Name______________________
Section ____________________
Lab 4
Flowers, Pollination and Fruit
Flowers are designed on plants for sexual reproduction. They contain organs that
produce gametes (sex cells), which, after fertilization, lead to the formation of seed and
fruit. Anthers found in flowers produce pollen grains, which ultimately make sperm
nuclei and egg cells are produced in ovules found in the ovaries. The transfer of pollen
between flowers can occur by a variety of mechanisms including wind, insects, birds,
bats and even water. Consequently, flowers are constructed to attract the appropriate
pollinators or, as in the case of wind pollination, to maximize the collection of pollen.
After successful pollination, seed develop from ovules within the ovaries, which then
mature into fruit (fruit contain seed). In today’s lab we will study the structure of a basic
flower as well the diversity of floral shape, color and size in flowering plants. We will
investigate the process by which reproduction occurs and study the overall shape of
pollen and pollination strategies. Finally, today’s lab will focus on the structure fruit and
the fruit types produced by flowering plants.
1. Floral Structure
Flowers are made up of four whorls of reproductive structures borne on a fleshy
platform called the receptacle. The
outermost whorl is called the calyx, which
is constructed of one to several leaf-like
structures called sepals. The calyx is
usually green, but may also be colored and
difficult to distinguish from the corolla.
The corolla occurs just inside the calyx and
is made up of colored petals. The shape
and color pattern of petals is often used to
attract animal pollinators. In wind and
water pollinated flowers, flowers may not
have petals or they may be significantly
reduced in size. Just inside the corolla are
the male reproductive structures, the
stamens. Stamens are made up of a long
stalk-like structure called the filament and
a sac-like anther which produces plooen. The center of a flower is occupied by the pistil
(composed of one or more carpels). The pistil has a bulbous ovary at its base that
contains the ovules, a sticky stigma, which is a receptive platform for pollen and a
slender style that connects the ovary with the stigma. Pollen, once deposited on the
stigma, will germinate and produce a pollen tube that grows down through the style
towards the ovules within the ovary. When fertilized by pollen, ovules develop into seed
and stimulate the ovary into developing into fruit. Necataries may be found at the base of
the flower often producing a small pool of sugary nectar, which serves as a reward for
animal pollinators.
Closely related plant species often share similar floral characteristics. However,
all plant species have unique flower structures that do not vary significantly between
individuals within the species. For this reason, flowers and floral structures are
commonly used in plant
identification keys.
Although flower
morphology within a species
is a very conservative trait,
the diversity of flower size
and shape can vary
tremendously between plant
species. For example floral
symmetry can be quite
different between species.
Actinomorphic flower
Zygomorphic flower
Flowers with regular
symmetry that have
morphologically similar petals that radiate out from the flower’s center are said to be
actinomorphic. Flowers that exhibit bi-lateral symmetry are said to by zygomorphic and
in contrast to actinomorphic flowers, have individual petals that may have very different
shapes.
The point of insertion of the floral parts on the receptacle relative to the ovary is
another floral characteristic that varies between species. If the sepals, petals and stamens
are all inserted below the ovary the flower is said to be epigynous and the ovary is
superior. In many flowers, however, the sepals, petals and stamens are inserted above the
ovary. In this situation the flower is
hypogynous and the ovary is said to be
inferior. Perigynous flowers have
superior ovaries but the sepals, petals
and stamens fuse along their lower
portions to form a cup that surrounds
(but is not attached to) the ovary. This
cup structure is called the hypanthium.
Dissect 3 flowers that differ in symmetry and ovary position. In the space
below, sketch and label all the floral parts observed on each dissected flower.
Inflorescence Types
Solitary flowers can be produced at the terminal end of a branch or in a leaf axil.
This, condition is relatively rare, however, and most plants produce flowers in groups on
flowering branches called inflorescences. Inflorescence types the first flower to open and
mature is at the top of the inflorescence it is said to be a determinant inflorescence. If the
first flower to open is at the bottom of the inflorescence it is said to be indeterminate.
The diagram below illustrates different determinant and indeterminate inflorescence
types. On display in lab today are many different inflorescence types. Use this figure to
help identify the inflorescence type for each flowering plant.
I
Study the plants on display in lab today. Use this table to organize your observations.
Species
Flower
shape/color
Flower
Symmetry
Ovary
position
Inflorescence
type
Fruit type
Pollination
Flowers are adapted, in part, to maximize the probability of successful pollination.
Consequently, floral shape and pollen structure can take on very different forms in plants
that use different pollination strategies. For example, wind pollinated flowers usually do
not produce showy flowers, but produce large numbers of pollen grains whereas insect
pollinated flowers will use both visual and olfactory cues to attract the appropriate insect
pollinator. Over 65% of flowering plants are pollinated by animals (insects, birds, bats)
and co-evolution between flower and pollinator can be so strong that neither flower nor
pollinator can persist without the other. Several pollination strategies that relate suites of
floral characteristics to complimentary characteristics in their pollinators have been
described, all of which underscore the co-evolutionary nature of this critical step in plant
reproduction. The chart below describes a few of these pollination strategies (for wind,
beetles, bees, butterflies, moths, hummingbirds and bats).
Some Pollination Strategies
Flower types
Petals usually lacking; large amounts
of pollen, often winged; large,
extensively branched stigmatic
surfaces
Dull or light colored with no special
shape, but often bowl shaped; no nectar
guides; strong fruit odor; produce
copious pollen or nectar
Often zygomorphic flowers with light
scent, not strong; yellow or blue flowers
with UV pigments; nectar guides often
present, moderate amounts of nectar
Mostly white or dull colored, heavily
perfumed; open during night, nectar in
tubes with copious nectar, nectar
guides absent
Vividly colored flowers, often red,
open in day; abundant nectar often in
tubes, no odor; flower often nodding
with no landing platform
Color usually drab or white, open at
night; large amount of nectar and
pollen; strong stale odor (musky),
usually large flowered or in strong
inflorescences
Pollen vectors
Random and affected by gravity and
wind direction. Wind
Clumsy fliers with poor vision; good olfactory
development; feed on pollen. Beetles
Good fliers; diurnal; vision shifted
into UV, does not see red; olfactory
sense well developed; feeds on
pollen and nectar. Bees
Good fliers, nocturnal; poor vision but good
olfactory sense, attracted to sweet odors;
feed on nectar. Moths
Good fliers, good vision especially in red, no vision
in UV; olfactory not well developed; feeds on
nectar and insects. Humming birds
Good fliers but large bodies, nocturnal; good
vision but color blind; good olfactory
development, attracted to musky odors; eats
nectar and pollen. Bats
Pollen type and Germination
Pollen also can be very different between species. Shape, size and texture can all
take on very important functional roles. Wind
dispersed pollen is usually small and may have winglike outgrowths to aid in their dispersal. Animal
dispersed pollen may be large and nutritious to act as
a reward for the pollinator. Some pollen is sticky to
better attach to a visiting pollinator. Once deposited
on the stigma, pollen must also contain enough
energy to grow a pollen tube towards the ovules to
complete fertilization.
In today’s lab we will study different kinds of
pollen grains, germinate pollen grains to observe
pollen tubes, and identify different pollination
strategies in plants growing on campus.
Germinating pollen. To observe pollen germinating
you will need to collect ripe pollen grains and spread
them on a fresh piece of onion epidermis. The onion epidermis acts like a receptive
stigmatic surface and “tricks” the pollen into making a pollen tube.
Procedure:
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From a fresh yellow onion, cut a
15 mm wide section about 5 mm
deep out of the onion
Remove the section and peel the
epidermis from the inner surface
Place the epidermal peel on the
slide, mesophyll side up (i.e. the
side towards the outside of the
onion) and trim to fit slide
Brush ripe pollen on epidermis
to deposit pollen
Place slide in a Petri dish lined with a damp paper towel and cover to keep moist
Observe slide every 20 minutes to check for pollen tube formation
Make a sketch of some of the tubes in different developmental stages in the space
below.
Compare pollen from different plant species. Sketch pollen grains making notes on the
relative size, texture and color of the pollen grains from different species. For each
type of pollen, list the properties that are related to their pollination vector (i.e. How is
size related to wind pollination or to their value as a reward to pollinator? Is sticky
pollen related to its pollinator type? etc.)
While your pollen is germinating, identify different pollination strategies from flowers
growing on campus. Use the table below to help you organize your observations.
Species
Color or
pattern
Shape
Odor
nectar
Nectar
guides
Pollinator
Fruit
Fruits develop from ripened ovaries, but may also contain other structures such as the
hypanthium and the receptacle. The ripened ovary wall is called the pericarp and can be
divided into three more or less distinct layers. The endocarp is the inner most pericarp
layer. The mesocarp,
which is the middle
pericarp layer and it
can be fleshy as in the
tomato or
indistinguishable from
other layers as in the
bean pod. The
exocarp is the
outermost layer and
forms the protective
skin on a fruit. Fruit
types are highly variable and the classification of fruit is largely artificial (i.e. not
reflecting taxonomic relatedness). The simplest classification schemes place fruit into
three categories: simple – fruit that develop from a single ovary; aggregate – fruit that
develop from multiple ovaries within a single flower; and multiple – fruit that develop
from several flowers coalescing into a single fruit.
Using the fruit available today for dissection, identify the different regions of the
pericarp. Make representative sketches of the fruit dissections below. Make sure you
label the sketches and indicate the fruit type that you are working on.
Use the key to fruit types to help fill in the table below.
Species
Fruit type
Key to common Fruit Types
1. Fruit derived primarily from ovary of the flower………………………………2
1. Fruit derived mostly from floral parts other than ovary. Accessory fruit: calyx or
receptacle fused with ovary wall; can be simple, aggregate or multiple…………..15
2. Fruit derived from a single flower…………………………………………..3
2. Fruit derived from multiple flowers whose ovaries have coalesced into a single
fruit.
Multiple fruit
3. Fruit derived from a single ovary…………………………………………4
3. Fruit derived from multiple ovaries………………..Aggregate Fruit
4. Pericarps flesy at maturity…………………………………………….5
4. Pericarps dry at maturity………………………………………………8
5. Pericarps fleshy throughout………………………………………..6
5. Pericarps divided into thin, skin-like exocarp; thick mesocarp; and
hard, stony endocarp……………………………..Drupe
6. Fruit with leathery exocarp………………………………..7
6. Fruit without a leathery exocarp; fleshy
throughout……………………Berry
7. Exocarp leathery and glandular; exocarp easily separated
from inner layers………………….Hesperidium
7. Exocarp leathery to woody, not glandular; inseparable from
inner layers……………………..Pepo
8. Fruit splits open when ripe (dehiscent); contains numerous seeds….9
8. Fruit doesn’t split when ripe (indehiscent); usually with only 1 or 2
seeds…………………………………………………………….……..12
9. Fruit one carpel………………………………………………..10
9. Fruit two or more carpels………………………………………11
10. Fruit splits along 1 seam….Follicle
10. Fruit splits along 2 seams…Legume
11. Fruit 2 or more fused carpels that separate, leaving a
persistent partition between…………………..Silique
11. Fruit 2 or more fused carpels that split in a variety of
ways………..Capsule
12. Fruits with wing-like outgrowths….Samara
12. Fruit without wing-like outgrowth…………………………..13
13. Fruit with hard exocarp; usually attached to extra material
(e.g. acorn cap)…………………………Nut
13. Fruit without hard exocarp and without extra material
attached …………………………………………………14
14. Fruit with one seed that is attached to pericarp at a
single point …………….Achene
14. Fruit with a single seed that is completely fused with
pericarp………..Caryposis (grain)
15. Simple fleshy fruit derived from inferior ovary surrounded by a fleshy
hypanthium …………………………………………..Pome
15. Most of fruit derived from receptacle; individual ovaries form berries, drupes, or
achenes (eg strawberry or blackberry)…..Accessory aggregate