Lab 4B: Plants 2- Vascular Seed Plants

Lab 4b: Plants 2- Vascular Seed Plants

Objectives:

Describe the relationship between microsporophyll, microsporangium, microspore, microgametophyte, and pollen in conifers
Describe the relationship between megasporophyll, megasporangium, megaspore, megagametophyte, ovule, and seed in conifers
Describe the structures of the “male” and “female” cones
Differentiate between pollination and fertilization

General Introduction:

We will continue our examination the Plant Kingdom as a series of 4 evolutionary stages. The nontracheophytes (mosses and liverworts) exhibit the first stage: embryos wrapped in parental tissue. The second stage, formation of vascular tissue is displayed in the vascular seedless plants (ferns and horsetails). Evolution of seeds is examined in the conifers, and the evolution of flowers in the anthophytes. (We will further explore the structure of seeds and fruits in a later lab this quarter.)

The evolution of the seed plants involved several major changes in the life cycle and life cycle structures from those of the seedless plants. We will look at these changes in lab including: the reduction of the gametophyte stages (and subsequently a sporophyte dominant alternation of generations), a heterosporous life cycle, and the evolution of the seed and pollen. The seed plants, unlike the seedless plants, have a gametophyte stage that is nutritionally dependent on the sporophyte generation. The reduced gametophyte stages are enclosed their entire lives by parental sporophyte tissue (whether on the plant itself or wrapped in a seed coat).

The gametophytes are the result of mitotic divisions of the haploid spores. In the heterosporous life cycle, microspores (produced through meiosis in the microsporangium) divide mitotically to become the multicellular microgametophytes (pollen grains). Within a pollen grain is at least one sperm nucleus. In that same heterosporous life cycle, megaspores (produced through meiosis in the megasporangium) divide mitotically to become the multicellular megagametophytes (ovules). Within an ovule is an egg nucleus. Transfer of the pollen grains to the ovule is termed pollination and union of a sperm nucleus and an egg nucleus is termed fertilization, with a resulting diploid zygote. The zygote will ultimately develop into the new sporophyte generation.

The zygote initially develops into an embryo within the megagametophyte. The embryo, the megagametophyte, and a seed coat (derived from diploid tissue surrounding the megagametophyte) are packaged together by the plant as a seed. The seed can be released from the plant to germinate in appropriate environmental conditions. Seeds provide several reproductive advantages for these plants. First, they can increase dispersal of the next diploid generation as the seed can be carried by the wind, water, or another organism. Second, the megagametophyte tissue serves as a carbon energy source for the developing embryo giving it an energy boost early in its life. With this energy boost, the embryo will produce leaves that allow for photosynthesis and metabolic independence. Third, the seed coat provides protection for the embryo while it waits to germinate until conditions are suitable.

General

Work in a group (the size of the groups will be determined by the size of the class and by the amount of equipment available
Examine the slides and the plant specimens provided in the lab.
Please do not hoard all slides at your lab station, and allow other students to have access to all slides.
Use the pictures in the photo atlas and your textbook to guide you through the slides and specimens.

Part 1: A Macroscopic and Microscopic Examination of Essential Structures

1) Phylum Coniferophyta

Background: The Phylum Coniferophyta represents a larger group of organisms known as gymnosperms. The gymnosperms bear naked seeds on sporophylls. The conifers are the largest of 4 phyla within this group. The conifers (“cone-bearing plants”) are represented in lab by several species found on campus (pines, cedars, etc.) and by prepared slides.

Almost all the conifers are “evergreen” holding their needle-like or scale-like leaves year round. This allows for growth year round, although this growth is reduced in the seasons of less sunlight. The reduced leaves are adapted to colder, drier climates with a thick cuticle, and stomata located in pits. Commercially, most of our lumber and paper pulp comes from the wood of conifers.

We will examine the pine tree structures in lab, and will focus on the life cycle stages discussed above. The diploid pine sporophyte (the tree) contains two types of cones. Staminate (or pollen) cones contain the microsporangia that give rise to the microgametophytes, pollen grains, containing the sperm nuclei. Ovulate cones contain the megasporangia that give rise to the megagametophytes, ovules, containing the egg nuclei.

Slides. Define and/or recognize: see structures listed below

Pinus, megaspore, l.s. Find megaspore mother cell and micropyle.
Pinus, archegonia,with egg, l.s. Find egg and archegonium.
Pinus, pollen (staminate) cone, l.s. and c.s. Observe microsporangium, microsporophyll, pollen.
Pinus, pollen grains, w.m. Observe pollen grain and haploid nuclei; note shape.

Live specimens:With your group, collect 1+ cones (including both staminate and ovulate) from a conifer. Sketch these examples in your notebook, providing an approximate scale on your sketchand labels of key structures.

2) Phylum Anthophyta

Background: The anthophytes are recognized as the “flowering plants”. The term angiosperm is also used to describe this group, but it more correctly refers to all plants that have fruiting structures. Not all angiosperms are anthophytes. (Having written that, however, we will focus only on the anthophytes as we also examine fruits). In a later lab, we will examine various types of flowers and fruits as well.

A flower is a stem tip with up to four whorls of modified leaves. The outermost whorl is composed of the sepals. Sepals are usually green and enclose the flower before it is revealed. The next inner whorl contains the petals. Petals are generally attractant colors (advertisements for pollinators) and along with the sepals are sterile structures in the flower.

The next inner whorl of a flower is the androecium: the stamens. These are essentially the “male” parts of the flower. A stamen is made up of an anther and filament. The filament positions the anther appropriately for the incoming pollinator, as the anther contains the pollen producing structures. The anther is similar to the pollen cone in a conifer as they both contain the microsporangium that produces the microspore that becomes the microgametophyte, the pollen grain.

The innermost whorl of the flower is the gynoecium, the carpels. A carpel is made up of three parts: a stigma, style and ovary. The stigma is the sticky end of the carpel that receives pollen. The style is the tube that connects the stigma to the ovary, and also positions the stigma to best receive the pollen. The ovary contains the “female” reproductive structures. The ovary contains the megasporangium that produces the megaspore that becomes the megagametophyte, the ovule.

Upon fertilization, the pollen grain produces a tube that grows through the style to an ovule in the ovary. Two sperm nuclei are delivered to the ovule for a unique double fertilization. One sperm nucleus fertilizes the egg nucleus to become the diploid zygote, and the second sperm nucleus fertilizes the two polar nuclei of the ovule becoming the triploid endosperm. The zygote will develop into the new sporophyte, and the triploid endosperm is the nutritive material of the seed.

In angiosperms, a fruit develops around the seed(s). The fruit is essentially a mature ovary. After fertilization, the ovary matures, ripens, and becomes the fruiting structure. Basically the fruit assists in dispersal of the seed(s). Some fruits are shaped like wings for wind-assisted dispersal, some are shaped like burrs that cling to animal fur, and some are edible relying on animals to consume the fruit and pass the seed(s) when they excrete their wastes.

Slides. Define and/or Recognize: see structures listed below

Pollen grains. (haploid nuclei, tube nucleus, generative nucleus)
Lilium: Ovary, uninucleate embryo sac. (megasporangium, ovule, megaspore mother cell)

Live specimens:With your group, collect 1+perfect flowers (containing all 4 whorls). Sketch these examples in your notebook, providing an approximate scale on your sketch and labels of key structures.

PostLab Questions to answer:

What are the evolutionary “advances” you observe in the seed plants? What additional advantages do the angiosperms possess?

Are conifers heterosporous or homosporous? What about the angiosperms?

In Part 1, you observed a “megaspore mother cell”. Is this cell haploid or diploid? What will this cell develop into? Would you also expect to find similar cells in an angiosperm? Would its fate be the same?

How does pollination occur in the conifers? (Who is the pollinator?) Are there any significant advantages ordisadvantages to this pollination strategy? Explain.

How may pollination occur in the angiosperms? Are there any significant advantages or disadvantages to this pollination strategy? Explain.