PLANTS: NONVASCULAR, VASCULAR, SEED AND SEEDLESS LAB 1 of 3 Objective: After completing this series of labs, you should be able to do the following: Describe adaptations that allowed Plants to colonize terrestrial habitats Describe the Alternation of Generations life cycle in plants using appropriate terminology Recognize and identify representative members of the Bryophyta, Hepaticophyta, Pteridophyta and Lycophyta Identify basic morphological structures (and their functions) of representatives of the Bryophyta, Hepaticophyta, Pteridophyta and Lycophyta Recognize and identify representative members of the Gymnosperms and Angiosperms. Describe the life cycle of the Gymnosperms and Angiosperms. Identify basic morphological structures (and their functions) of representatives of the Gymnosperms and Angiosperms. Summarize major trends in the evolution of land plants and provide evidence from your laboratory investigations. Introduction Among the seven major lineages of the domain Eukaryota is the lineage Plantae. Plantae consists of the algae and the green plants. Green plants dominate terrestrial and freshwater habitats. Although green algae have traditionally been considered protists, it is logical to study green algae along with land plants for two reasons: (1) they are the closest living relatives to land plants and (2) the transition from aquatic to terrestrial life occurred when land plants evolved from green algae. Land plants were the first organisms that could thrive with their tissues completely exposed to the air instead of being partially or completely submerged in water. The most ancient groups in the lineage are dependent on wet habitats, while the more recently evolved groups can live and reproduce in dry or even desert conditions. The phylogenetic tree in Figure1 reflects a recent phylogeny that has emerged from laboratories around the world. Figure 1. A phylogeny of Plantae 1
In order for the transition to land to be successful four key adaptive steps occurred (figure 2): 1) Land plants had to prevent water loss. This was accomplished by the formation of a waxy cuticle that covers the aboveground parts of plants. To allow for gas exchange stoma evolved. Stomata consist of an opening surrounded by specialized guard cells. The ability to open and close the stomata provides the plant with a way to regulate gas exchange (figure 3). 2) Land plants had to protect themselves from UV light. This was accomplished by forming UV-absorbing compounds called flavonoids. 3) Land plants had to move water from tissue in direct contact with water to areas without access to water. This was accomplished by means of specialized cells composing vascular tissue. 4) Land plants had to protect their gametes from desiccation. This was accomplished through: Figure 2: A series of evolutionary Innovations Allowed Plants to Adapt to Life on Land. a. The formation of spores, highly protected structures that contain the gametes. b. The development of the gametangium, a multicellular reproduction organ that produces and protects gametes. A gametangium that produces sperm is called an antheridium and a gametangium producing eggs is called an archegonium. Land plants reproduce undergoing a process called alternation of generations. Individuals exist as a multicellular haploid organism (a gametophyte) or a multicellular diploid organism (a sporophyte). As their names imply, gametophytes produce gametes and sporophytes produce spores (figure 4). 2
stoma Figure 3. Structure of a leaf Figure 4. Alternation of Generations in Land Plants Based on morphology land plants are traditionally clustered into three broad categories: Nonvascular plants which lack vascular tissue. Seedless vascular plants which have vascular tissue but do not make seeds. Instead, they make spores that are carried by wind to a new habitat. Seed plants which have vascular tissue and produce seeds. During this lab we will investigate the microanatomy of nonvascular and vascular plants (including both seedless and seed plants). In the following 2 labs, you will investigate the macro-anatomy of these organisms in the field. You must bring this lab with you as a reference for the following 2 labs. A. NONVASCULAR PLANTS (BRYOPHYTES) The nonvascular plants are composed of three groups that share some key characteristics: mosses (Bryophyta), liverworts (Hepaticophyta) and hornworts (Anthocerophyta). These are small plants generally lacking a true vascular tissue (xylem and phloem). In fact, they do not have true roots, stems or leaves. Because of this, we refer to the plant body as a thallus. Nonvascular plants have a cuticle but generally lack stomata. 3
Because they are nonvascular, these plants are restricted to moist habitats for their reproductive cycle and have never attained the size of most land plants. The gametophytes remain close to the ground, enabling the motile sperm to swim from the antheridium (male reproductive structure) to the archegonium (female reproductive structure) in order to fertilize the egg. Unlike other land plants, the nonvascular plants are characterized by a dominant gametophyte and a small, short-lived sporophyte that is dependent on the gametophyte (figures 5 and 6). Since the gametophyte produces sperm (male gametophyte) or egg (female gametophyte), self-fertilization does not occur. Asexual reproduction can occur by fragmentation of the gametophyte. Figure 5: Mosses Life Cycle Figure 6: Liverwort Life Cycle Procedure for the Bryophyta: 1. Refer to page 45 and 46 in your photo atlas. 2. Obtain a compound microscope, prepared slides of Mnium antheridia, archegonia and capsule (sporangium). 3. Draw and label the following terms in your notebook: antheridial head, antheridia, archegonial head, archegonia, sporangium, and spores. 4. Review the life cycle of mosses. 5. Fill in the Bryophyta information on the summary table for plants and include this table in your notebook. (you can do this as homework) 4
Procedure for the Hepaticophyta: 1. Refer to pages 41 through 43 in your photo atlas. 2. Obtain a compound microscope, prepared slides of Marchantia male gametophyte and female gametophyte. 3. Draw and label the following terms in your notebook: thallus, rhizoid, antheridiophore, antheridia, archegoniophore, archegonia, sporophyte 4. Review the life cycle of liverworts. 5. Fill in the Hepaticophyta information on the summary table for plants and include this table in your notebook. (you can do this as homework) B. SEEDLESS VASCULAR PLANTS Seedless vascular plants consist of the Lycophyta (club mosses), Psilotophyta (whisk ferns), Pteridophyta (ferns) and Equisetophyta (horsetails). Seedless vascular plants are analogous to the first terrestrial vertebrate animals, amphibians, in their dependence on water for external fertilization and development of the unprotected, free-living embryo. Because of that, they are well suited for swampy areas. The first green plants possessed a cellulose-based cell wall that also contained lignin. Lignin provided firm structural support and the condition for evolution of tracheids. Tracheids are long, thin, tapered vascular cells that have a lignin containing secondary cell wall in addition to the cellulose-based primary cell wall. The cell wall contains pits that allow for water and nutrients transport. Today, all vascular plants contain tracheids. The life cycle is a variation of alternation of generations in which the sporophyte is the dominant plant while the gametophyte is much reduced. The sporophyte is still dependent on the gametophyte for early development. Eventually, the gametophyte is Figure 7. Fern Life Cycle Figure 7. Fern Life Cycle 5
totally consumed by the sporophyte (figure 7). These plants generally have well-developed roots, stems, and leaves. The specialized leaf that bears the sporangia is called a sporophyll. Procedure the Pteridophyta: 1. Refer to pages 51-53 in your photo atlas. 2. Obtain a compound microscope and prepared slides of fern gametophyte and sori. 3. Draw and label the following terms in your notebook: gametophyte, archegonia, antheridia, rhizoids, sorus, pinna, sporangium 4. Review the life cycle of ferns. 5. Fill in the Pteridophyta information on the summary table for plants and include this table in your notebook. (you can do this as homework) Procedure the Lycophyta: 1. Refer to page 48 in your photo atlas 2. Obtain a compound microscope and prepared slides of Lycopodium strobilus. 3. Draw and label the following terms in your notebook: sporophyll, sporangium, spores 4. Based on what you know about the life cycle of plants in general and the life cycle of seedless vascular plants specifically, draw a picture of a proposed life cycle of club mosses in your notebook. (you can do this as homework) 5. Complete table 1 (Summary of structures and functions involved with the life cycle of plants) and put a copy in your notebook. (you can do this as homework) 6. Fill in the Lycophyta information on the summary table for plants and include this table in your notebook. (you can do this as homework) C. SEED PLANTS: GYMNOSPERMS AND ANGIOSPERMS Seed-producing plants are probably the most familiar plants to most people because, unlike mosses, liverworts, horsetails, and most other seedless plants, which are overlooked because of 6
their size or inconspicuous appearance, most seed plants are large or showy. Gymnosperms include pines, firs, yew, redwood, and many other large trees. Angiosperms include plants whose flowers are showy as well as many plants with reduced flowers, such as the oaks, grasses, and palms. The seed plants are the monophyletic group consisting of the gymnosperms and angiosperms. The group is defined by two key synapomorphies: the production of pollen and seeds. In the life cycle of these plants (Figures 8 and 9), the gametophyte have been greatly reduced. The male gametophyte is a multinucleated pollen grain. The female gametophyte is small and retained within the sporangium in the ovule of the sporophyte generation. Figure 8. Life Cycle of Pinophyta Figure 9. Angiosperm life cycle 7
The pollen grain is desiccation resistant and adapted for pollination, removing the necessity for fertilization in a watery medium. The pollen grain grows a pollen tube to convey the sperm nucleus to an egg cell. The resulting zygote develops into an embryonic sporophyte which develops within the gametophyte tissue that is itself protected by the sporophyte generation. The seed includes three primary regions: the embryo, nutritive tissue, and seed coat (Figure 10). The embryo is the young sporophyte plant. This is what will grow into the new tree, shrub, vine, etc. The embryo is surrounded by nutritive tissue which will feed it during its early growth; until it can establish its own root system and leaves to support itself. Nutrients in the tissue are absorbed into Figure 10. Anatomy of a bean seed the developing embryo by specially modified leaves called cotyledons. In some plants, the cotyledons may absorb all the nutrients before the seed is even dispersed, storing the food inside them. Around the whole seed is a layer called the seed coat. This layer may be thick or thin, depending on the species, but it often contains light-sensitive chemicals. When the environment has appropriate light and water the seed coat may trigger germination of the seed. The seed develops from structures called ovules. The ovule is an immature seed, which does not yet contain a viable embryo. It is only after the egg cell inside the ovule is fertilized by sperm that the ovule is called a seed. The ovule is surrounded by integument tissues that produce the seed coat. The resulting seed is not only protected from environmental extremes but also is packed with nutritive materials and can be dispersed away from the parent plant. 1. GYMNOSPERMS 150 million years ago Earth became warmer and drier and the swamp forests declined. The landscape became dominated by a great variety of seed-bearing plants called gymnosperms ( naked seed plants ). Gymnosperms were the first to gain reproductive independence from water through the development of internal fertilization via pollen. Unlike the angiosperms, gymnosperms do not produce flowers that attract pollinators. They rely on wind pollination. Gymnosperms do not produce flowers. Gymnosperms consist of Ginkgophyta (Ginkgoes), Cycadophyta (Cycads), Cupressophyta (Redwood, Junipers, and Yews), Pinophyta (Pines, 8
Spruces, Firs) and Gnetophyta (Gnetophytes). In this exercise we will closely observe examples of the Pinophyta. Procedure for the Pinophyta: 1. Refer to pages 55-57 in your photo atlas. 2. Observe a prepared slide of Pinus female gametophyte using a compound microscope 3. Draw and label the following terms in your notebook: ovuliferous scale, ovule, megasporangium, megaspore, micropyle, integument. 4. Observe a prepared slide of a Pinus male gametophyte using a compound microscope. 5. Draw and label the following terms in your notebook: microsporophyll, microsporangium, pollen grains. 6. Review the pine life cycle. 7. Fill in the Gymnosperm information on the summary table for plants and include this table in your notebook. (you can do this as homework) 2. ANGIOSPERMS Angiosperms occupy well over 90% of the vegetated surface of Earth and contribute virtually 100% of our agricultural food plants. The evolution of the flower resulted in enormous advances in the efficient transfer and reception of pollen. Whereas gymnosperms are all wind-pollinated, producing enormous amounts of pollen that reach appropriate species by chance, the process of the flower pollination is mediated by specific agents insects, birds, and bats in addition to water and wind. Angiosperm reproduction (Figure 9) follows the trend for reduction in the size of the gametophyte. Flowers contain two key reproductive structures: the stamen and the carpel (pistil). The stamen includes an anther, where microsporangia develop. Meiosis occurs inside the microsporangia, forming microspores. Microspores divide by mitosis to form pollen grains. The carpel contains a protective structure called an ovary where the ovules are found. As in gymnosperms, ovules contain the megasporangia. A cell inside the megasporangium divides by meiosis to form the megaspore, which then divides by mitoses to form the female gametophyte (embryo sac). When a pollen grain lands on the stigma, it germinates, grows a pollen tube to the embryo sac and release 2 sperm. A double fertilization follows producing a zygote that will develop into the embryo and the endosperm nucleus which will develop into nutritive endosperm tissue. The ovule and its contents will develop into the seed. The ovary or other tissue surrounding the ovule will develop into another unique feature of angiosperms: fruit. 9
Procedure for the Magnoliophyta: 1. Refer to pages 59-62 in your photo atlas. 2. Use a compound microscope to observe a slide of a cross section through a lily anther. 3. Draw and label the following terms in your notebook: pollen grain, microsporangium, anther 4. Use a compound microscope to observe a slide of a cross section through a lily ovary. 5. Draw and label the following terms in your notebook: ovary, carpel, ovule, embryo sac 6. Fill in the Angiosperm information on the summary table for plants and include this table in your notebook. (you can do this as homework) Writing Assignment (see syllabus for due date): Complete this assignment in Google Docs. Describe the alternation of generations life cycle. Using that description, compare and contrast the life cycle of the nonvascular, seedless vascular and seed vascular plants. Be sure to use appropriate terminology and to use the examples you observed in class as examples throughout. 10
Complete the table and place it in your notebook. Table 1 Summary of structures and functions involved with the life cycle of plants Structure Function Haploid or diploid? Produced by mitosis or meiosis? Antheridium Examples of organisms that have this Archegonium Spore Gamete Sporangium Gametophyte Sporophyte 11