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Biology 172L – General Biology Lab II Lab 02: Plant Life Cycles and Adaptations I: Mosses, Ferns and Their Relatives

Introduction

Members of Kingdom Plantae share the following characteristics: autotrophy through photosynthesis, multicellular, eukaryotic cells with chloroplasts and cell walls composed of the carbohydrate cellulose, nonmotile (except for the flagellated gametes in seedless plants), cells lacking cilia and flagella (except for the flagellated gametes in seedless plants), and complex reproductive cycles involving spore-forming (sporophyte) and gamete-forming (gametophyte) generations (see discussion below).

Many biologists distinguish members of Kingdom Plantae from the multicellular plant-like members of the protists. Seaweeds, in particular, resemble conventional plants in their possession of leaf-like, stem-like, and root-like structures. However, these structures are not considered true leaves, stems and roots because they lack true vascular tissues. In addition, like other algae, seaweeds differ from conventional plants by forming motile zoospores and lacking waterproof cuticles. Yet there also appears to be a clear evolutionary relationship between Kingdom Plantae and green algae (Kingdom Chlorophyta).

TABLE I

PLANT ADAPTATIONS TO THE LAND ENVIRONMENT

• Can obtain enough water without

being immersed in water. • Vascular tissues to transport water

and nutrients. • Waxy waterproof coating, a cuticle,

to minimize evaporative water loss. • Structural support tissues for

standing upright in air. • Carry out reproduction without

dependence upon external water.

Nearly all members of Kingdom Plantae

are terrestrial organisms. Thus they exhibit varying degrees of adaptation to living in the terrestrial environment (Table 1). Vascular tissues connect the photosynthetic parts of the plant with the absorptive structures (roots) that collect water and dissolved inorganic nutrients (e.g., nitrates and phosphates) from the soil. A

waterproof cuticle minimizes evaporative water loss from tissue surfaces. If spores are released, they are not motile zoospores, but are nonmotile spores that may be dispersed from the parent plant by the wind. On the other hand, seed plants, having dispensed with dispersal by windblown spores, are dispersed by embryo-containing seeds that can withstand desiccation.

Both the algae of Kingdom Chlorophyta and the plants of Kingdom Plantae exhibit complex reproductive cycles involving spore-forming (sporophyte) and gamete-forming (gametophyte) generations (Fig. 1). The cells of sporophytes possess chromosomes in pairs (diploid condition). Mature sporophytes produce spore-containing structures called sporangia. Inside the sporangia, cells destined to become spores undergo a division process, called meiosis, whereby the resulting daughter cells do not have chromosomes in pairs. Rather, each cell possesses only one member from each pair of chromosomes (haploid condition). These haploid spores may be carried away from the parent plant by the wind and may survive long periods of time under suboptimal conditions.

Figure 1. Typical plant life cycle.

Under the appropriate conditions the

spores germinate and undergo normal cell division (mitosis) to produce multicellular gametophyte plants. Since mitosis yields clones of genetically identical daughter cells, every cell of the gametophyte plant is haploid. The gametophyte plant is responsible for producing

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the gametes, which are similarly haploid. When two gametes fuse (fertilization), the two haploid sets of chromosomes are combined to yield a diploid zygote. This diploid zygote grows (through mitosis) into the diploid sporophyte plant, completing the cycle.

In many types of algae, the gametes are morphologically indistinguishable from each other (isogamy). Thus no male-female distinction is possible. However, plants of Kingdom Plantae, as well as other kinds of algae, exhibit oogamy, whereby a motile sperm (male gamete) fertilizes a nonmotile egg (female gamete). Gametophyte structures responsible for the production of sperm are called antheridia, while those gametophyte structures producing eggs are archegonia. When a male-female distinction exists, there may be separate male and female gametophyte plants (separate sexes), or a single plant may produce both male and female gametes.

In seedless land plants, such as mosses and ferns, the motile sperm, released from the antheridium, swims to the nonmotile egg protected within the archegonium. Thus successful fertilization is dependent upon external water, indicating that these primitive plants are not as well adapted to a terrestrial environment as the seed plants in which fertilization does not involve swimming sperm, but involves pollen tube formation (to be discussed later).

In seedless plants, fertilization of the egg takes place inside the archegonium. As the resulting zygote grows into the sporophyte, the sporophyte grows out of the gametophyte's archegonium. In the mosses, the sporophyte remains attached to the gametophyte throughout its growth and maturation, while, in the ferns, the sporophyte eventually outgrows the gametophyte which eventually dies.

In most terrestrial plants, the sporophyte generation is the most well adapted for survival out of the water, because the sporophyte usually possesses vascular tissues, while the gametophyte lacks them. Thus there is a trend towards sporophyte dominance, as opposed to gametophyte dominance, in terrestrial plants.

In this laboratory exercise, we will examine seedless plants from primarily two plant divisions: Phylum Bryophyta (the mosses) and Phylum Pterophyta (ferns). Our focus will be on their adaptations for a terrestrial existence. In this light, we will compare mosses and ferns to each other and to seaweeds exhibiting no adaptations for a terrestrial existence.

Bryophytes are plants exhibiting incomplete adaptation to the terrestrial

environment. The gametophyte generation is dominant and often forms dense, low-lying mats in moist environments. The mat acts much like a sponge to absorb and retain water. Usually, they produce separate male and female gametophyte plants (see attached diagrams). To achieve fertilization, the flagellated sperm must swim to the egg contained within the archegonium. The sporophyte embryo begins life surround by the gametophyte tissues of the archegonium. The mature sporophyte remains attached to the top of the female shoot. In most mosses, the sporophyte lacks vascular tissues, limiting how tall above the female shoot it can grow. The sporophyte produces a sporangium in which the haploid spores develop. Eventually the sporangium opens up to release these spores which may be carried away by the wind.

Tracheophytes are plants that possess true vascular tissues. The presence of vascular tissues allows for spatial separation between the photosynthetic parts of the plant (the leaves) and the absorptive part of the plant (the roots). Thus vascular plants may get very tall. Since only the sporophyte generation produces vascular tissues, these plants are sporophyte dominant.

These vascular tissues consist of two types, xylem and phloem. Xylem consists of dead tube-shaped cells that conduct water and dissolved minerals from the roots through the stems to the leaves and other organs. Phloem is composed of living cells arranged into tubes that transport the organic products of photosynthesis throughout the plant.

Another adaptation of vascular plants for living in the terrestrial environment is the presence of lignin embedded in the cellulose matrix of their cell walls. Lignin is a hard material that provides structural support for these plants.

Tracheophytes include both seedless and seed plants. Among the seedless plants are ferns (Phylum Pterophyta) and their allies, including horsetails, whisk ferns, and club mosses (Phylum Lycophyta). Note that the modern wisk ferns resemble the earliest vascular plant fossils (ca. 400-440 million years before the present). Seed plants include the cone-bearing gymnosperms and the flowering plants (the angiosperms). Seeds are actually sporophyte embryos in protective packages containing stored food reserves to assist in the initial stages of germination.

While well adapted to the terrestrial environment, ferns and their allies are restricted to living in relatively moist environments. They produce swimming sperm rather than the pollen

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of seed plants. As in the mosses, the sporophyte embryos grow out of the gametophyte archegonium (note that in the ferns, individual gametophytes possess both antheridia and archegonia). However, the sporophyte eventually outgrows the diminutive gametophyte which eventually dies. Thus the sporophyte generation is dominant.

Procedures and Assignments Read the appropriate information in your BIOL 172 textbook, Campbell and Reese (2005) Chapter 29, before coming to the laboratory session. Also be sure to bring you Biology Photoatlas to the lab session. I. PHYLUM BRYOPHYTA (The Mosses) A. Female Shoot with Attached Sporophyte

Examine the living and preserved moss

specimens on display. Note that because mosses lack true vascular tissues, like seaweeds, they do not possess true roots, true stems, nor true leaves. Yet they can stand upright in the air. Draw and label a diagram of a typical female shoot with an attached sporophyte illustrating the following features: female shoot, rhizoids, sporophyte stalk, and sporophyte capsule.

B. Archegonium

Examine the prepared slide of the top of

the female shoot. Find and identify the vase-shaped archegonia. How many eggs occur in each archegonium? Draw and label a diagram of a typical archegonium illustrating the features observed (label eggs, neck, neck canal, venter, and stalk). C. Antheridium

Examine the prepared slide of the top of

the male shoot. Find and identify the antheridia. How does the amount of sperm in the antheridium differ from the number of eggs in the archegonium? Draw and label a diagram of a typical antheridium illustrating the features observed (label sperm, sterile jacket, and stalk).

D. Sporangium Capsule

Examine the prepared slide of the

sporangium capsule. Draw and label a diagram of a typical capsule illustrating the following

features: spores, columella, operculum, capsule wall, and seta (stalk). Indicate which of these tissues are haploid (n) and which are diploid (2n).

E. Questions to Answer

Based upon your observations and on the

information presented answer the following questions.

1. Which generation, sporophyte or

gametophyte, is represented by the green mats that mosses typically form?

2. When spores germinate, what

generation develops? 3. What is produced in the sporangium? 4. For each of the following moss stages,

determine whether it is haploid or diploid: gametophyte, spore, zygote, sporophyte, sperm, and egg.

5. How are mosses dispersed? 6. State whether each of the following

moss characteristics is an indication that mosses are either well adapted or poorly adapted to life on land: (a) lack of vascular tissues; (b) body covered by a cuticle; (c) swimming sperm; (d) egg and embryo protected by the female shoot; and (e) spores are windblown.

II. SEEDLESS VASCULAR PLANTS

(Ferns)

A. Whole Fern Plant Examine the fern specimens on display.

The large, divided leaf (the frond) arises from horizontal stems called rhizomes. If the frond is mature it will bear many sori on its undersurface. Each sorus contains a cluster of sporangia. Draw and label a diagram that illustrates the following features: fronds, stems, rhizomes, and roots. What generation, sporophyte or gametophyte, is being illustrated in your diagram?

B. Fern Leaflet Sorus

Examine an individual sorus under the

dissecting microscope. Tease it apart with a dissecting needle to understand its structure. Compare these observations with those made

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by examining a cross section of a fern sorus under the compound microscope. What is being produced in the sporangia?

C. Fern Prothallus

Examine the whole mount slides of the

gametophyte prothallus and attempt to identify the locations of the antheridia and archegonia. How do fern gametophytes differ from moss gametophytes?

D. Fern Generation Dependency

Examine the diagram plastomount

specimens illustrating the life cycle of a fern. In addition, examine the whole mount slides illustrating juvenile sporophytes arising from gametophytes. Is either generation in the fern life cycle dependent for any length of time on the other generation?

E. Fern Adaptations to the Terrestrial Environment What feature(s) of a fern indicates that it is

better adapted to the terrestrial environment than a moss?

F. Fern Dependency on Moist Environments

What feature(s) of a fern indicates that it is

not fully adapted to life on land? III. FERN RELATIVES

Examine the specimens of whiskferns, clubmosses and horsetails on display. Draw and label diagrams of each. IV. MOSS-FERN SUMMARY

Complete the table on the following page with the appropriate descriptive information.

TABLE II

COMPARING MOSSES TO FERNS

Dominant

Generation

Vascular

Tissue

Swimming

SpermDispersal of Species

Moss

Fern

sporophyte or

gametophyteyes or no yes or no explain how

VOCABULARY sporophyte gametophyte vascular tissue zoospore cuticle spore diploid haploid mitosis meiosis gamete fertilization zygote isogamy oogamy sperm egg antheridium

archegonium thallus holdfast stipe blade pneumatocyst xylem phloem lignin cellulose tracheophyte rhizome frond sorus

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Lab Summary Your lab summary should consist of the following:

1. Descriptive title 2. Short introduction identifying main

objectives of the lab activity. 3. Brief description of methods employed. 4. Results and Discussion section

including labeled diagrams and tables as described in the Procedures and Assignment section above and following the appropriate protocols for presenting figures in a laboratory report. Be sure to follow all of the rules from producing figures for lab reports (one figure per page). Corresponding to each figure, there should be a short paragraph that describe the significant features of the figure.

5. In addition, be sure to answer all questions asked.

6. Please organize your report with numbered sections corresponding to the numbered sections in the Procedures and Assignment section of this lab description.

7. Short conclusion summarizing what was learned.

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Figure 2. Moss life cycle.

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Figure 3. Fern life cycle.