Illustration: FIGS. 270-274.—Stamens: 270, a typical stamen with the
terminal anther, _b_, surmounting the filament, _a_, and opening in
the normal manner down the outer side of each cell; 271, stamen of
tulip tree, with adnate extrorse anther; 272, stamen of an evening
primrose (_Œnothera_) with versatile anther; 273, stamen of pyrola,
the anther cells opening by chinks or pores at the top; 274, stamen
of a cranberry, with the anther cells prolonged into a tube and
opening by a pore at the apex. (_After_ GRAY.)]
=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.)
[Illustration: FIGS. 275-278.—Forms of pollen: 275, from _mimulus_;
276, star cucumber; 277 wild balsam apple; 278, _hibiscus_. (_After_
GRAY.)]
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.
[Illustration: FIGS. 279, 280.—Ovary of yucca, a hypogynous
monocotyl, dissected: 279, vertical section; _ov_, ovules; 280,
diagram of a horizontal section of the same, enlarged, showing the
three carpels and six locules; _ds_, dorsal sutures; _vs_, ventral
sutures; _ov_, ovules; _pl_, placenta.]
[Illustration: FIGS. 281-283.—Different kinds of placenta: 281,
parietal; 282, central and axial; 283, free central. 281 and 282 are
horizontal sections; 283, vertical.]
[Illustration: FIG. 284.—Horizontal diagram of a flower of the lily
kind. The dot represents the growing axis of the plant.]
=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.
[Illustration: FIGS. 285-288.—A flower of the cress family: 285,
side view; 286, view from above; 287, diagram of parts: _p_, petals;
_s_, sepals; _st_, stamens; _pi_, pistil; _cl_, claw of petal; +, +,
position of the missing stamens; 288, pistil and stamens, enlarged.
(_After_ GRAY.)]
[Illustration: FIG. 289.—Section of a tomato flower, showing the
hypogynous arrangement: _cx_, calyx; _c_, corolla; _s_, stamens; _p_,
pistil; _o_, ovary; _st_, stigma. (Twice natural size.)]
=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.
[Illustration: FIG. 290.—Iris flower: _sp_, spathes; _s_, sepals +
_p_, petals = perianth.]
[Illustration: FIG. 291.—Vertical section of iris flower: _ov_,
ovules; _pl_, placenta; _tu_, tube of the perianth inclosing the
style; _sta_, stamen; _sti_, stigma: _o_, ovary. (_After_ GRAY.)]
[Illustration: FIG. 292.—Vertical section of iris flower, with
perianth removed, showing a stamen and three stigmas: _su_, stigmatic
surface.]
[Illustration: FIG. 293.—Cross section of ovary of iris flower: _c_,
_c_, carpels; _l_, _l_, locules; _ov_, ovules; _pl_, placenta.]
=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.
[Illustration: FIG. 294.—Horizontal diagram of iris 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.
[Illustration: FIGS. 295-296.—Evening primrose, dicotyl flower with
inferior ovary: 295, exterior view; 296, longitudinal section,
showing vertical arrangement of parts.]
=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?
[Illustration: FIGS. 297-300.—Flower and sections of pear: 297,
cluster of blossoms, showing inflorescence; 298, vertical section
of a flower; 299, ground plan of a flower; 300, vertical section of
fruit.]
[Illustration: FIG. 301.—Vertical section of an almond blossom with
petals removed, showing the perigynous arrangement.]
=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.
[Illustration: FIGS. 302-304.—Diagrams showing arrangement of parts
with reference to the ovary: _bd_, receptacle; _k_, calyx; _kr_,
corolla; _st_, stamens; _fr_, ovary; _g_, style; _n_, stigma; 302,
perigynous; 303, hypogynous; 304, epigynous.]
=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.
[Illustration: FIGS. 305-308.—An oxeye daisy: 305, a flower head;
306, vertical section of a head; 307, disk flower; 308, ray flower,
enlarged.]
=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?
[Illustration: FIGS. 309-314.—Akenes of the composite family:
309, mayweed (no pappus); 310, chicory (pappus a shallow cup);
311, sunflower (pappus of two deciduous scales); 312, sneezeweed
(_Helenium_, pappus of five scales); 313, sow thistle (pappus of
delicate downy hairs); 314, dandelion, tapering below the pappus into
a long beak. (_After_ GRAY.)]
[Illustration: FIGS. 315-317.—Flowers of _Arnica montana_, showing
successive stages in pollination: 315, pistil just extruding from
anther tube, covered with pollen, but with stigmatic surfaces closed;
316, stigma opened and mature; 317, stigma recurved to receive pollen
from its own or neighboring anthers if foreign pollen has not reached
it.]
=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?
[Illustration: FIGS. 318-322.—Dissection of a papilionaceous flower:
318, front view of a corolla; 319, the petals displayed: _v_,
vexillum, or standard; _w_, wings; _k_, keel; 320, side view with
all except one of the lower petals removed, showing the essential
organs protected in the keel: _l_, loose stamen; _st_, stamen tube;
321, side view, showing how the anthers protrude when the keel is
depressed; 322, ground plan. (_After_ GRAY.)]
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
[Illustration: FIGS. 323, 324.—Salvia: 323, a newly opened flower,
showing the pollen-covered anther striking the back of a visiting
bee; 324, an older flower, with the protruding pistil rubbing against
the back of a bee covered with pollen from a younger flower.]
=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.
[Illustration: FIGS. 325, 326.—Salvia: 325, longitudinal section
through a flower, showing the rocking connective which is struck at
_a_ by a visiting insect; 326, section of the same flower after being
visited, showing the lower arm of the connective pushed back and
lowering the anther.]
=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.)
[Illustration: FIG. 327.—Staminodia, transformed stamens of canna
simulating petals: _pet_, petals; _st_, staminodia.]
[Illustration: FIG. 328.—Flower of a cactus (_cereus greggii_),
showing transition from scales to petals.]
=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.
[Illustration: FIG. 329.—A pollen grain emitting a tube (magnified).]
=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.
[Illustration: FIG. 330.—Diagram of a simple flower, showing course
of the pollen tube: _a_, transverse section of an anther before its
dehiscence; _b_, an anther dehiscing longitudinally, with pollen;
_c_, filament; _d_, base of floral leaves; _e_, nectaries; _f_, wall
of carpels; _g_, style; _h_, stigma; _i_, germinating pollen grains;
_m_, a pollen tube which has reached and entered the micropyle of the
ovule; _n_, stalk of ovule; _o_, base of the inverted ovule; _p_,
outer integument or testa; _q_, inner integument; _t_, cavity of the
embryo sac; _u_, its basal portion; _z_, oösphere.]
=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?
[Illustration: FIGS. 331-333.—Flower of Lorillard tomato: 331, newly
opened bud, showing stage in which the stamens should be removed;
332, mature flower: _cx_, calyx; _c_, corolla; _s_, stamens; _st_,
stigma; 333, flower with stamens removed for pollination. (Natural
size.)]
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.
[Illustration: FIGS. 334-335.—Seeds of Bartlett pear, showing the
advantage of cross-fertilization: 334, cross-fertilized; 335,
self-fertilized.]
=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.
[Illustration: FIG. 336.—Showing the effect of in-breeding on corn in
one generation. The two left-hand rows are from self-fertilized seed.]
=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).
[Illustration: PLATE 11.—Hybrid between a red and a white carnation,
showing characters intermediate between the two parents.]
=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_.
[Illustration: FIG. 337.—Effect of hybridization between related
species in imparting superior vigor to offspring: _M_, Californian
black walnut (_Juglans californica_), male parent; _F_, Eastern black
walnut (_J. nigra_), female parent; _H_, hybrid.]
=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.
[Illustration: Diagram illustrating Mendel’s Law.]
=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.
=260. Fixing the type.=—It is the tendency of plants to vary under
the influence of climate, soil, food supply, crossing, and other
causes perhaps unknown to us, that makes the plant breeder’s art
possible. When a horticulturist sets out to produce a new fruit
or vegetable, he first forms in his mind a clear idea of what he
wants—whether increase of yield or size, resistance to cold, drought,
or disease, improvement in flavor, color, shape, etc., or change
in the time of maturing or flowering (early and late varieties).
Suppose, for instance, he wishes to produce an oxeye daisy with
all the disk florets changed to white ones like the rays. He will
begin by selecting plants with the greatest number of rays and the
most conspicuous ones that he can find, and sowing the seeds of
the flowers which show the greatest tendency to the development of
these qualities. He will continue this process from generation to
generation, rigorously destroying all specimens that do not approach
nearer the ideal sought, until all disposition to “rogue,” as the
tendency to revert is called, has been eliminated. When variations
cease to occur and the seed of the new variety always “come true,”
the type is said to be _fixed_; though some care will always be
necessary to keep it so, as the influence of changed surroundings and
the danger of mixture with foreign pollen must always be provided
against.
[Illustration: FIG. 338.—A field of pumpkins grown from selected
seed.]
=261. Survival of the fittest.=—In the fierce struggle continually
going on among both plants and animals for food, shelter, and elbow
room in the world, any individual that happens to vary in a way which
adapts it to its surroundings a little better than its rivals, has an
advantage that will enable it to survive when less favored members of
the species will perish. Its offspring, or some of them, may inherit
this quality and transmit it, with the attendant advantage, to their
posterity, and so on, till that particular breed outstrips all
competitors, and in time, as the less favored intervening forms die
out, becomes differentiated as a new species. This is, in brief, the
doctrine of natural selection and the survival of the fittest.
=262. Artificial selection.=—Artificial selection enables the breeder
to accomplish more quickly what nature appears to do by the slow
process of natural selection. It is by this means that our choicest
fruits and vegetables have been developed from greatly inferior, and
sometimes inedible, wild forms. Plants respond so readily to the
influence of selection, and the changes brought about by it are so
rapid, that new styles of fruits and flowers succeed each other in
the market with almost as great frequency and in as ready response
to demand as the new styles of women’s bonnets and gowns in the shop
windows.
[Illustration: FIG. 339.—Variation in blackberry leaves due to
hybridization.]
=263. Causes of variation.=—While man cannot directly force plants
to vary in any given direction, he can hasten the process of
variation by crossing, or by changing the conditions under which
they are growing. This is called “breaking the type.” Hybridization
furnishes the readiest means to this end. Change of food supply,
especially if accompanied by excess of nourishment, is probably the
expedient that ranks next in effectiveness. Light, temperature,
moisture, character of the soil, exposure to wind, and the like,
also have their influence; and in adapting themselves to changes
in these various conditions, plants are apt to exhibit an unusual
number of variations, when removed from one locality to another,
especially if the difference in soil and climate is very marked.
Now comes the breeder’s opportunity. By taking advantage of such
variations as may occur either spontaneously, or as the result of
his efforts to break the type, he will generally find some that will
meet his requirements; and knowing the effect produced by different
conditions, he can, to a certain extent, influence the course of
variation in the direction desired, by subjecting his specimens to
the conditions that tend to produce it. If he wishes to develop a
dwarf variety, for instance, he will take notice that overcrowding,
lack of nourishment, and cold tend to produce that result in
nature, and by acting on this hint he can direct his efforts more
intelligently. He will learn, too, not to waste time in trying to
breed a plant contrary to its nature. He must not expect to gather
figs from thistles by any art of selection or skill in culture. By
attention to Mendel’s law, a still further saving of time and labor
may be effected.
It is obvious, from what has been said, that a breeder’s chance of
finding what he wants will be greater in proportion to the number
of individual plants he has to choose from. For this reason, a
horticulturist sometimes uses thousands and hundreds of thousands of
specimens of a single kind in conducting his experiments. In this way
he compresses into a short space of time the advantage that nature
can gain only by spreading her random experiments over a long series
of years, or even centuries.
[Illustration: FIG. 340.—Mutation in twin ears of corn, showing the
sudden variations that sometimes occur, by which a new type may be
provided without the labor of selection.]
=264. Mutation and variation.=—There are at least two ways in which
changes in vegetable and animal forms are thought to occur: (1)
by the preservation and fixation through selection and heredity,
of slight differences that may appear from time to time, such
divergences being called “fluctuating variations”; (2) by the
appearance now and then, due to causes as yet unknown, of definite
and sudden changes creating a new form at a single, though perhaps
small, leap. When such a change is temporary and passes away with
the individual in which it first appeared, it is called a “sport,”
and leads to no important results; but when it is inherited by the
offspring, so that it is capable of giving rise to a new species,
it constitutes a “mutation.” The value of a mutation to breeders
in saving time and trouble is obvious. Professor Hugo de Vries, a
Dutch botanist, was the first to call attention to the importance of
mutation and its bearing upon the production of new species.
=265. Factors in the evolution of species.=—Variation, heredity, and
selection are the three principal agents underlying all changes,
whether for the improvement or deterioration of living organisms.
The influence of external surroundings in keeping up a variation
once begun, or in starting new ones, is also a factor that cannot
be disregarded. It is for this reason that natural species are so
much more stable than those brought about by man. The former, being
evolved in response to natural conditions, are liable to change
only as alterations in their surroundings are brought about by the
slow operation of natural causes. But the types resulting from the
breeder’s art, produced as they often are in response to human
demands and in direct opposition to the requirements of natural
conditions, are in a sense purely artificial, and can be preserved
only by keeping up the artificial surroundings by which they were
developed. Hence, the importance of diligent cultivation and constant
care and tillage, without which the most carefully selected stocks
may quickly “run out” and degenerate into worthless forms.
Practical Questions
1. Which are the more pliable to the breeder’s art, annuals or
perennials? Why? (91, 93, 262, 263.)
2. What advantage is gained by using buds and grafts instead of
seedlings in making new varieties of fruit trees? (257, 259, 260.)
3. Would it be practicable to breed new varieties of slow-growing
forest trees, like oak, cypress, redwood, from seeds? Why or why
not? (93, 262, 263.)
4. Can you account for the existence of the numerous intermediate
forms between the different species of oaks found in nature? (255,
257.)
5. If a breeder wished to produce a sweet-scented daisy or pansy,
how would he make his selections? (260.)
6. Which would be the more useful for his purpose, a plant that
showed a general tendency to variability, or one that remained
steadily fixed to its type? (260.)
7. What could he do to break the type? (263.)
8. Would an intelligent breeder set out to produce edible roots and
tubers from wheat or barley? (263.)
9. Would he think it worth while to try to develop a fleshy fruit
from a filbert or a walnut tree? From a haw? From sheepberry and
black haw? From tupelo (ogeechee lime)? (263.)
10. Suppose a florist should wish to change the color of a rose
from pink to deep red; how could he hasten the process? (257, 263.)
11. Explain why it is so much easier to produce new varieties of
plants when there are already many kinds in existence, as, for
example, the rose, peach, and chrysanthemum. (255, 256; Exps. 78,
79.)
VIII. ECOLOGY OF THE FLOWER
A. THE PREVENTION OF SELF-POLLINATION
MATERIAL.—Any kind of unisexual flowers obtainable. Some good
examples for illustrating points mentioned in the text are: for
spring and early summer, catkins of almost any of our common forest
trees,—oak, hickory, willow, poplar, etc.; tassels and young ears
of early corn; for summer and early fall, flowers of late corn, and
of melon, squash, pumpkin, or others of the gourd family. Examples
of _dichogamy_ are: evening primrose, showy primrose (_Œnothera
speciosa_), willow herb (_Epilobium_), dandelion, artichoke,
sunflower, or any of the composite family; of _dimorphism_:
English primrose (_Primula_), loosestrife (_Pulmonaria_), bluets
(_Houstonia_), partridge berry; _cleistogamic_: fringed polygala,
violets. Peanuts, while not technically classed as cleistogamic,
are strictly close-fertilized, and approach the type so nearly that
they may be used as an illustration.
=266. Ecology= is the study of plants and animals in relation to
their surroundings. The principal modifications that flowers undergo
in this respect are in adapting themselves for (1) pollination, and
(2) protection.
[Illustration: FIGS. 341, 342.—Unisexual flowers of willow: 341,
staminate; 342, pistillate.]
[Illustration: FIG. 343.—Twig of oak with both kinds of flowers:
_f_, fertile flowers; _s_, _s_, staminate; _a_, pistillate flower,
enlarged; _b_, vertical section of pistillate flower, enlarged; _c_,
portion of one of the sterile aments, enlarged, showing the clusters
of stamens.]
=267. Unisexual flowers.=—The advantages of cross fertilization were
shown in the last two sections. It was also shown that the first
step taken by the breeder to secure this result is to render the
flower incapable of self-fertilization, by removing the stamens.
Nature accomplishes the same purpose by the more effectual expedient
of providing imperfect, or _unisexual_ flowers, in which stamens
only, or pistils only, occur in the same flower. When the stamens
alone are present, the flower is said to be staminate, or _sterile_,
because it is incapable of producing seeds of its own, though its
pollen is a necessary factor in seed production. If, on the other
hand, the ovary is present and the stamens absent, the flower is
pistillate and _fertile_; that is, capable of producing fruit when
impregnated with pollen. Sometimes both stamens and pistils are
wanting, as in the showy corollas of the garden “snowball,” the
hydrangea, and the rays of the sunflower. Such blossoms are said to
be _neutral_, from the Latin word _neuter_, meaning neither, because
they have neither pistils nor stamens. They can, of course, have no
direct part in the production of fruit, but are for show merely.
(231.)
=268. Monœcious and diœcious plants.=—When both kinds of flowers,
staminate and pistillate, are borne on the same plant, as in the oak,
pine, hickory, and most of our common forest trees, they are said
to be _monœcious_, a word which means “belonging to one household”;
when borne on separate plants, as in the willow, sassafras, and black
gum, they are _diœcious_, or “of two households.” Draw a flowering
twig of oak, pine, or willow. Where are the fertile flowers situated?
Notice how very much more numerous the staminate flowers are than the
fertile ones. Why is this necessary? (275.)
[Illustration: FIGS. 344, 345.—Flower of fireweed (_Epilobium
angustifolium_): 344, with mature stamens and immature pistil; 345,
the same a few days older, with expanded pistil after the anthers
have shed their pollen. (_After_ GRAY.)]
=269. Dichogamy= is the name applied to a condition where the stamens
and pistils mature at different times, as in the evening primrose,
oxeye daisy, and most of the composite family. It is a very common
method in nature for preventing self-pollination, and quite as
effective as the monœcious arrangement, since it renders the flowers
practically unisexual.
[Illustration: FIGS. 346-347.—Flower of pulmonaria: 346, long styled;
347, short styled.]
=270. Dimorphism= denotes a condition in which the stamens and
pistils are of different relative lengths in different flowers of
the same species, the stamens being long and the pistils short in
some, the pistils long and the stamens short in others. Flowers
of this sort are said to be _dimorphous_, or _dimorphic_, that
is, of two forms; and some species are even _trimorphic_, having
the two sets of organs long, short, and medium, respectively, in
different individuals. Examples of dimorphic flowers are the pretty
little bluets (_Houstonia cœrulea_), the partridge berry, the swamp
loosestrife, and the English cowslip. Of trimorphic flowers we have
examples in the wood sorrel and the spiked loosestrife (_Lythrum
salicaria_) of the gardens. These flowers were a great puzzle to
botanists until the celebrated naturalist, Charles Darwin, proved
by experiment that the seeds produced by pollinating a dimorphous
flower with its own pollen, or with pollen from a flower of similar
form, are of very inferior quality to those produced by impregnating
a long-styled flower with pollen from a short-styled one, and _vice
versa_.
[Illustration: FIGS. 348-350.—Three forms of loosestrife (_Lythrum
salicaria_).]
=271. “Nature abhors self-fertilization.”=—These are the
three principal methods by which nature provides against
self-fertilization. Other cases occur in which the relative position
of the two organs is such that self-pollination is difficult, or
impossible, as in the iris and bear’s grass; or the pollen may be
incapable of acting on the stigma of the flower that produced it.
This aversion to self-fertilization is so great that many flowers,
even when capable of it, will give preference to the pollen of
another plant of the same kind, if dusted with both. From his
observations on the behavior of plants in reference to this function,
Charles Darwin drew the conclusion that “Nature abhors perpetual
self-fertilization.”
=272. Cleistogamic flowers.=—Apparent exceptions to this rule are
the hidden flowers found on certain plants which seem to have been
constructed with a special view to self-fertilization. They are
called _cleistogamic_, or closed, because they never open, but are
fertilized in the bud; and those of the fringed polygala do not
even rise above ground at all. Flowers of this kind can be found on
several species of violet, concealed under the leaves, close to the
ground; and the flowers of the peanut, found in the same situation,
while they open slightly, are close-fertilized and practically
cleistogamic. They are much more prolific than ordinary flowers,
but are not common, and seem to be a provision against accident, for
the plants producing them are generally provided with other flowers
of the usual kind,—some, as the violet, having elaborate special
adaptations for cross fertilization.
Practical Questions
1. Why does a strawberry bed sometimes fail to fruit well, although
it may flower abundantly? (267, 268.)
2. Are berries found on all sassafras trees? On all buckthorns?
Hollies?
3. Would a solitary hop-vine produce fruit? A solitary ash tree?
(267.)
4. Why is a mistletoe bough with berries on it so much harder to
find than one with foliage merely? (267, 268.)
B. WIND POLLINATION
MATERIAL.—In spring, catkins of forest trees, staminate and
pistillate flowers of pine. At nearly all seasons, heads of grain
and panicles of various kinds of grass can be obtained. For
experiment, a potted plant of any kind, just about to bloom, may be
used.
EXPERIMENT 80. TO TEST THE EFFECT OF SHUTTING OUT EXTERNAL
AGENCIES.—Tie paper bags over flower buds of different kinds when
nearly ready to open and leave until the flowers have withered.
On removing the bags, mark with colored threads the flowers that
had been covered, and watch until seed time. Do you notice any
difference in the number, size, or weight of the seed produced by
them and by those of the same kind left exposed? How do you account
for the difference, if there is any? By what agencies could foreign
pollen have been carried to the stigmas of the exposed flowers? If
any of the covered specimens wither and drop their seed vessels
without any attempt to fruit, examine a fresh flower, and see if it
is capable of self-pollination.
As already explained, experiments of this kind, to be conclusive,
should be tried on as many specimens as possible. The greater the
number of species and individuals included, the better. Where it is
not practicable to carry on experiments by the class, pupils who
are interested can make them at home.
=273. The problem of pollination.=—When a plant has provided against
self-pollination, its problem is only half solved, as it must now
depend upon the conveyance of pollen to the stigma by extraneous
means.
[Illustration: FIG. 351.—Feathery stigmas of a grass adapted to wind
pollination.]
=274. Adaptations to wind pollination.=—A very large number of
plants, among which are included nearly all our principal forest
trees, grains, and grasses of every kind, depend exclusively upon
the wind for the distribution of their pollen. This being the case,
it is, of course, an advantage to them to get rid of all unnecessary
appendages that might hinder a free play of the wind among their
flowers, and so they consist, as a rule, of essential organs only
(Figs. 341, 342). Such flowers are often distinguished, however,
especially among grasses and low herbs, by large, feathery stigmas
that are well adapted to catch and hold any stray pollen grains which
may be floating in the air. Place a stigma of oat or other grass
under the microscope and you will probably see a number of pollen
grains clinging to its branches.
=275. The disadvantages of wind pollination.=—This is a very clumsy
and wasteful method, however, for so much pollen is lost by the
haphazard mode of distribution that the plant is forced to spend its
energies in producing a vast amount more than is actually needed,
and great masses of it are frequently seen in spring floating like
patches of sulphur on ponds and streams in the neighborhood of pine
thickets. Like those that are self-pollinated, wind-pollinated
flowers are generally very inconspicuous, devoid of odor, and of all
attractions of form or color, because they have no need of these
allurements to attract the visits of insects. Besides being wasteful,
wind pollination is very uncertain. The pollen cannot be blown about
very well unless it is dry, and in rainy weather it may all be rotted
or washed away before it can reach the stigmas that are ready to
receive it.
Practical Questions
1. Why do the flowers of oak, willow, and other wind-fertilized
plants generally appear before the leaves? (274.)
2. Can you account for the showers of “sulphur” sometimes reported
in the newspapers? (275.)
3. Do you see any connection between the feathery stigmas of most
grasses and their mode of pollination? (274.)
4. Why are house plants not apt to seed so well as those left in
the open? (Exp. 80.)
5. Why are the tassels of corn placed at the tip of the stalk?
(274.)
6. Can you trace any connection between the winds and the corn
crop? (274.)
7. If March winds should cease to blow, would vegetation be
affected in any way? (274.)
8. Why are wind-fertilized plants generally trees or tall herbs?
(274.)
9. Is it good husbandry to plant different varieties of corn or
other grain in the same field, if it is desired to keep the strain
pure? (255, 274.)
10. Is water a good pollen carrier? (275.)
11. What is the only class of plants it is likely to reach?
12. What is the only other agency, besides wind and water, by which
this office can be performed?
C. INSECT POLLINATION
MATERIAL.—Half a dozen panes of glass, about 6 × 9; squares of
bright-colored cloth or paper; a few spoonfuls of honey or sirup;
perfumes of various kinds, preferably flower extracts; fetid and
disagreeable smelling substances, such as a bit of decaying animal
or vegetable matter. Observations on living plants can best be made
out of doors or in a greenhouse, as opportunity offers.
EXPERIMENT 81. HAS THE COLOR OF FLOWERS ANY ATTRACTION FOR
INSECTS?—Place half a dozen panes of ordinary window glass out of
doors or in an open window to which insects can have free access.
Lay under the first pane a piece of black paper or cloth, and under
the others bright-colored pieces of red, blue, white, yellow, and
purple. Drop on the center of each pane a little honey or sirup,
and watch. Do insects show any color preferences? Which color
attracts fewest visitors? Which most?
EXPERIMENT 82. DOES ODOR INFLUENCE INSECTS?—Try the same experiment
with different odors, removing the bright colors and sprinkling
some kind of perfume on each pane. Try also the effect of decaying
meat and other malodorous substances. Are any insects attracted
by these? What kinds? Does this account for the noisome smells
of the “carrion-flower” and skunk cabbage? What kinds of insects
are attracted by sweet-smelling substances? Do the greater number
appear to be attracted by these, or by foul odors? Are flowers of
the sweet-smelling or the foul-smelling kind more common in nature?
Do insects seem to be more strongly influenced by colors or by
odors?
=276. The color of flowers=, being an adaptation to changing external
conditions, is a very unstable quality, and varies greatly within
the limits of the same species. Even on the same stem, flowers of
different colors are often found, due, probably, to hybridization.
Yet, notwithstanding all this apparently random intermingling of
hues, the range of color for each species is confined, approximately,
within certain limits. Nobody has ever seen a blue rose or a yellow
aster; and though the florist’s art is constantly narrowing the
application of this law, it still remains true that in a state of
nature, certain colors seem to be associated together in the floral
art gamut. Yellow is considered the simplest and most primitive color
in flowers, and blue the latest and most highly evolved. Yellow,
white, and purple, in the order named, are the commonest flower
colors in nature; blue, the rarest. Do you see any relation between
these facts and the color preferences of insects?
=277. Advantages of insect pollination.=—It is evident that this is
a much more certain as well as a more economical method of securing
pollination than through the haphazard agency of wind or water.
In probing around for the nectar or the pollen upon which they
feed, these busy little creatures get themselves dusted with the
fertilizing powder, which they unconsciously convey from the stamen
of one flower to the pistil of another. Insects usually confine
themselves, as far as possible, to the same species during their
day’s work, and since less pollen is wasted in this way than would
be done by the wind, it is clearly to the advantage of a plant to
attract such visitors, even at the expense of a little honey, or of a
liberal toll out of the pollen they distribute.
=278. Special partnerships.=—Some plants have adapted themselves
to the visits of one particular kind of insect so completely that
they would die out if that species were to become extinct. The
well-known alliance between red clover and the bumblebee was brought
to light when the plant was first introduced into Australia. It grew
luxuriantly and blossomed profusely, but would never set seed till
the bumblebee was introduced to keep it company.
[Illustration: FIG. 352.—Pod of _yucca_ pierced by the _Pronuba
yuccasella_.]
[Illustration: FIG. 353.—Pronuba pollinating pistil of yucca.]
[Illustration: FIG. 354.—Moth resting on yucca blossom.]
A remarkable partnership of this kind exists between the _pronuba_,
or yucca moth, and the flowering yuccas, of which the bear’s grass
and Spanish bayonet are familiar examples. The pods of these plants
are never perfect, but all show a constriction at or near the middle,
such as is sometimes seen in the sides of wormy plums and pears. This
is caused by the larvæ of the moth, which feed upon the unripe seeds.
A glance under the nodding perianth of a yucca blossom (Fig. 354)
will show that the short stamens are curved back from the pistil in
such a manner that, under ordinary circumstances, the pollen cannot
reach the stigma except by the rarest accident. But the yucca moth,
as soon as she has deposited her eggs in the seed vessel, takes care
to provide a crop of food for her offspring by gathering a ball of
pollen in her antennæ and deliberately plastering it over the stigma
(Fig. 353). In this way fertilization of the ovules and maturing of
the fruit is secured. The larvæ feed on the unripe seeds for a time,
but so few are destroyed in proportion to the number matured that the
plant can well afford to pay the small toll charged in return for the
service rendered.
[Illustration: FIG. 355.—Upper boughs of a caprifig tree, showing an
abundant crop of spring fruit.]
[Illustration: FIG. 356.—Female wasps issuing from the galls of
caprifigs, in which the eggs are laid.]
=279. Caprification of the fig.=—A more complicated case of
specialization is that of the Smyrna fig of commerce—the only one
of the species that is capable of perfecting seeds. The staminate
flowers are borne on a separate tree, the caprifig, which grows wild
in the countries bordering on the Mediterranean. The caprifigs,
as the fruit of this tree is called, are worthless except as the
breeding and nesting places of a small insect, the fig wasp. This
insect is the necessary agent in conveying pollen from the stamens of
the caprifig to the pistils of the Smyrna fig, which it penetrates at
certain seasons of the year in the effort to lay its eggs. In order
to insure _caprification_, as this process is called, the caprifigs
are strung by hand on fillets of cord or raffia and hung about on the
trees which are to be fertilized. In this case we have an example
of a threefold partnership between man, the fig tree, and the wasp,
which is necessary to the existence of two of the parties.
D. PROTECTIVE ADAPTATION
EXPERIMENT 83. ARE THE FLORAL ENVELOPES OF ANY USE?—Carefully
remove the calyx and corolla from a young flower bud on a growing
plant and see what will happen. Remove them from a flower just
unfolding. Mark each by tying a colored thread lightly around the
petiole and see if it sets as many seeds, or as good ones, as the
unmutilated flowers on the same plant.
EXPERIMENT 84. IS THE POSITION OF A FLOWER ON THE STEM OF ANY
IMPORTANCE?—Invert a blossom of pea or sage, and see what parts
would come in contact with the body of a visiting insect. How would
its chances for pollination be affected? Try to make a flower grow
in an inverted position by tying or weighting it down, and watch
the effect on seed production.
EXPERIMENT 85. IS THE POSITION OF FLOWERS ON THE STEM INFLUENCED
BY LIGHT?—Place a potted plant with expanding flower buds near a
window so that the light will reach it from one side only, and
notice the position of the buds. After a day or two reverse the
position with regard to light, and watch whether any change of
position takes place.
[Illustration: FIGS. 357-359.—Flower of monkshood, showing the
changes by which it returns to its original position under the
influence of geotropism after the axis of inflorescence, s, has
been inverted: 357, inverted position; 358, change due to negative
geotropism; 359, change due to lateral geotropism.]
EXPERIMENT 86. IS THE POSITION OF FLOWERS ON THE STEM INFLUENCED
BY GEOTROPISM?—Lay a potted plant of lily of the valley, larkspur,
gladiolus, or digitalis in a horizontal position, tie the main stem
to keep it from changing its direction of growth, and leave for two
or three days in a place where it is lighted equally on all sides.
How do the individual flowers behave? What part bends to turn them
up? Vary the experiment by turning the pot bottom upwards so
that the flowering axis will point downwards. This can be done by
inclosing the pot in a bag of strong cheesecloth, with the string
tied loosely but firmly around the foot of the stem to prevent the
contents from falling out, and suspending the whole bottom upwards.
In making these experiments, use flowers that grow in a long
cluster, or raceme, and hold the main axis in a vertical position
by tying or weighting it down. Watch the behavior of the individual
flowers. Arrange another pot containing the same kind of plant, in
the same way, and suspend one in a dark place, keeping the other
in the light. Does the same movement take place in both? Is it in
response to light, or to gravity?
[Illustration: FIGS. 360, 361.—Protection of pollen in the thistle:
360, position at night, or during wet weather; 361, position in
sunshine.]
[Illustration: FIGS. 362, 363.—A bell flower: 362, position in
daylight; 363, position at night, or during wet weather.]
=280. Means of protection.=—Where plants have adapted themselves to
insect pollination, it is, of course, important to shut out intruders
that would not make good carriers. In general, small, creeping
things, like ants and plant lice, are not such efficient pollen
bearers as winged insects, and hence the various devices, such as
hairs, scales, and constrictions, at the throat of the corolla, by
means of which their access to the pollen is prohibited. To this
class of adaptations belong the hairy filaments of the spiderwort,
the sticky ring about the peduncles of the catchfly, the swollen
lips of the snapdragon, the scales or hairs in the throat of the
hound’s-tongue, the velvet petals of the partridge berry, and the
recurved edges of corollas like those of the morning-glory and
tobacco, over which small crawling insects cannot easily climb.
Of flowers that are pollinated by night moths, some close during
the day, as the four-o’clock and the evening primrose; and _vice
versa_, the morning-glory, dandelion, and dayflower (_Commelyna_)
unfold their beauties only in the sunlight. For similar reasons,
night-blooming flowers are generally white or very light-colored, and
shed their fragrance only after sunset. A nodding position is assumed
by many flowers at night, or during a shower, to keep the pollen from
being injured by dew or rain.
[Illustration: FIG. 364.—A flower of the trumpet vine (_Tecoma
radicans_) adapted to pollination by humming birds and humming bird
moths, which has been pierced by a bee or bird for honey.]
[Illustration: FIG. 365.—Head of the swordbill, a bird adapted to
feeding on nectar from long, tubular corollas.]
=281. Insect depredators.=—The secretion of honey is a common
means of attracting insects, and various adaptations, such as
spurs, sacs, and pockets, are provided for protecting it against
unwelcome intruders. In general, plants that have long, tubular
flowers, like the trumpet honeysuckle (_Lonicera sempervirens_) and
the trumpet vine, are reserving their sweets for humming birds, or
long-tongued moths and butterflies. This protective device is not
always successful, however, against insect depredators, for it is not
uncommon to find such corollas with a puncture near the base, made by
wasps or bees, and sometimes by humming birds themselves, in their
impatience to get at the feast before the flower is open. Through the
breach thus made, a rabble of petty thieves can then find entrance.
Practical Questions
1. Of what use is the brilliant coloring of the camellia? The large
flowers of the magnolia? The perfume of the rose and the violet?
The fetid odor of the ailanthus? (277; Exps. 81, 82.)
2. Are the tastes of insects in regard to odors always the same as
ours? (Exp. 82.)
3. Have flowers any economic value except for decorative purposes?
4. Can you name any that are used as food or beverages? Any that
furnish spices and flavorings? Drugs, medicines, or dyes?
5. What commercial food product is obtained almost entirely from
flowers?
6. Name some of the flowers that are most valued by the beekeeper.
7. Mention another important industry that is entirely dependent on
flowers.
8. Name some of the flowers that are most important to the perfumer.
9. Why do the seeds of fruit trees so seldom produce offspring true
to the stock? (256, 257, 271, 277.)
10. Would you place a beehive near a field of buckwheat? Of clover?
Near a strawberry bed? In a peach orchard? Near a fig tree? Under a
grape arbor?
11. Why are very conspicuous flowers, like the camellia, hollyhock,
and pelargoniums, so frequently without odor?
12. Why is the wallflower “sweetest by night”? (280.)
13. What advantage can flowers like the morning-glory gain by their
early closing? (280.)
14. Of what use to the cotton plant, Japan honeysuckle, and
hibiscus is the change of color their blossoms undergo a few hours
after opening? (277, 278, 280.)
15. Why does the Japan honeysuckle, which has run wild so
abundantly in many parts of our country, produce so few berries?
(278, 280.)
16. If the trumpet vine grows in your neighborhood, examine a
number of corollas and account for the dead ants found in them.
Account also for the large hole (sometimes three quarters of an
inch in diameter) often found near the base of the tube. (281.)
17. Do you see any connection between the greater freshness and
beauty of flowers early in the morning, and the activity of bees,
birds, and butterflies at that time?
18. The flowers most frequented by humming birds are the trumpet
honeysuckle, cardinal flower, trumpet vine, horsemint (_Monarda_),
wild columbine, canna, fuchsia, etc.; what inference would you draw
from this as to their color preferences?
Field Work
1. The ecology of the flower is so suggestive a subject and so
peculiarly appropriate to outdoor work that it seems hardly
necessary to point out the many attractive fields of inquiry it
opens to the student of nature. In this way alone can experiments
in insect pollination be carried on to the best advantage. Try
the effect of enveloping buds of various kinds in gauze so as
to exclude the visits of insects, and note the result as to the
production of fruit and seed. Envelop a cluster of milkweed
blossoms in this way and notice how much longer the flowers so
protected continue in bloom than do the others; why is this? Try
the same experiment upon the blooms of cotton and hibiscus, if you
live where they grow, and see whether the characteristic change in
color occurs in flowers from which insects have been excluded, and
whether good seed pods are produced by them. Try the effect upon
fruit production of excluding insects from clusters of apple, pear,
and peach blossoms.
2. Make a list of all the outdoor plants, both wild and cultivated,
that are found blooming in your neighborhood, keeping a record of
the earliest specimens of each as you find them. The best way is to
keep a sort of daily calendar, and at the end of each month give a
summary of the species found in bloom during that period. In this
way a fairly complete annual record of the flowering time of the
different plants for that vicinity will be obtained. The record
should be kept up the whole year round. Don’t stop in winter,
but go straight on through the coldest as well as the hottest
season, and you will make some surprising discoveries, especially
if the record is continued year after year. Give the common name
of each plant, adding the botanical one if you know it. Any facts
that you may know or may discover in regard to particular plants,
such as their medicinal or other uses, their poisonous or edible
properties, the insects that visit them, and in the case of weeds,
their origin and introduction, will greatly enhance the interest
and value of the record.