Plant Biology - Seed Plants - Quick Guides | Biology

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Study GuidePlant Biology–Seed Plants1. The Gymnosperm PhylaGymnosperms are seed-producing plants thatdo not form flowers or fruits. The wordgymnospermmeans“naked seed.”This refers to the fact that theirovules and seeds are exposed, usually oncone scales, rather than being enclosed inside a fruit.Unlike angiosperms (flowering plants), which all belong toone single evolutionary lineage,gymnosperms are divided intofour different phyla, each representing aseparate evolutionary line.1.1Phylum Coniferophyta (The Conifers)Conifers are thelargest and most familiar group of gymnosperms. They are mostlywoody,evergreen treeswithneedle-shaped or flattened leaves.Habitat and DistributionConifers are especially well adapted tocooler and drier environments. Today, they dominate manyforests of theNorthern Hemisphere, but they are also found in parts of theSouthern Hemisphere.Common examples include:•Pines•Firs•Spruces•Hemlocks•Yews•Junipers•Araucaria (Southern Hemisphere)Some conifers are record holders:•Sequoiasare among thetallest trees on Earth•Bristlecone pinesare among theoldest living plants

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Study Guide1.2Pines (GenusPinus)Pines are one of the most studied conifers, withmore than 90 species.Leaves (Needles)•In young seedlings, needles growsinglyalong the stem•In mature trees, needles grow inbundles called fascicles•Each fascicle is ashort branch with limited growth, which is important evolutionarily1.3Adaptations of Pine Needles (Xeric Adaptations)Pine needles are adapted toxeric conditions, where water is scarce or frozen.Key features include:•Thick cuticleto reduce water loss•Epidermis and hypodermiswith thick-walled cells•Sunken stomata(helps reduce transpiration)•Resin canalsfor protection•Vascular bundles surrounded by transfer tissueNeedles are not shed all at once. Instead:•Each needle lives2–4 years (or more)•Because some needles remain, the tree staysevergreen1.4Wood Structure in ConifersConifer wood is commonly calledsoftwood.Important features:•Made mainly oftracheids(no vessels)•Tracheids havecircular bordered pits•Very little parenchyma tissue•Vascular cambium is bifacial, producing:oSecondary xylem (wood) inwardoSecondary phloem outward

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Study Guide1.5Life Cycle of Pine (Very Important Exam Topic)1.6A Two-Year Life CycleThe life cycle of pine takesabout two yearsto complete.Year 1: Pollination (Spring)•Pollination occurs inspring•Wind carries pollen to theovulate cone•The pollen grain germinates and forms apollen tube•At this stage:oNo egg cells are present yet

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Study GuideoNo archegonia existoThe megagametophyte has not formedThis lack of synchronization is not a problem because:•The pollen tube takesover one yearto grow through the nucellus•During this time:oThe megaspore mother cell dividesoThe megagametophyte developsoArchegonia with eggs are formedoThe generative cell divides to formtwo sperm cellsYear 2: Fertilization and Seed FormationIn thespring of the following year:•Fertilization occurs•Eggs in2–3 archegoniamay be fertilized (polyembryony)•Usually,only one embryo survives1.7Structure of the Pine SeedA pine seed contains tissues fromthree generations:1.Seed coat–parent sporophyte (2n)2.Embryo–new sporophyte (2n)3.Megagametophyte–nutritive tissue (1n)Bylate summer, the seed:•Matures•Is released from the cone•Germinates if conditions are favorable•Grows into a new pine tree, continuing the cycle

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Study Guide1.8Other Conifers (Beyond Pines)Not all conifers look or behave like pines.Deciduous ConifersSome conifersshed their leaves, such as:•Larch (Larix)•Bald cypress (Taxodium)•Dawn redwood (Metasequoia)Yews•Haveflattened leaves•Do not form typical cones•Produce afleshy red structure called an aril1.9Global Distribution and Evolutionary History•Conifers are found inboth hemispheres•Many grow in areas unsuitable for angiosperm trees•Some genera exist only insmall, isolated populationsExamples:•Dawn redwood•Wollemia pineInterestingly:•Botanists discovered some of these speciesonly recently•17 conifer speciesgrow only along theCalifornia coast

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Study Guide1.10Evolutionary DeclineConifers reached theirpeak during the Mesozoic Era, when they dominated Earth’s forests.Since then, their diversity has slowly declined, a trend that continues today.Summary•Gymnosperms = naked seeds•Conifers are the most important gymnosperms•Pines show strong xeric adaptations•Pine life cycle takestwo years•Seeds contain tissues fromthree generations•Conifers once dominated Earth but are now declining2. Other Living Gymnosperm Phyla

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Study GuideLiving gymnosperms today are asmall but diverse group of plants. Most of them survive inspecialhabitatswhere flowering plants (angiosperms) struggle to grow. These places may bevery hot, verydry, or very cold.Many gymnosperms showancient featuresin their structure and life cycles. Some of these featureslook like early experiments in plant reproduction—ideas that were later refined and successfully usedby flowering plants. In this way, gymnosperms give us awindow into plant evolution.2.1Phylum CycadophytaCycads are gymnosperms thatlook like palm trees, but instead of flowers, they producecones.They are found mainly intropical and subtropical regionsaround the world. Interestingly,twocycad species grow naturally in Florida.Some cycads grow15 meters or morein height, while others haveshort trunksand form arosetteshape, with leaves spreading out from the center.Theovulate (female) conesof cycads are very large—some can weighover 30 kilograms. Thesecones growuprightand develop among the leafy structures calledmegasporophylls.Male and female cones are produced ondifferent plants, meaning cycads aredioecious.Even though cycads do not look like pine trees, theirlife cycle is very similar to that of pines. Oneimportant difference is their sperm. The sperm are delivered to the archegonia through a pollen tube,but they aremultiflagellate, meaning they havehundreds of flagellathat help them move.

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