Thought questions:

1. What features are found only in plants, which distinguish them from members of other kingdoms?

2. Are there structures similar to a mouth and an anus in a plant?

3. Where on the overall plant body do plants have their greatest surface area?

4. Over evolutionary history, what have been trends in Dependency on water for survival? For fertilization? Vascular tissue? Leaves? Sporophyte size? Gametophyte size? “Male” and “female” specialization of spores and gametophytes?

Learning objectives:

1. List characteristics that distinguish plants from organisms in other kingdoms.

2. Describe the life cycle of a non-vascular plant, a moss (Phylum Bryophyta).

3. Describe the life cycle of a seedless vascular plant (Phylum Pterophyta).

4. Describe the life cycle of a gymnosperm, a pine (Phylum Coniferophyta).

5. Describe the life cycle of angiosperm, a type of seed plant (Phylum Anthophyta)

6. Recognize examples of plants from 1 non-vascular phylum (Bryophyta) 1 seedless vascular Phylum (Pterophyta) 4 seed plant Phyla (Cycadophyta, Gingkophyta, Coniferophyta, Anthophyta)

7. Recognize major evolutionary trends in sporophytes and gametophytes.

8. List the parts of the flower and the fruit, and their functions.

9. Understand how plant bodies and growth are organized into root and shoot systems.

Readings: Campbell et al 6th edition: Chapters 29-30. Ch.38 pp.783-790, and Ch 35 pp. 729-734.

fall 2004, Lab 7-1

Lab 7PLANT FORM AND FUNCTION

OCTOBER 25-29, 2004

LAB OVERVIEWBASIC PLANT BIOLOGY:

LIFE CYCLES, EVOLUTION TRENDS,

In lecture and recitation you will learn about how to relate plant structure with plant function, particularly the functions that are essential for plant survival and growth. Today we will review these topics in lab. We will also add to this knowledge by studying plant reproduction from an evolutionary

point of view, emphasizing how it has become less dependent on water during millions of years of plant evolution.

BEFORE CLASSPrepare for today’s quiz by reading the lab manual, attending recitation, and reading the recommended pages in your textbook

DURING LAB, IN PAIRS

Make measurements of your results from Lab 6 and record your data on the appropriate worksheet: Embryo Weight, Shoot Length, Root Length, Number of Lateral Roots.

Enter your group’s Lab 6 data into the group database.

Examine the fresh specimens and prepared slides that will allow you to see key portions of the life cycle of mosses, ferns, pines, and angiosperms.

Look at some of the diversity of seedless vascular plants (ferns and fern allies) and the diversity of seed plants (cycads, Gingko, and conifers in addition to diverse angiosperms.)

ASSIGNMENT (due at beginning of lab next week)

Individually: complete worksheets 1 (Life Cycles of Mosses, Ferns, Pine, and Angiosperm) 2 (Evolutionary Trends)

Per group: typed summary of your assigned Phylum of plants; we’ll begin lab next week with short (<5 minute) group presentations.

Lab 7-2

Overview of the Kingdom Plantae

What makes a plant a plant? Non-biologists usually think its photosynthesis. By the end of today’s lab, you should be able to criticize this answer and offer better ones. For example:

an essential feature of all members of the plant kingdom is a life cycle that alternates between a haploid (n) and a diploid (2n) phase – the gametophyte and the sporophyte (alternation of generations);

also, the term embryophyte is a good synonym for all taxa in the plant kingdom, because it emphasizes the multicellular embryo as the most important morphological and ecological feature shared by all plants. The embryo is not found in the group that shares a common ancestor with plants -- the photosynthetic green algae (Phylum Chlorophyta) (you saw examples of these protists in lab 5: Oedogonium and Chlamydomonas).

In plants, embryos are physiological and morphologically connected with and dependent upon at least one previous generation. In the seed plants embryos are connected to and dependent upon the previous two generations.

The Kingdom Plantae consists of many Phyla. We will begin today with Phylum Bryophyta (mosses) and Phylum Pterophyta (ferns). We will also examine several seed plant phyla: Cycadophyta (cycads), Gingkophyta (Ginko), and Coniferophyta (conifers); culminating with the flowering plants in the Phylum Anthophyta (a synonym for the flowering plants or angiosperms).

Toward the end of lab, your instructor will assign one or a few phyla to your group. Each group will prepare a one-page typed summary reviewing the most important features of any given phyla for the rest of your classmates, and also be prepared to critique the reviews offered by your peers. Each group should also prepare a definition of what makes a plant a plant and what makes members of her phylum different from other plants.

Overview of Plant Evolution

Plants

Non-seed-bearing plants Seed-bearing plants

Non-flowering plants Flowering plants

Monocots Dicots

fall 2004, Lab 7-3

TRENDS IN PLANT EVOLUTION

Non-vascular plants: Mosses and moss allies

Fresh specimens and prepared slides are available for tracing the complete life cycle of the typical moss species in the Phylum Bryophyta. As you study, read the brief notes below and make sketches in the empty boxes on the life cycle diagram on Worksheet One. You should also fill out the “Non-vascular plant” column in Worksheet Two. Note that not all non-vascular plants are exactly like the mosses.

1. Gametophyte . The gametophyte is the dominant stage in the life cycle of mosses and moss allies. “Dominant” means that this stage is larger and lives longer and is the one most easily noticed. Gametophytes are multifunctional. They photosynthesize, manufacturing foods for their own growth, which occurs by mitosis. They often can reproduce asexually, which also involves mitosis.

2. Prepared slides of gametangia. The mature gametophytes of mosses and liverworts produce eggs and sperm in complex, multicellular structures known as gametangia (from the Greek, “gamete-holding vessel”). The sperm-producing gametangium is called an antheridium while the egg-producing gametangium is known as an archegonium. (See life cycle diagram, and make sketches in the boxes provided.) Recall that the entire plant is haploid at this stage. The gametangia develop in specialized locations on the gametophyte, i.e. the tips of stems in mosses. Since the gametophyte is already haploid, gametes are the products of mitosis (NOT meiosis) within the gametangia.

Sperm released by the antheridium are flagellated and reach the egg in an archegonium by swimming. Archegonia release sugars, proteins, acids, and other substances that attract the sperm. Therefore, for sexual reproduction to occur, water is essential (usually in the form of rain). In general, you will not find mosses in extremely dry areas (deserts), although there are a few exceptions.

Examine the prepared slides of thinly sectioned “male” and “female” gametophytes and draw examples of antheridia and archegonia in the appropriate boxes of the life-cycle diagram.

3. Sporophytes. After an egg inside an archegonium is fertilized by the motile, flagellated sperm, it forms a diploid zygote. This cell starts dividing inside the archegonium to form the multicellular embryo, which continues developing to form the mature sporophyte. As the sporophyte grows, some of the tissue develops into a foot that maintains connection with the supporting gametophyte. In this manner, the gametophyte’s photosynthesis is essential not only for nourishing the gametophyte itself, but also for nourishing the next generation.

When a sporophyte is mature, certain specialized cells within it will undergo meiosis. This occurs in a structure called a sporangium (plural, sporangia). (This is true for moss sporophytes and for all plant sporophytes.) The single-celled, haploid products of meiosis are packaged for dispersal, with a coating of a protein to protect them from drying out. These haploid cells are called spores.

Although mosses are short in stature, and although moss sporophytes are smaller than moss gametophytes, the sporangium, from which spores are released, is taller than the gametophyte.

Lab 7-4

Seedless vascular plants: Ferns and Fern Allies

Contrast the ferns with the mosses. In ferns, the sporophyte phase is dominant — it is larger and longer in duration and definitely the phase most easily noticed. However, when it begins life as a zygote and embryo, it is nutritionally dependent on the smaller, less noticeable gametophyte; as its development continues, it becomes photosynthetic and its maternal gametophyte slowly dies—resulting in an independent sporophyte. Each fern sporophyte bears multiple sporangia. Is this an advantage? Why?

There are living specimens of both fern sporophytes and fern gametophytes for you to study, as well as prepared slides to add to your understanding. Worksheet One provides you with a life cycle diagram with empty boxes, where you should make sketches of important structures in the different phases of the life cycle.

As you make your sketches, be sure to include notes about how large the structures are. A common source of confusion is diagrams that show the fern sporophyte and fern gametophyte at approximately the same size. By the time you finish your observations today, you should be aware that this is misleading and inaccurate. Also, be sure to complete the “Seedless vascular plant” column on Worksheet Two. Note that not all seedless vascular plants are exactly like the ferns in the Phylum Pterophyta.

1. Fern sporophytes . Many fern sporophytes (2n) have been lent to your lab from the Barnard Greenhouse. Some of the large, flat leaves of ferns are purely vegetative. They have cells and tissues specialized for photosynthesis. Some leaves are also reproductive, and these are referred to as sporophylls. To identify sporophylls, look on the lower side of the leaves for small dots, called sori (sorus, singular). The sporangia of a fern are found inside the sori. There, meiosis occurs to produce haploid single-celled fern spores. Like the spores in mosses, they are coated with a protein to help prevent desiccation after dispersal. In some species, the fertile sporophylls are highly modified and do not resemble the vegetative (non-sporangia bearing) leaves.

2. Fern gametophytes . Fern spores (1n) that land on a suitably moist substrate will germinate, beginning to grow into a multicellular gametophyte (1n) by the cell divisions of mitosis. Remember that meiosis cannot occur in a haploid organism (why?). If there is sufficient light to support photosynthesis, fern gametophytes have a source of nutrition and will survive and grow to maturity. Fern gametophytes are an independent part of the life cycle. They do not depend on the dominant sporophytes for nutrition.

We have ordered spores on nutrient media in petri dishes for you to examine them with your dissecting microscopes (see below). You may also want to look at the moist soil in the various pots from the greenhouse to see if there are any gametophytes growing there.

We also have prepared slides that allow you to use the compound scope to examine a fern gametophyte in greater detail. Most ferns are homosporous, producing just a single type of spore. That spore grows into a gametophyte by mitosis, and subsequently that gametophyte produces both sperm-producing and egg-producing gametangia, also by mitosis.

On the prepared slide, identify the sperm-producing gametangium, or antheridium. This is the structure that produces flagellated sperm (male gametes). As in the moss gametophyte, the male gametes are produced by mitosis. Make a sketch in the appropriate box in your life cycle diagram.

Also examine the egg-producing gametangium, the archegonium. There, large nonmotile eggs (female gametes) are produced by mitosis. As in the mosses, the egg cells release sugars and pheromones to attract sperm. Sperm cannot swim from the antheridium to the egg in the archegonium unless water is present. Again, make a sketch in your life cycle diagram.

fall 2004, Lab 7-5

3. Sperm release from a 14-day old fern gametophyte. You can take one of the Petri dishes and use your dissecting scope to examine a fern gametophyte at as high magnification as possible. When you have the gametophytes well focused, look at the dish from the side and use a pipette to add just a single drop of water to the clump of gametophytes. Don’t move the dish! (You don’t want to lose your fine focus.) Go back to observing the gametophytes and wait a few moments. You should see the sperm release. Do these sperm have flagella? How can you tell (the flagella would be much too small to see under the dissecting scope)?

Non-flowering seed plants: three gymnosperm Phyla

Members of the seed plant phyla obviously produce seeds. They also produce pollen. The origins of seeds and pollen are in two types of sporangia that produce two types of spores (they are heterosporous). In contrast, only a single type of sporangium, and spore (homosporous), is produced in most mosses, moss allies, almost all ferns, and many fern allies.

‘Female’ or megasporangia produce spores by meiosis, and these spores are not released from the plant. Rather, they grow by mitosis while attached to the sporophyte, growing into a female gametophyte. At maturity, the female gametophyte will produce an egg cell that requires fertilization. Then, the fertilized ovule matures into a seed.

‘Male’ or microsporangia also produce spores by meiosis, and these spores are also never released directly from the plant. They go through a small number of mitotic divisions before maturing into multi-cellular pollen grains. Thus, a synonym for a pollen grain is microgametophytes, or male gametophyte. One of the cells in the pollen grain is the sperm cell. Its nucleus will fuse with the nucleus in the egg cell at fertilization (there are variations among the seed plant phyla).

The greenhouse has lent us sporophyte specimens from three different seed plant phyla. You should also examine “male” and “female” pine cones and the micro- and mega-gametophytes produced in pines. (Pines are members of the Phylum Coniferophyta.) After you complete your observations, complete the “Non-flowering Seed Plants” column in Worksheet 2 and the life cycle drawings in Worksheet 1.

1. Sporophyte of the cycad. The cycads (Phylum Cycadophyta) are an ancient lineage of mostly tropical plants. The cycad flora, which were quite extensive during the age of the dinosaurs (known to botanists as the Age of Cycads), are rare today and many species are endangered. Many of them look like ferns, but they produce true seeds rather than single-celled spores.

Cycads have separate male and female individuals. The specimens that we have to show you are males. On these plants, where are the sporangia? What cells or structures are produced there? What happens to them?

Lab 7-6

2. Gingko sporophyte branches and reproductive structures. Another phylum of seed plants is the Phylum Gingkophyta, represented by the single species Gingko biloba.

This species been a common ornamental tree in urban areas for a century or two, and in recent decades it has proven to be remarkably tolerant of air pollution. There are many along Claremont Avenue in the blocks just north of Barnard’s campus. The fossil record of Gingko extends back hundreds of millions of years, and 150-million-year-old fossil Gingko leaves look exactly like the distinctive, flat, fan-shaped leaves of modern trees. G. biloba was once widely distributed in the northern hemisphere, but it now occurs in the wild in only a few locations in eastern China. Its disappearance from the wild is somewhat of a puzzle.

Individual Gingko trees are either male or female, much like the cycads. Any given Gingko tree is either “male” (producing only pollen, not seeds) or “female” (producing ovules and seeds, but not pollen). The seeds of the female trees are renowned for their putrid odor (and are referred to by some non-scientists as Ginko stinko). The Ginko trees along Claremont Avenue currently have ripening fruits; you might visit them to have a sniff.

3. Pinus “ male” pollen-bearing cones and “female” ovule-bearing cones . Both types of cones bear the sporangia, where meiosis occurs to produce spores. The two types of cones produce different types of sporangia and spores, “micro-” and “mega-”. Microspores mature into small, numerous, “male” microgametophytes (pollen grains) and larger, less numerous megaspores mature into “female” megagametophytes (ovules).

Pinus “ male” microgametophytes . The male or microgametophyte in a pine is more commonly referred to as a pollen grain. Like all plant gametophytes, pollen grains grow directly from a single-celled haploid spores; they are not produced inside of an antheridium. At maturity, the pine pollen grains released into the wind are multicellular – each pollen grain has four cells. Only one of these cells is a gamete, the cell that will fertilize an egg cell in the female gametophyte.

Pinus “ female” megagametophytes . We observe the very last phase of development in a female gametophyte, found inside of a pine seed. You should be able see the sporophyte embryo, which is the next generation, surrounded by background tissue. This background tissue is all haploid, part of the female or megagametophyte. At this stage in development, it is not possible to see the egg cell, but prior to fertilization it was surrounded by a “jacket” of cells, which is the female gametangium or archegonium.

fall 2004, Lab 7-7

Angiosperms are flowering plants

The most diverse and most dominant plants on the planet today are the flowering plants, the angiosperms belonging to the Phylum Anthophyta. Most of the material in lab today is from the sporophyte generation, starting with starting with the vegetative plant body, and proceeding to the reproductive structures (flowers and fruits). As you study these vegetative and reproductive features, you will fill in the final column in Worksheet Two and the life cycle drawings in Worksheet 1.

As you work through the lab, you will encounter many different structures, tissues, and cell types. While you are expected to recognize these anatomical features and learn the terms used to identify them, it is even more important to learn their function. Do not pass over any of the structures or tissues referred to in bold without reviewing their functional significance.

Flowers

Flowers are the reproductive structures characteristic of angiosperms. Under certain environmental conditions, the shoot apical meristem is transformed into a floral meristem. Instead of growing into a leafy branch, the cells of the meristem differentiate into a modified branch bearing up to four types of floral organs – sepal, petal, stamen and carpel.

The size, shape, color, and scent of flowers is intimately related to their mode of pollination. Wind-pollinated plants, for example, have reduced petals; larger ones might block air-borne pollen grains. Animal-pollinated plants, in contrast, have showy floral parts and scents that attract pollinators. Pollination systems have played a key role in the evolution of angiosperms and are largely responsible for the enormous variety of flowers.

Petals and sepals. The most recent evidence from molecular and developmental genetics supports what botanists have long believed: the organs of a flower are modified leaves. You can see this most easily in the parts that are usually the most visually attractive to pollinators, the sepals and petals. The sepals are located outside the petals and enclose and protect the flower bud. Often they are green, but they may also be colored and showy like the petals. Identify the sepals and petals, and draw and label these parts on the generalized flower on the life-cycle diagram. Sepals and petals are often called “sterile” flower parts because they do not bear microsporangia (pollen) or megasporangia (ovules).

Reproductive parts of a flower The stamen and carpel are referred to as “fertile” floral organs because they bear microsporangia and megasporangia, respectively. Inside sporangia, sporocytes undergo meiosis to produce the first cells in the gametophyte portion of the plant life cycle.

Male floral partsThe male floral organ is the stamen, and consists of a thin filament that supports the anther. The anther contains the microsporangium containing the microsporocytes that produce the microspores. Locate the stamens and their constituent parts on the flowers available in lab today. As in gymnosperms, the male gametophyte in angiosperms is greatly reduced (compared to the “lower plants”). The mature microgametophyte, the pollen grain, contains three nuclei: two sperm nuclei in one cell (this is called the generative cell; mitosis often does not occur until after the pollen tube has begun to grow through the maternal tissue) and a third nucleus in the tube cell (that will direct the growth of the pollen tube).

Lab 7-8

Female floral partsThe female floral organ is referred to as a carpel or pistil. Evolutionarily, it is thought to be a highly modified sporophyll. It is swollen at the base to form an ovary that contains the ovules (the ovary will develop into the fruit, and the ovules will develop into the seeds). The additional layer of maternal, sporophyte tissue in the ovary provides an extra layer of protection for the megametophytes and the developing embryo. It is unique to angiosperms and gives the group its name – the ovary is the “vessel” (angio) that contains the “seeds” (sperma).

The other end of the carpel consists of a flattened or branched, sticky surface on which pollen lands and is called the stigma. It is most often connected to the ovary by a tube of tissue, the style. Identify each of these structures on the sample flowers.

Now cut open the ovary to see the ovules (it is best to cut a cross-section) under the dissecting scope. Some ovaries, like those of corn, have only a single ovule while others, like those of orchids, may contain hundreds or thousands. An ovule consists of the megasporangium tissue and surrounding layers of integuments. A single megasporocyte (also called a microspore mother cell) in each megasporangium undergoes meiosis to produce four haploid megaspores. Typically, just one of these cells (the single functional megaspore) develops into a mature female gametophyte.

In angiosperms, this female gametophyte is extremely reduced (again, compared to the “lower plants”). An archegonium is no longer produced, and at maturity the gametophyte, called an embryo sac, consists of only seven cells and eight nuclei (note that this is ONE specific example of an embryo sac. There is a great diversity of embryo sac structure found throughout the angiosperms. This example of seven cells/eight nuclei that we study is the most common form of embryo sacs.) To understand the stages of pollen and embryo sac formation, study the diagrams in your textbook.

Pollination and fertilizationThe enclosure of the ovules in an ovary means that they are not directly accessible to the sperm nuclei in the pollen grain. Instead, the pollen grain lands some distance away, on the stigma, and the pollen tube that will deliver the sperm nuclei must first grow through the sporophytic tissue of the carpel. This allows the flower being pollinated to exert considerable control over the pollen tubes, even keeping some of them from reaching the ovules. Differences in mode and timing of pollination and the potential for interactions between the style and the pollen tube allow angiosperm flowers to exert considerable passive but effective “control” over the process of mating. Recall from your observations of gymnosperms that pollination, the delivery of pollen to a stigma, is not the same as fertilization, the union of sperm and egg nuclei.

In fact, the process of fertilization in angiosperms is another of their defining features. It involves a process called double fertilization. One of the sperm nuclei delivered by the pollen tube fuses with a single nucleus in an egg cell (within the multi-cellular egg sac) to form the usual diploid zygote.

However, the second sperm nucleus fuses with the two polar nuclei in the egg sac. Together, these three nuclei form the triploid (3n) endosperm. (See the summary, on the life-cycle diagram.) The endosperm, a tissue unique to angiosperms, grows and becomes the food reserve for the young sporophyte, either directly or indirectly through the cotyledons. The zygote develops into an embryo, and the mature, fertilized ovule becomes a seed.

**Watch the video on double fertilization in angiosperms.

fall 2004, Lab 7-9

Fruits

Under the stimulus of the growing seed or seeds, the ovary wall expands and thickens to become a fruit. The fruit protects the seeds, and more importantly, aids in their dispersal.

The development of a fruit as a greatly expanded ovary, can be readily observed in the pods of snow peas. Draw an opened snow pea fruit in the box, and label the parts in bold face below.

Snow peas are actually the entire carpel from the pea flower, and at the soft, edible stage, are still growing. With the help of your TA or instructor, find the stigma and style, and at the other end, the remains of the sepals. Observe the greatly expanded ovary, and feel the immature seeds (peas) inside. Notice the "seam" running along one side. Split the pod along the seam and observe the seeds attached (by the placenta) along the inside of the ovary. The unfolded ovary should now look distinctly leaf-like, like a leaf that bears reproductive structures, i. e., a sporophyll.

Reviewing the angiosperm life cycle and reproduction in angiospermsIf you fully understand the angiosperm life cycle, you should be able to describe the functions of each part of a flower and fruit and identify the ploidy (haploid, diploid, or triploid) of all the different tissues and stages. You should also be able to do your own “narration” of the film loop on double fertilization.

GROUP ASSIGNMENT DUE AT BEGINNING OF NEXT WEEK’S LAB

Each group of 3-4 students will be assigned a phylum within the plant kingdom. Each group should prepare the following to hand in at the beginning of lab next week (approximately one typed page, not including any diagrams):

1) description of what makes a plant a plant and different from other organisms;2) summary of what makes a member of the assigned phylum different from other plants;3) labeled diagram(s) and description of a specific or generalized life cycle; and4) description of the interesting features of the assigned phylum.

Next week’s lab will begin with short (less than 5 minutes each) presentations by each of the groups. The summary sheets for each group will be photocopied and distributed to all members of each lab section.

**** IMPORTANT: Worksheet 1 is four pages-long and cannot be printed from CourseWorks. It consists of hand-drawn figures on 11 x 17-inch paper. You can pick up a copy at recitation or from Altschul 911.

Worksheet 2 is a separate Word file. You must print this out (or pick up a copy from Altschul 911).

Lab 7-10

Please draw in the appropriate space on worksheet 1.

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fall 2004, Lab 7-11

Name _________________________________________________ Day/Time/Instructor _______________________________BC Bio 2003 Fall 2004

LAB 7, WORKSHEET 2: TRENDS IN PLANT EVOLUTION

Non-vascular plants(Mosses and allies)

Seedless vascular plants(Ferns and allies)

Non-flowering seed plants (Gymnosperms)†

Flowering plants(Angiosperms)

Formal names of phylum(-a)studied in lab?Dominant phase (gametophyte or sporophyte?)Vascular tissue present?

Antheridium present?

Archegonium present?

Are spores produced?

Homosporous or heterosporous (micro- & megas-pores)?

Are spores released from sporophyte?*

Isogamous or oogamous?

Are pollen grains produced?

Is water required for fertilization?

Are seeds produced?

Are flowers and fruits produced?

Does double fertilization occur?

† For the ‘gymnosperm’ column, fill in answer based on your observations of the pine, a conifer. * If there are both microspores and megaspores produced, specify what happens to each.*

Lab 7-12

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Lab 7-13

Lab 7-14