Lab 4 - Gymnosperms Spring 2024 PDF

Introduction to the Seed Plants— Gymnosperms Introduction The gymnosperms are a large group of related plants, consisting of four phyla of seed plants: Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta. Gymnosperms have vascular tissue and seeds. A seed is a reproductive structure that contain...

Introduction to the Seed Plants— Gymnosperms Introduction The gymnosperms are a large group of related plants, consisting of four phyla of seed plants: Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta. Gymnosperms have vascular tissue and seeds. A seed is a reproductive structure that contains a young sporophyte (in embryo stage). Another major evolutionary advance that differentiates the gymnosperms from the seedless vascular plants is the development of pollen, which represents the male gametophytes in the plant life cycle. The word “gymnosperms” has its origins in the Greek language and means “naked seed.” Here, seeds are exposed on the megastrobilus. This “naked seed” feature is one of the main things that distinguishes gymnosperms from flowering plants, the angiosperms. There are several other structures and new terms that you will see this lab that you didn’t see in the seedless plants lab. They are: “Micro-” – is a prefix for male structures, such as microsporangium and microspore. “Mega-” – is a prefix for female structures, such as megaspore and megagametophyte. Cones – Ovules – specialized strobili in which the sporangia, spores, and gametophytes that develop from the spores, are produced. Female cones are also referred to as seed or ovulate cones, while male cones are also called pollen or staminate cones. megasporangia that develop within the female cone and contain female gametophytes. The female gametophyte completes its life entirely enclosed within the ovule, which eventually matures into the seed. 1 Gymnosperms are heterosporous, producing both megaspores and microspores. The two different spores develop into two different gametophytes—megaspores develop into megagametophytes inside seed (female; ovulate) cones, while microspores grow into microgametophytes inside pollen (male; staminate) cones. When both spore types are borne on the same plant, a sporophyte is said to be monoecious (“one house”). Members of the Coniferophyta are monoecious. When the spore types are borne on separate plants, a sporophyte is dioecious (“two houses”). Members of the Cycadophyta, Ginkgophyta, and most Gnetophyta, are dioecious. What you should be able to do after this lab 1. Place representative plants in their appropriate phyla. 2. Recognize and differentiate seed and pollen cones of Coniferophyta and of Pinus sp., and relate these to the alternation of generations life cycle. 3. Describe the life cycle of Pinus sp., and be familiar with the structures in which spores, gametophytes, gametes, and embryos develop. 4. Describe a seed of Pinus sp. Part I. The leaves of gymnosperms The sporophyte generation is the dominant of the two in the gymnosperms. They have all the major plant organs: roots, stems, and leaves. Some of the leaves are needle-like, while some are scale-like and others fanlike. EXAMINE the sporophytes (or their representative leaves) of the different phyla on display and SKETCH their leaves. Cycadophyta 2 Gnetophyta Ginkgophyta Coniferophyta Leaves of Pinus sp. The leaves of Pinus sp. (Phylum Coniferophyta) are borne at the end of reduced branches only a few millimeters long. The needles of pines are in bundles of up to 7, but most commonly 2-5, depending on the species. Most pines retain their needles for 3-4 years before they are replaced. EXAMINE the pine branches and needles on display. QUESTION #1: Do all pines have the same number of needles in each bundle? QUESTION #2: How many needles does the Longleaf pine have in each bundle? EXAMINE the cross-section of a pine needle on display under the microscope. SKETCH the pine needle cross section below, then IDENTIFY and LABEL the epidermis, mesophyll, xylem, phloem, and a resin duct (see Fig. 8.48 and 8.49 in atlas). 3 QUESTION #3: How many needles are in view in this section? QUESTION #4: What is resin and why do conifers produce it? Part II. The Cones of Gymnosperms The cones are made of specialized leaves, called sporophylls, that are modified to bear the sporangia and the spores. Some cones are fleshy, like a fruit, and others are woody. EXAMINE the cones and/or photographs of cones on display. Can you recognize both fleshy and woody cones? The blue cones of the juniper on display look like fruits. QUESTION #5: Fleshy cones are NOT fruits. What structure is needed to produce a fruit? Comparison of pollen and seed cones of Pinus sp. (Atlas p. 116-118) Woody cones are made of small sporophylls and are green in their early development. By the time the pollen and seeds are released from either cone they have dried and turned brown. EXAMINE the pine cones on display. Be sure to be able to distinguish pollen cones from seed cones. QUESTION #6: Which of the two are commonly seen on the ground or on the branches? COMPARE the female cones of different ages. QUESTION #7: How much larger is a mature seed cone (one that has released its seeds) than a first-year cone (produced that spring)? 4 Pollen cones of Pinus sp. Inside pollen cones are microsporangia, which are borne on microsporophylls. EXAMINE the preserved pollen cones of Pinus sp. on display under the dissecting microscope. Notice that each cone consists of many microsporophylls. The microsporangia may appear as swollen areas on the bottom of the microsporophyll. LOOK for pollen that was released from a broken microsporangium. QUESTION #8: How many microsporangia are on each microsporophyll? In the microsporangia, microspores would have been produced. These are not visible—by the time the slide was made, the microspores had developed into the microgametophytes (pollen grains) by mitosis. In pines (as well as other gymnosperms) the gametophyte matures inside the sporangium. QUESTION #9: How is this different from where a gametophyte matures (i.e., grows) in the seedless plants? EXAMINE the longitudinal section of a pollen cone under the compound microscope. Because the cone was cut length-wise instead of cross-wise, you see only one of the two microsporangia that are attached to each microsporophyll. You will also see pollen grains. SKETCH a pollen cone; IDENTIFY and LABEL at least one microsporophyll, microsporangium, and pollen grain. (See p.118 in atlas) 5 QUESTION #10: What type of cell division gave rise to microspores? To the microgametophytes? After pollen grains are released from the microsporangia, a pollen grain that is to participate in the continuation in the life cycle will have to land on the appropriate spot on the female pine cone. When it does this, it will produce a pollen tube and two sperm cells. EXAMINE the pine pollen on display. QUESTION #11: What color is it? EXAMINE the same pine pollen on display under a compound microscope. Note the distinctive shape of each grain (it looks like a Mickey Mouse hat). Each immature pollen grain consists of two bladderlike wings and four cells: two prothallial cells, one tube cell, and one generative cell. SKETCH a pollen grain; IDENTIFY and LABEL the tube cell, generative cell, a prothallial cell, and air sacs of the pollen grain. QUESTION #12: What role do the air sacs of the pollen grain play in the life-cycle of the pine? After pollen lands on an ovule in a seed cone, the tube cell forms the pollen tube and the generative cell eventually divides (via mitosis) to produce 2 sperm cells. Refer to the illustration provided next to the microscope. 6 Seed Cones of Pinus sp. Inside seed cones are megasporangia, which are borne on megasporophylls (also called scales). On the scales of seed cones are ovules. Each ovule contains a single megaspore mother cell (2n) that undergoes meiosis to form megaspores (n). Only one of the megaspores will develop into the megagametophyte. Archegonia with eggs develop within the megagametophyte, still contained in the ovule. QUESTION #13: What type of cell division produced the megagametophyte? EXAMINE the preserved immature seed cones of Pinus sp. that are on display under the dissecting microscope. Find the megasporophylls on either side of the central axis of the cone. Notice that the entire cone is