BIO 152 Lab Exercise 4 - Flowers, Fruits, and Seeds PDF

Preview

Summary

This is one of the lab exercises...

Description

BIO 152 Lab Exercise 4 FLOWERS, FRUITS, AND PLANT REPRODUCTION OBJECTIVES OF LABORATORY: 1. Become familiar with flower structure and its related functions 2. Relate flower and fruit structure to the angiosperm life cycle 3. Gain an understanding of embryo development and germination in angiosperms Angiosperms (flowering plants) are characterized by two unique reproductive structures, flowers and fruits. Within the angiosperms, there are two dominant groups, the monocots and the eudicots. In this exercise, you will be studying the angiosperm life cycle and the characteristics of flowers and fruits in both groups. In this exercise, you will be introduced to many new terms. Because flower and fruit structures are highly variable, precise terminology is especially important to allow for consistent reference to structures with quite different appearances but from a common developmental origin. CONCEPT TO THINK ABOUT: After studying two labs on plant life cycles, you should be able to see the trend towards reduction of the gametophyte stage in more derived plants. In gymnosperms and angiosperms, the female gametophyte is so reduced that it requires a microscope to see it. Can you think of other trends toward reduction in other evolutionary lineages? For example, the lineage leading to placental mammals (the reduction does not have to be reproductive in nature)? Can you identify common factors that lead to reductions? I. INFLORESCENCES Flowers are attached to plants in various arrangements. If a plant has only one flower (like a tulip), the flower is solitary and is attached to the stem by a stalk, the peduncle. Such a flower represents a developmental transformation of a single shoot tip. Flowers may occur in clusters or inflorescences (like lilacs). In this case, the stalk to the cluster of flowers is called the peduncle and the stalks of each individual flower is called a pedicel. Inflorescences are categorized by their pattern of growth (determinate or indeterminate), their overall shape (elongate, flat-topped, etc.), and whether individual flowers are stalked or stalk-less. Below are some common types of inflorescences.

spike

corymb

raceme

simple umbel

panicle

compound umbel

1

cyme

head

II. FLOWERTERMINOLOGY III. Floral parts An angiosperms flower’s basic design has four whorls of floral organs. As you will learn later, there is tremendous diversity in this basic plan. The four whorls consist of the following: a) The outermost floral appendages are sepals (collectively, the calyx) and are usually green in appearance. They are often mistaken for leaves. b) Next inward are the petals (collectively, the corolla). Petals are modified leaves, and usually brightly colored to attract pollinators. c) Next inside the petals, are the stamens (collectively, the androecium), consisting of a filament (the stalk) and an anther. The anther contains the microsporangia in which pollen cells are produced by meiosis. Stamens carry out the “male” function in the flower. d) Lastly, in the center, is the gynoecium, the “female” part of the flower consisting of carpels. A carpel contains a terminal stigma that is receptive to pollen, an elongated style through which the pollen tubes will grow, and an expanded basal ovary containing ovules. Ovules contain the megasporangia in which the female gametophyte develops; they are attached to the ovary wall by a stalk (the funiculus). The ovary will mature into the fruit, the ovule into seeds. 1)

A diagram of a typical angiosperm flower is provided to the right. Label: sepals, petals, calyx, corolla, stamen, anther, filament, carpel, stigma, style, and ovary.

Reference Textbook page 485, 643

2

Carpels are modified ovule-bearing leaves (sporophylls) that are folded and fused to enclose the ovules (fused carpels are sometimes called pistils). This feature of enclosing the ovules is the basis of the term “angiosperm” (which translates into seed vessel”). 2) What are the advantages to this enclosed seed for plant survival?

Flowers possessing all four whorls (see floral parts above) are called complete flowers. If one or more of the whorls is absent, then the flower is incomplete. If both sexes are present in a flower, it is bisexual or perfect; if one sex is absent, the flower is unisexual or imperfect. A plant species that possesses both male and female flowers on the same individual plant is termed a monoecious plant, while a species in which male and female flowers occur on different individual plants is a dioecious plant. Note: The terms monoecious and dioecious apply to an individual plant, not a flower.

Floral symmetry Flowers have a characteristic symmetry when viewed from above, due mainly to the shape of the calyx and corolla. Flowers with one plane of symmetry (i.e. one mirror-image plane) are called bilateral, zygomorphic, or irregular. Those with two planes of symmetry are isobilateral, and those with three or more planes of symmetry are radial, actinomorphic, or regular.

Bilateral

Isobilateral

3

Radial

IV. ANGIOSPERM LIFE CYCLE Flowers and fruits are the reproductive structures characteristic of the angiosperms. To appreciate plant reproduction, it is important to develop an understanding of the life cycle. The flowering plant life cycle is characterized by a heteromorphic alternation of generations between a small, reduced haploid gametophyte generation and a dominant diploid sporophyte generation. The angiosperm sporophyte is the familiar “plant” with roots, stems, and leaves. Angiosperms have distinct male and female gametophytes that result from meiosis in specialized cells of the sporophyte plant, followed by several mitotic cell division events. The male gametophyte (microgametophyte) is called the pollen grain, and the female gametophyte (megagametophyte) is called the embryo sac. Flowers are the sporophyte organs that produce these gametophytes.

MALE (1N)

SPERM

FEMALE GAMETOPHYTE (1N)

MITOSIS

(1N)

(1N)

SPORE MOTHER CELLS (2N)

2N ZYGOTE

2N

2N

See also textbook pg. 489

4

EMBRYO

Male gametophyte development: In angiosperms, both gametophyte plants are structurally reduced in comparison to other plant groups. The male gametophyte begins its development in the anther. In the anther, diploid cells undergo meiosis to form masses of haploid microspores. These microspores occur in tetrads (groups of four). Nourished by the surrounding layer of cells, each microspore will develop into a pollen grain, containing the immature male gametophyte plant. The outer wall of angiosperm pollen is made up of sporopollenin, one of the most chemically inert biological polymers on the planet. It is chemically very stable and is usually well preserved in soils and sediments. Each microspore divides to form two cells, the tube cell and the generative cell, both within the original spore wall. The generative cell is mostly a nucleus that will divide to produce two sperm, the male gametes. The tube cell is a nucleus surrounded by cytoplasm and will produce the pollen tube, after the pollen has landed on a receptive stigma surface. This simple arrangement of cells comprises the entire mature male gametophyte plant! 3)

Observe the scanning electron micrographs of angiosperm pollen. What is a possible function of the highly sculptured outer wall of a pollen grain?

4)

Examine a prepared slide of a mature lily anther cross section. First, under lowest power, find the anther. Notice the two large chambers on either side of the anther. These are the pollen sacs where microspores arise by meiosis. Each microspore develops into a pollen grain. Examine these microspores under higher power. Draw the anther with pollen sacs below and label anther, pollen sacs, and microspores. Label the tube cell and generative cell on the diagram of a pollen grain in cross section below. Reference textbook pg. 643

5

Female gametophyte development The ovule of an angiosperm contains the megasporangium is surrounded by two layers of cells called the integuments. An opening in the integuments, the micropyle, provides access to the megasporangium inside. In the ovule, diploid cells within the megasporangium undergo meiosis to produce four haploid megaspores. Usually only one of the four megaspores will continue to develop into the embryo sac, the mature female gametophyte plant. At maturity, the female gametophyte will usually have seven cells. Most important are the egg cell (the female gamete situated near the micropyle) and the central cell (containing two polar nuclei). The egg cell is usually surrounded by two synergids (these three cells collectively are referred to as the egg apparatus). The three cells opposite the egg apparatus referred to as the antipodal cells. 5)

Examine a prepared slide of a lily ovary cross section using low power. The three chambers within the ovary contain ovules. Examine the interior of the ovules under higher power. If sectioned properly, you may be able to see the female gametophyte (embryo sac).

6)

On the drawing of the mature female gametophyte to the right, label: female gametophyte (embryo sac), egg, synergids, polar nuclei, antipodal cells, central cell, micropyle, and integuments.

Reference textbook pg. 643-644

6

7)

Label the following structures in the diagram of the angiosperm life cycle below: flower, carpel, stigma, megaspore mother cell, megaspores (three usually abort), female gametophyte (megagametophyte or embryo sac), anther, microspores, pollen grain, male gametophyte (microgametophyte), pollen tube, egg, endosperm, embryo, seed. Indicate which stage is haploid (1N) and which stage is diploid (2N) and where meiosis occurs in the life cycle.

Reference textbook pg. 489

7

V.

FLOWER STRUCTURE OF MONOCOTS AND EUDICOTS

Obtain flowers of both a monocot and a eudicot from your instructor. Using the dissection procedure provided below, identify the structures indicated and answer all relevant questions on the next two pages. Make rough sketches of your dissections as indicated.

Dissection Procedure: Before dissecting your flower, observe the type of inflorescence from which your flowers come. A dissection microscope may aid in your dissection below. a. Obtain an entire open or partially open flower by breaking it off at its attachment point to the stem. b. The outermost structures on the flower are the sepals. In monocots, the sepals may be green or colorful like the petals. In eudicots, they are usually green. Identify the colorful petals to the inside of the sepals. c. Use a razor blade to cut off, one at a time, the sepals and petals at their point of attachment to the flower. The remaining structures left are the true reproductive structures. d. Locate the stamens surrounding the carpel. Each stamen consists of a long slender filament and an anther. Draw a stamen in the space provided and label the filament and anther. e. Pollen (the male gametophyte) is produced in the anther. Remove the stamens. Slice across the anther to expose the pollen. Draw the cross section of the anther in the space provided. f. Centermost in the flower is the carpel or carpels (also known as pistil). It consists of the ovary toward the base of the flower, the style, and the stigma. Draw this structure and label the ovary, style and stigma. g. The stigma is the receptive part of the carpel, where the pollen will land and germinate. Look at the stigma under the microscope and observe its morphology. h. Use a razor blade to cut a very thin cross section from the center of the ovary. Place the section flat and examine under the microscope. Note the small, whitish ovules (immature seeds) within the ovary. Draw this section in the space provided on the following pages, labeling the ovary and ovules.

8

8)

MONOCOT FLOWER DISSECTION

a. Name of species: b. Inflorescence type: c. Floral symmetry : d. # of sepals: e. # of petals: f.

# of anthers:

g. When you slice across the anther, what do you find inside?

h. Flower type (circle all that apply): complete, incomplete, perfect, imperfect i.

Which structure contains the female gametophyte with egg?

j.

In eudicots, floral parts typically occur in multiples of 2 (4) or 5, while in monocots, parts are in multiples of 3. Do your flowers follow this rule?

Stamen and cross section of anther

Carpel and cross section of ovary

9

9)

EUDICOT FLOWER DISSECTION a. Name of species: b. Inflorescence type: c. Floral symmetry : d. # of sepals: e. # of petals: f. # of anthers: g. When you slice across the anther, what do you find inside?

h. Flower type (circle all that apply): complete, incomplete, perfect, imperfect i. Which structure contains the female gametophyte with egg? j. In eudicots, floral parts typically occur in multiples of 2 (4) or 5, while in monocots, parts are in multiples of 3. Do your flowers follow this rule?

Stamen and cross section of anther

Carpel and cross section of ovary

10

II. HORMONAL REGULATION OF GROWTH AND DEVELOPMENT (Week 1) A hormone is an organic compound produced by one tissue and transported to other tissues, where it exerts a physiological effect on target tissues; it is usually active in very small quantities. In plants, five general classes of hormones appear to play major roles in regulating plant growth and development: auxins, cytokinins, gibberellins, abscisic acid and ethylene. These hormones may exert their effects individually or in interaction with each other (synergism). Today you will set up several experiments that test some effects of two of these hormone classes: auxins and gibberellins. You will record and interpret your results during the next lab period. Experiment A: Effects of Auxins on Root Elongation Indoleacetic acid (IAA) is a naturally-occurring plant auxin produced primarily in shoot tips and transported downwards throughout the plant. Smaller amounts are also produced in root tips. In general, auxins promote cell elongation in growing tissues, but they also influence patterns of cell differentiation. In normal concentrations, IAA appears to stimulate cell growth but very high concentrations may inhibit growth, especially in sensitive tissues such as roots. Using intact cucumber seedlings you will study the effect of differing concentrations of auxin (IAA) on growth of root tissue immediately after germination. 15)

Predict what do you think will happen. Draw what you think the response of root length will be to increasing IAA concentrations:

IAA Root Test - Week 1 Procedure: 1. Working in groups as directed by your instructor, prepare 10 cm petri dishes with the following concentrations of IAA: 0 (control), 0.1, 1.0 and 10 mg/L. 2. Place a piece of filter paper into the bottom half of each Petri dish and label it with a permanent marker to indicate the concentration contained within. 3. Add 3.5 ml of solution to each dish. Be sure not to mix up the pipettes and to put the correct solution into the proper dish. Next, add ten cucumber seeds into each dish, spacing them evenly.

4. Close the petri dishes, and tape them into a stack. Label the stack of petri dishes with your lab section and group name. Place your plates in the designated area. 5. Incubate the seeds for one week at 18.5°C.

11

Experiment C: Environmental Regulation of Growth and Development While the complex processes of plant growth and development are regulated internally by plant hormones, these activities must also respond to the external environment so growth will take place at the appropriate time and under the most favorable circumstances. There are a variety of growth responses to environmental conditions, the most familiar of which are the tropisms. These are “turning” or bending responses to environmental stimuli that result from differential growth. One such tropic response is gravitropism, a growth response to gravity. Gravitropism may be positive (toward the direction of gravitational pull) or negative (away from the direction of gravitational pull). The mechanism of this process is not fully understood, but evidence implicates the differential inhibition of cell elongation caused by auxin. We will study the gravitropic response in the radicles of sprouted corn seedlings. Root Gravitropic Test - Week 1 Procedure: 1. Working in groups as directed by your instructor, obtain a plastic cup and line it with a wet filter paper. Add some crumpled moistened paper towels to the center of the cup. 2. Using permanent marker, label the outside of the cup with your section number and group name. 3. Obtain four corn seeds and place them between the filter paper and edge of the cup. The corn seeds should be oriented in different directions. 4. Cover the cup with plastic wrap secured with a rubber band, and place in the specified area. The seedlings will be grown for one week at 18.5°C.

Lab developed by numerous botanists at University of Wisconsin-Milwaukee, Department of Biological Sciences; edited by S. Hoot, E. Young & P. Engevold. ©Department of Biological Sciences, University of Wisconsin-Milwaukee

12...