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Seed Plants – the Gymnosperms

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Seed Plants – the Gymnosperms Seed Plants – The Gymnosperms The seed plants evolved from fern-like non-seed ancestors. Several changes occurred to make this novelty possible. First, two types of spores, large megaspores and small microspores, appeared. This change is illustrated in Selaginella and some aquatic ferns. In both, the gametophytes are reduced in size, developing within the spore walls. The male gametophyte developing within the microspore wall became the pollen. The female gametophyte developed within the spore wall, and the spore was retained within the megasporangium. For fertilization to occur pollen was carried by wind to the megasporangium, the grains germinated as a tube and the male gametes moved to the egg cell. After fertilization, the embryo developed inside of the megasporangium, now called the ovule. The fertilized ovule then became a seed, with an embryo inside. This new life cycle had several advantages. First, the protected pollen grain was blown by wind to the site of germination, reducing the requirement for water and permitting these plants to sexually reproduce in much drier conditions. Secondly, the development of the seed provided a means of protecting the embryo against dessication and the storing of the embryo in dormancy until ideal conditions would trigger germination. Finally, modifications of the seed promoted dispersal by wind or animals. Plants with this life cycle are called gymnosperms because the ovule/seed is produced on a leaf-like structure and is unprotected, or naked. Gymno- = naked and -sperm = seed. We will see details of this modification and this new life cycle in two plants native to south Florida. There are four groups of living gymnosperms: the conifers, the cycads, Gingko, and Gnetum and its relatives. Ginkgo is represented by a single species: Gingko biloba. It does not grow in south Florida, but it is sold in health food stores as a tonic to improve cerebral circulation and memory in aging. The Conifers These trees are enormously important, as the source of softwood timber used in wood-based construction and the source of fiber in producing most of the world’s paper. They are dominant in certain forests at temperate and higher latitudes, as in northern latitudes and the northwest of the United States. Most of the coastal areas of south Florida, on the limestone ridge, were covered with a pine forest, called the pine rocklands. Little of this forest remains, having been replaced by agriculture and then commercial development. Conifers vary in their leaves and particularly in their female cones; some being reduced to look more superficially like a berry. They all share the same basic life cycle, that of a pine tree given as an example below. Three conifers native to south Florida are described in detail, and several exotic species are mentioned briefly. Dade County Pine— Pinus elliottii var. densa. This extreme southern variety of the slash pine grows in a few remaining stands on the rock ridge of south Florida. Its wood is very resinous, extremely hard when dry, and very resistant of termite attack. It was used in the construction of homes and boats well into the last century. Dade County pine grows in the Ecosystem Preserve and the parking lot just to the east. A couple of trees also grow in the small conifer collection NE of the north parking lot. Dade County Pine, continued The fragile male cones and small purplish female cones develop in January-February. In a forest the air is yellow with the wind- carried pollen. After fertilization the female cone scales swell and close. Then the cones develop for the entire year, and open to release the winged seeds prior to the rainy season (May) the next year. The trees always have female cones in some stage of development, but the male cones soon fall off the tree after they shed their pollen. The Dade County Pine is a member of the pine family (Pinaceae) along with all pines and the firs, such as the Frazer’s fir from the Appalachian Mountains on sale before Christmas. Phylum Gingophyta: The Ginkiphyta Procedure – examine pine twigs and consist of one species, Ginkgo biloba leaves (Maindenhari plant), a large dioecious tree that 1. Examine pine twigs having leaves does not bear cones. Ginkgo are hardy plants (needles) and a terminal bud. Notice in urban environments and tolerate insects, the number of needles; the length and fungi, and pollutants. Males are usually number of leaves distinguishes many planted because females produce fleshy, of the species Pinus. smelly, and messy fruit that resembles cherries. Ginkgo has not been found in the wild and would Questions probably be extinct but for its cultivation in 1. How are the needles arranged? ancient Chinese and Japanese gardens. 2. How many leaves are in a bundle? 3. How are pine leaves different from Phylum Gnetophyta: This gnetophytes (71 those of deciduous plants? species in 3 genera) include some of the most 4. Why are pines called evergreens? distinctive (if not bizarre) of all seed plants. They 5. How do the structural features of pine have many similarities with angiosperms, such as leaves adapt the tree for life in cold, flowerlike compound strobili, vessels in the dry environment? secondary xylem, loss of archegonia, and double fertilization. Pine life cycle In seed plants, the gametophyte gneration is greatly reduced. A germinating pollen grain is the mature microgametophyte (male cones) of a pine. Pine microsporangia are borne in pairs on the scales of the delicate pollen-bearing cones. Megagametophytes (female cones), in contrast, develop within the ovule. The familiar seed-bearing cones of pines are much heavier than the pollen-bearing cones. Tow ovules, and ultimately two seeds, are borne on the upper surface of each scale of a cone. In the spring, when the seed-bearing cones are small and young, their scales are slightly separated. Drops of sticky fluid, to which the airborne pollen grains adhere, form between these scales. These pollen grains geminate, and slender polled tubes grow towards the egg. When a pollen tube grows to the vicinity of the megagametophyte, sperm are released, fertilizing the egg and producing a zygote there. The development of the zygote into an embryo occurs within t the ovule, which mature into a seed. Eventually, the seed falls from the cone and germinates, the embryo resuming growth and becoming a new pine tree. Pine Life Cycle diagram Procedures and questions about conifer reproduction Procedure – examine pine cones Procedure – examine a pine seed 1. Examine young living or 1. Examine a prepared slide of a preserved ovulate cones. These pine seed. Locate the embryo, cones will develop and enlarge seed coat, and food supply. considerable before they are Seeds are released when the mature. cone dries and the scales 2. Examine a prepared slide of a separate. This usually occurs young ovulate cone ready for 13-15 months after pollination. pollination. Each ovuliferous 2. Examine some mature pine scale of the female cone bears seeds, noting the winglike two megasproangia, each of extensions of the seed coat. which produces a diploid megaspore mother cell. Each Questions megaspore mother cell undergoes 1. On which surface of the scale meiosis to produce a megaspore are the seeds located? that develops into a megagametophyte. A 2. How large in a staminate cone megagametophyte and its compared to a newly pollinated surrounding tissues constitute an ovulate cone? A mature ovulate ovule and contains at least one cone? archegonium with an egg cell. 3. Examine a prepared slide of an 3. What is the make gametophyte? ovulate cone that has been sectioned through an ovule. An 4. What is the female ovule develops into a seed. gametophyte? 4. Examine a mature ovulate cone and notice its spirally arranged 5. What is the function of the ovuliferous scales. These scales winglike extensions of a pine are analogous to seed? microsporophylls of staminate cones, but ovuliferous scales are 6. How are other gymnosperms modified branches rather than similar to pines? modified leaves. At the base of each scale you’ll find two naked 7. How are they different? seeds. Notice that the seeds are exposed to the environment and supported (but not covered) by an ovuliferous scale. Bald Cypress—Taxodium distichum. This is a conifer in another family, the Taxodiaceae. Its female cones are much smaller and the individual scales are rounded to produce a round cone. It is a swamp tree, growing in stands throughout the southeast. It was once common in a strip of swamp forest down the southeast coast of Florida, and more common along the west coast, as in the Big Cypress National Preserve. Bald Cypress trees were planted on pond margins at FIU soon after it opened. We now have some bald cypress “domelets”, with cypress knees (the pneumatophores that assist in oxygen uptake to the roots) and Everglades wading birds sitting on branches. The bald cypress is unusual among conifers in that it loses its short needle foliage during the winter months. Few of the original cypress domes remain; the majority of these swamp forests were logged before and during the Second World War, partly for the construction of PT boats. Phylum Cycadophyta: the cycads These gymnosperms are no longer widely distributed, only found in mostly dry tropical regions, but they were once dominant plants. These were the primary food of the large herbivorous dinosaurs. Most cycads are extremely tough, thorny, and often very toxic. Fairchild Tropical Garden, and the adjacent Montgomery Botanical Center, have the largest cycad collection in the world. Cycads have life cycles similar to the conifers, but certain details (as the flagellate male gametes) are different. Cycad plants are female (producing long-lived female cones) or male (producing ephemeral male cones).
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