Biology 3B Lab Vascular Seed Plants Page 1 of 7 Biology 3B Laboratory Vascular Seed Plants – Gymnosperm & Angiosperm Objectives • To understand the general systematic relationships of gymnosperms and angiosperms • To describe the general features of gymnosperms and angiosperms • To understand the life cycles of gymnosperms and angiosperms • To compare the significant features of life cycles for various gymnosperms and angiosperms and state the particular evolutionary importance • To be able to differentiate between representative organisms in each group: pine, cycad, ginkgo, Ephedra and Welwitschia. Introduction In the last laboratory you studied nonvascular and seedless vascular plants. If you recall, there are four natural extant groupings for plants: the Bryophytes, Pteridophytes, Gymnosperms and Angiosperms. In this laboratory, you will study seed plants: the Gymnosperms and the Angiosperms. Both seed plant groups are thought to have risen from a group of extinct phylum Progymnospermophyta. These progymnosperms had intermediate characteristics between ancestral ferns (an extinct seedless vascular group) and seed plants. Progymnosperms reproduce via spores like seedless vascular plants. However, progymnosperms also possess secondary vascular structures (secondary xylem and phloem) that are characteristic of seed plants. Therefore, it is thought that this secondary vascularity evolved first in progymnosperms. Embryophytes Vascular plants Seed plants Bryophytes Pteridophytes Progymnosperms Gymnosperms Angiosperms Figure 1: A simplified summary showing the phylogenetic relationship between the major groups of Embryophytes, vascular plants and seed plants. Currently, taxonomists have seed plants grouped into one of five phyla. Four phyla are commonly grouped together as the gymnosperms (naked seeds – 720 living species). These four phyla all Zygote producing a multicellular embryo with early development in archegonium or embryo sac Vascularity Secondary xylem & phloem Seeds FlowersBiology 3B Lab Vascular Seed Plants Page 2 of 7 include living representatives: Cycandophyta (cycads), Ginkgophyta (ginkgo), Gnetophyta (gnetophytes) and Coniferophyta (conifers). The most successful plants now are angiosperms in the phylum Anthophyta (flowering plants – 235,000 living species). Both groups have adaptations which allowed members in these groups to survive in a drier environment, thus they are not restricted to damp habitats like the plants studied in the last lab. In addition, angiosperms have unique set of characteristics that distinguishes them from gymnosperms that have contributed to their success. Both gymnosperms and angiosperms exhibit alternation of generations like the “lower plants.” However, the gametophyte generation is reduced further than what you examined with the seedless vascular plants. The gametophyte generation of seed plants is dependent on the sporophyte generation to support them. Gymnosperms Gymnosperms have been around since the Carboniferous period (362 MYA). During this time, vascular seedless plants dominated the swamp forest landscape. As the environment became warmer and drier, gymnosperms began to diversify. One of the reasons for this diversification was the reduced (microscopic) gametophyte stage. Gymnosperms are heterosporous, producing two different types of spores. Male cones (microsporophyll) produce the male gametophyte (pollen or microspore) that is resistant to desiccation and often wind dispersed due to its microscopic size. As a result, this eliminated the need for water as the dispersal mechanism. Pollination occurs when the pollen arrives at a female cone (megasporophyll) that contains the female gametophyte (ovule or megaspore). A pollen tube develops so that the pollen grain can reach the microscopic ovule. At this point, fertilization occurs and the developing embryo is dependent on the mature sporophyte in becoming the seed. The evolution of seeds is one of the most significant events for the plant kingdom. A seed typically consists of an embryo, seed coat, and stored food. The seed coat and stored food is what enables seeds (small sporophyte) to remain dormant until conditions are more favorable for growth. Phylum Cycadophyta – Cycads The once widely diverse Cycadophyte has been reduced to ten genera and approximately 100 species. Cycads may look like miniature palms due to their unbranched leaves and are found in tropical and subtropical regions. However, cycads produced flagellated sperm in the conelike structure near the apex of the plant. Ovulate cones produce ovules on separate plants. Organisms that have separate male and female reproductive parts are referred to as dioecious (two house). • Examine demonstration examples of cycads and be able to recognize the representatives.Biology 3B Lab Vascular Seed Plants Page 3 of 7 Phylum Ginkgophyta – Gingko or maidenhair tree The only living representative in this phylum is the Gingko biloba. The last wild stands of gingkos were most-likely in China. Today, gingkos can be found worldwide as it has been introduced as a landscape tree due its resilience to air pollution. Present day gingkos have change very little over the past 150 million years. • Examine the leaves from G. biloba. Notice the bi-lobed leaves with branching venation. Between the two doors outside SM 202 is a living gingko. Unlike other gymnosperms, gingkos are deciduous (loses its leaves). Phylum Coniferophyta The phylum Coniferophyta has the most numerous (550 species), widespread and ecologically important gymnosperms. Coniferophyta also has the oldest living individual organism, a 5,000 year old bristlecone pine and the tallest vascular plant, the redwoods with heights up to 117 meters. This group includes pine trees, junipers, cedars, firs, spruces, etc.; which are of economic value. As mentioned before, gymnosperms are heterosporous. Male cones (microsporophyll) have specialized tissues (microsporangium) that produce pollen via meiosis resulting in four microspores. Each of these microspores develops into a winged pollen grain which consists of two prothallial cells, a generative cell and a tube cell. The smaller male cones are typically located on the lower branches of the tree where