Download Objectives A. Flowers

BIODIVERSITY LABORATORY
REPRODUCTIVE DIVERSITY OF PLANTS: FLOWERS, FRUITS, SEEDS
Objectives
Observe and understand the development and diverse morphology of flowers and fruits.
A. Flowers
Flowers are the sexual organs of plants. Pollen produced in flowers contains sperm cells, while egg
cells form in the ovary portion of flowers. Though many plants are self-fertile (i.e., a sperm cell and an
egg cell from the same plant can successfully unite), cross-pollination (union of gametes from different
plants) is usually more advantageous for maintaining genetic diversity. Aquatic plants, and terrestrial
plants living in moist environments, like mosses and ferns, can utilize water as a medium that allows
male gametes to swim to female gametes. Most terrestrial plants, however, face special challenges in
bringing sperm and egg together. The solution is to employ some agent to facilitate the transfer of
gametes. These ‘agents of pollination’ accomplish the transfer of pollen from one plant to another. The
earliest evolving terrestrial plants, e.g., conifers and grasses, utilized wind for pollination. As discussed
earlier this semester, wind is generally available but is not particularly efficient at delivering pollen.
Thus a wind-pollinated plant must make a large investment in producing huge amounts of pollen in
order to guarantee that some find receptive eggs. To increase pollination efficiency, plants have
evolved schemes employing animals to deliver pollen in a much more discriminating manner than the
wind. These animals include bees, butterflies, other insects, bats, and birds.
When animals serve as pollinators, a plant needs to: 1) attract the pollinator to the flower; 2)
accommodate the pollinator physically so that its body or head and mouthparts can fit into the flower;
and 3) reward the visiting animal so that it will learn to "expect" similar rewards when it visits another
flower of the same species (thus completing pollen transfer). You have already seen some of the more
interesting, bizarre, and spectacular examples of plant attractants, morphologies, and rewards (or false
rewards!) in the video "Sexual Encounters of the Floral Kind". In this part of today's lab you will dissect
two different types of flowers to familiarize yourself with basic floral anatomy.
Parts of a flower
Flowers of two different types of plants, monocot and dicot, will be available in lab. You will need
to compare and contrast characteristics of the two types. In your comparison, focus upon how the two
types differ in terms of multiples of flower parts.
1) To become familiar with floral parts it is helpful to note that a flower consists of concentric whorls
of modified leaves (Fig. 1). Obtain a fresh flower from one of the types provided. Starting at the
outermost whorl working towards the center, identify the sepals, petals, stamens and carpels. Because
they are often fused, the carpels may appear as a single structure rather than a whorl of modified
leaves. Make a longitudinal section of the flower (cut it in half from top to bottom) so that you can view
the internal structures.
2) Examine a stamen of your flower using the hand lens. The stalk is called the filament and the
head is called the anther--it contains the pollen grains.
3) Next examine a pistil from your flower using the hand lens. A pistil is composed of one or more
carpels (modified leaves) and is differentiated into three parts. The upper portion, called the stigma,
often has a sticky surface to which pollen will adhere. Below the stigma is the neck-like style. At the
base of the pistil is a swollen portion, the ovary, which houses the ovules, which are the immature
seeds.
Note that as the ovule develops into a seed, the ovary will mature into a fruit.
Flower and Fruit Diversity
Page 1
4) Repeat the above examination for the other flower type.
5) How do these examples of monocot and dicot flowers compare to one another?
What features do they share in common?
How are they different?
Figure 1. A generalized flower.
B. Fruit Development
The stimuli of pollination and fertilization initiate changes in the flower ovary that result in the formation
of a fruit. A fruit is defined as the ripened ovary (or ovaries) of a flower, along with its contents and, in
some species, other closely associated parts such as a greatly enlarged receptacle or bracts. There are
numerous kinds of fruit, ranging from small, dry, seed-like fruits to the larger, fleshy types (Fig. 2a). The
correct botanical classification of a fruit may at first be confusing. For instance, a tomato is really a
berry, while a raspberry is really an aggregate simple fruit formed from drupelets. There can be no
argument about what is a fruit and what is a vegetable, however; if it is derived from the ovary of a
flower, it is botanically a fruit.
The parts of a fruit are the ripened ovary wall, or pericarp (which may be differentiated into outer
exocarp, intermediate mesocarp, and inner endocarp); seeds; placental tissues (the lining of the
ovary to which seeds are attached); partitions; receptacle (sometimes called the floral tube); and axis of
the stem (Figs 2a and 2b).
Figure 2a. Examples of dry and fleshy fruits.
Flower and Fruit Diversity
Page 2
The making of a fruit: the tomato
The tomato is type of fruit known as a berry. As such, it is fleshy at maturity and is derived entirely
from the ovary of a flower. The tomato has a “superior” ovary that develops above the whorls of sepals
and petals.
Carefully slice a tomato in half along its horizontal axis (equator). Many flowers have compound
carpels, single structures that were formed by the fusion of several separate carpels. (Some texts use
the general term pistil to describe a single carpel or several fused carpels.) Is there any evidence to
suggests that the tomato flower has a compound carpel? How many seeds (generally speaking) does
the tomato contain? How many ovules did the ovary of the tomato flower contain?
The making of a fruit: the apple
The apple, although fleshy like the tomato, is classified as an accessory fruit. Accessory fruits
develop from the ovary of the flower, but also include other floral parts as well. In the case of the apple,
the fleshy part of the fruit arises from the floral tube (receptacle)--only the “core” arises from the original
ovary. The "core line" marks the location of the outer portion of the ovary wall, the exocarp. The
leathery endocarp portion of the ovary wall is located closest to the seeds.
In contrast to berries, accessory fruits typically develop from epigenous flowers (those with an
“inferior” ovary--one that is situated below the whorls of sepals and petals.) In the apple flower, the
ovary is buried in the floral tube (receptacle) tissue. Where on the apple do you find the remnants of
sepals, stamens, and stigma?
Figure 2b illustrates changes in the floral parts that accompany the processes of pollination,
fertilization, and subsequent development of the apple fruit. Study the diagram carefully. Note in
particular that each ovule must be fertilized if that ovule is to develop into a mature seed.
Carefully slice an apple in half along its horizontal axis (equator). Does the apple have a compound
carpel? If so, how many carpels fused to form the ovary of the flower from which the apple fruit
developed? How many ovules were in each carpel?
Figure 2b. Development of an apple fruit.
Flower and Fruit Diversity
Page 3
C. Outline of the Classification of Fruits
I.
SIMPLE FRUIT (Figs. 3, 5, and 6) - arises from the single ovary of one flower.
A. SIMPLE FLESHY FRUIT(Fig. 3) - a portion or all of the PERICARP is fleshy at maturity.
1a. True Berry - always develops from a superior ovulary; entire pericarp fleshy at maturity. Tomato,
grape, avocado, date, eggplant, persimmon.
1b. Hesperidium - pericarp leathery, pulpy, juice sacs develop from inner carpel walls. Citrus
2. Drupe - Pericarp divided into three parts: outer exocarp, middle mesocarp (fleshy), and inner
endocarp (a stone or pit enclosing a seed). Plum, peach, apricot, almond, coconut.
3. Simple Accessory Fruit - fruit includes ovulary wall and other flower parts or closely-related
structures. Most develop from inferior ovularies, include ripened floral tubes.
a. False berry - floral tube and pericarp (entire fruit) ripens fleshy. Banana, gooseberry, currant,
cranberry, and pepo-type fruits(pumpkin, cucumber, melons).
b. Pome - fleshy receptacle, exocarp, and mesocarp; endocarp leathery. Apple.
B. SIMPLE DRY FRUIT (Figs. 5 and 6) - Pericarp is leathery, papery, or woody.
1. Dehiscent (Fig.5) - opens when ripe.
a. Follicle - develops from a simple pistil, opens along one side only. Milkweed, columbine
b. Legume - develops from a single pistil, opens along two sides. Bean, pea.
c. Capsule - develops from a compound pistil with two or more carpels. Opens in a variety of ways.
Poppy, iris, daylily, jimson weed, horse-chestnut, Brazil nut.
2. Indehiscent (Fig. 6) - fruit does not open at maturity.
a. Achene - small, one-seeded; seed coat does not adhere to pericarp. Dandelion, sunflower,
buttercup.
b. Grain, or Caryopsis - seed coat is fused with pericarp. Corn, rye, oats, barley, rice.
c. Samara, - usually one-seeded, has a wing formed by an extension of the ovary wall. Maple, elm.
d. Nut - a rather large, one-to-two-seeded fruit, bracts usually wholly or partially enclose the hard,
stony or woody pericarp. Hazelnut, beech, oak, hickory.
II. AGGREGATE FRUIT (Fig. 4) - arises from a number of separate ovaries attached to the single receptacle of
one flower.
A. AGGREGATE-SIMPLE - individual ovularies form druplets, these adhere to one another and separate
from the dry receptacle. Raspberry
B.
AGGREGATE-ACCESSORY - receptacle becomes fleshy, remains part of the fruit.
1. Drupelets adhere to elongated, fleshy receptacle. Blackberry.
2. Simple pistils ripen into achenes on fleshy, greatly enlarged receptacle. Strawberry.
III. MULTIPLE FRUIT (Fig. 4) - develop from a number of individual ovaries of flowers grouped closely together. A
fruit is produced from each flower; these fruits remain together in a single mass. Pineapple, mulberry, osage
orange, breadfruit, fig.
D. Observations of Fruit Types
Stations featuring the various fruit types are set up around the perimeter of the lab. Visit these stations to make
observations, and using the outline above and the figures on page 6, answer the questions posed below.
1. Legume. (Fig. 5.1) Examine the pods of a pea and of a soybean. Along how many sides does this fruit split?
Identify the floral structures that are represented. The pod is composed of one carpel. The fact that this carpel
may represent a modified leaf is more apparent in the pea than in many fruits, since the venation is not obscured.
Remove one of the peas and examine the hilum, or scar of the seed stalk, and close by it the micropyle, a tiny
opening through which the pollen tube entered the ovule and which admits water as a preliminary to germination.
Flower and Fruit Diversity
Page 4
2. Follicle. (Fig. 5.2) A follicle differs from a legume in that it splits along one side only. Examine the follicles of
milkweed. In the milkweed each seed has a large tuft of hairs by which it may be carried by air currents. Does the
milkweed ‘parachute’ arise from part of the seed or part of the fruit?
3. Capsule. (Fig. 5.3) Examine capsules of poppy and Iris. How do these differ from follicles and legumes? How
many carpels are there in the Iris capsule? How many in a poppy capsule? (Dissect the poppy capsule along its
‘equator’ to reveal the answer.)
4. Achene. (Fig. 6.1) Examine achenes of buckwheat and sunflower. Crack the outer coat and remove the seed.
Are the seed coats attached to any part of the pericarp? Peel off the seed coats and remove the embryo. Are
these plants monocotyledons or dicotyledons? Explain. Does the dandelion ‘parachute’ arise from part of the
seed or part of the fruit?
5. Samara. (Fig. 6.3) The broad-winged fruits of elm and maple are known as samaras. Drop one of these fruits
from the height of six feet. Explain how the behavior of this fruit as it falls might benefit the plant. Does the samara
‘wing’ arise from part of the seed or part of the fruit?
6. Nuts. (Fig. 6.4) Acorns, filberts, and hazelnuts are examples of true nuts, and are not to be confused with such
commercial "nuts" as almonds, walnuts, and hickory nuts, which are really like the stones of drupes, or with pine
nuts and Brazil nuts, which are hard-walled seeds. The pericarp is stony in a true nut. Examine an acorn, note the
cup-like involucre (a series of bracts) at the base. How does a nut differ from an achene?
7. Caryopsis. (Fig. 6.2) The caryopsis is the typical fruit of the grasses. Examine a grain of corn (one of the
largest grasses), and find the embryo or germ. Section the grain longitudinally, perpendicular to the flat sides.
Identify the pericarp, the large endosperm (divided into hard and starchy portions), and the embryo. How does
this fruit differ from an achene?
8. Berry and False Berry. Examine the fruit of grape or tomato (Fig. 3.4). Cut it into sections, and determine the
number of carpels and the arrangement of seeds. Such a fruit is a true berry.
The hesperidium (Fig. 3.6). is a type of berry with a leathery rind. The orange and the other citrus fruits fall into
this category. From what is the pulp derived?
The pepo, a fruit type peculiar to the melon family, is a false berry (Fig. 3.5). Study a cucumber, squash,
pumpkin, melon, or gourd. Cut it into sections, and observe the arrangement of the seeds. A fruit of this type is
developed from a flower with an inferior ovary, so the fruit wall and rind are composed in part of tissues from the
receptacle. The flesh is mostly mesocarp and endocarp, but the placental tissues may also be well developed
(especially in watermelon), which forms much of the pulp.
9. Drupe. (Figs. 3.2 & 3.3) Cut longitudinally through a cherry, peach, plum, or olive. Remove the flesh from one
side, leaving the stone in place. Identify the exocarp or skin, the fleshy mesocarp, and the bony endocarp. Is the
seed easily visible? Why not? What is necessary to obtain the seed?
10. Pome. (Fig. 3.1) Examine an apple. Cut longitudinally and another cut transversely. Identify the sepals,
ripened ovary, receptacle (fused floral tube) tissues, and vestiges of stamens. How many carpels are there? Was
the ovary inferior or superior?
11. The strawberry is an aggregate fruit (Fig. 4.1) in which the fleshy part consists of the enlarged receptacle.
The small, seed-like structures (fruitlets) scattered over the surface of the receptacle are small achenes.
Raspberries and blackberries are also aggregate fruits (i.e., not true berries). How do raspberries and
blackberries differ from one another?
12. The pineapple is a multiple fruit (Fig. 4.2). How does this differ from an aggregate fruit? To help with your
investigation, ask your instructor to demonstrate the proper method for carving a pineapple.
13. Fruit type ‘unknowns’. In order to leave lab today, you must correctly identify to fruit type three of the fruit
‘unknowns’ of your choosing. Have your lab instructor check your identifications. For each incorrect
identification, you must add an additional unknown to the number required to escape lab!!
Flower and Fruit Diversity
Page 5
Figure 3. Simple fleshy fruits
Figure 5. Dry dehiscent fruits
Flower and Fruit Diversity
Figure 4. Aggregate and Multiple fruits
Figure 6. Dry indehiscent fruits
Page 6