Lab 9 – Gymnosperms andAngiosperms
When mosses and liverworts first evolved, they dominated theterrestrialenvironment. But they were soon challenged by the more advancedtracheophytes.The ferns and “fern allies” formed the great planetary forests of thelatePaleozoic. By the end of the Paleozoic, a new group of plants waschallengingthe 150 million-year domination of the ferns and fern allies. The seedplants protected the embryonic sporophyte from drying up byencasingit in a tough waterproof seed coat.
You are watching: Which of the following statements is true of the pine life cycle?
The evolution of the seed is as profound a step as theevolutionof the shelled egg in reptiles. Just as the evolution of the amnioticeggenabled reptiles to become the first truly terrestrial vertebrates, tobreak that final link with their aquatic heritage, so did the evolutionof the seed allow plants to escape the limitation of growing in verymoistenvironments. These gymnosperms soon became the dominantplants.The Mesozoic is sometimes called the Age of Cycads.
But their success was short-lived. During the mid to late Mesozoic,the first flowering plants or angiosperms appeared. Theyrapidlydominated the more primitive gymnosperms, and are the dominant plantsonEarth today. These waves of competition are typical of the history oflife.The survivors are relegated to scattered populations in restrictedhabitats,where they live in the shadows of their successful competitors. Amongthegymnosperms, only the conifers are major competitors with floweringplants.Having evolved in a dryer, cooler climate, conifers are better adaptedto dry or cool habitats, and dominate forests in northern latitudes, athigh elevations, and on sandy soils.
Today we will examine both gymnosperms and angiosperms, and comparetheir complex life cycles. The trend toward a dominant sporophyte stageis now complete. The gametophytes of seed plants are microscopic. Thefemalegametophyte consists of a handful of cells buried in the tissues of thesporophyte. The male gametophyte, the pollen grain, has a brieffree-living stage while it is carried from plant to plant by wind,water,or animals. No longer relying on flagellated sperm, and with theirdevelopingembryos protected from desiccation, seed plants break the last linkwiththeir aquatic ancestors.
Introduction to Gymnosperms
The first seed plants evolved relatively early on, in the lateDevonian.By the end of the Paleozoic they were competitive enough to replace theclub mosses, horsetails, and whisk ferns, and become the dominantvegetationof the Mesozoic, the era of the dinosaurs. By the end of the Mesozoic,they too would be swept aside by the newly evolved angiosperms, thefloweringplants. There are only 720 living species of gymnosperms, a paleremnantof a once diverse and dominant race.
Living gymnosperms are a diverse group of plants, most of which beartheir sporangia in large, prominent strobili or cones.Thesestrobili are similar to those of lycopsids and horsetails. Strobiliconsistof a shortened stem with several modified leaves (sporophylls)thatbearsporangia. Like all seed plants, gymnosperms are heterosporous.The sporangia that generate the male microspores andfemalemegasporesare usually borne on separate cones. Male cones (staminate cones)are typically much smaller than female cones (ovulate cones).Sporophyllsthat bear microsporangia are called microsporophylls.Sporophyllsthat bear macrosporangia are called macrosporophylls. The pinelifecycle is typical of gymnosperms, and is described in detail below.
Division Gnetophyta – Ephedra, Gnetum,Welwitschia
Division Cycadophyta – cycads (Cycas revoluta)
Division Ginkgophyta – Ginkgo biloba
Division Coniferophyta – conifers (Pinus)
Terms staminate cones ovulate cones sporophylls strobilus microsporophylls macrosporophylls heterosporous ovules seeds megasporangium megaspore mother cell megaspore microsporangium microspore mother cell microspore pollen grainsCharacteristics of Divisions
Division Cycadophyta – (~100 sp., 9 genera, fr. Gr.kyos=palm,phyton=plant) – cycads
Cycads have very thick leaves, that look like very tough versions offern fronds. These palm-like plants have unbranched stems, with aterminalcrown of leaves. These leaves are incredibly well defended with sharptipsand with complex secondary compounds, including potent neurotoxins andcarcinogenic compounds. They reached their peak during the Mesozoic,withspecies reaching from 6-60 feet. The Mesozoic is sometimes called theAgeof Cycads. A giant cycad today might reach 9-10 feet max.
They are unisexual or dioecious, having separate male and femaleplants.Dioecious means two houses, vs. monoecious = one house (bisexual, bothsexes in one). Only one genus of cycad (Zamia) is native toNorthAmerica. The Seminoles ate the starchy roots of Zamia pumila,foundin southern Florida. In India, Japan, and Sri Lanka, sago flour isoftenmade from cycad stems (it is also made from real palms, which areangiosperms).
Cycads are widely grown as ornamental landscape plants. Cycads alsoenrich the fertility of barren soil, because they are symbiotic withnitrogen-fixingcyanobacteria. Cycads are extremely slow growing, and can live 1,000yearsor more. They are wind pollinated, a strategy which requires immenseamountsof airborne pollen. A few may have been pollinated by beetles attractedto the edible pollen grains. This may be the humble beginnings of thecomplexanimal pollination developed by flowering plants. The pollen sacs andovulesare born on scalelike sporophylls in compact cones. Unlike pine cones,the cones of cycads are often very large in relation to the plant.
Division Ginkgophyta – one sp., Ginkgo biloba(maidenhairtree)
Ginkgo trees are commonly seen in cities today. They are attractiveshade trees, reaching 100 feet or more, with beautiful yellow foliageinthe Fall. They are very resistant to air pollution and insects. You cansee these trees right on campus (Richardson and the Gibson Hall“loop”).
That the sole remaining species did not join its brethren inextinctionwe owe to the ancient Chinese and Japanese, who cultivated it in theirtemple gardens for centuries. Their may no longer be a single livingwildtree. It is a popular tree for bonsai, because the leaves will readilyminiaturize, and the branches are easy to shape. The species name bilobacomes from the two distinct lobes of its fan-shaped leaves, verydifferentfrom the straplike or needle shaped leaves of other gymnosperms. Thecommonname maidenhair tree comes from the similarity of ginkgo leaves tofrondsof the maidenhair fern.
Ginkgos and cycads show a transitional stage between the primitivefernsand the more advanced conifers and flowering plants. They haveflagellatedsperm, but the male gametophyte grows a pollen tube, a long filamentthroughwhich the sperm can safely swim to the egg. The pollen grains of otherseed plants grow similar tubes. The megasporangia, which contains theeggs,form tiny female strobili on the tips of special branches on the femaletree. The microsporangia, which produce the pollen grains, are in malestrobili that hang down like little pine cones on the male tree.
The seed that forms on the female trees is covered with a thickfleshycoat which makes the seed look like a little fruit (which it istechnicallynot). They have an incredible odor when they ripen, which one otherwisestodgy botany text describes as “rotting dog vomit”. So be very carefulif you plant one of these wonderful trees and select a male tree!!Althoughin fairness to the female tree, its seed is prized in China as a sourceof medicinal drugs.
Division Gnetophyta – (70 sp. in 3 genera), Gnetum,Ephedra,Welwitschia
This odd little group of gymnosperms are mainly xerophytes, plantsthatare adapted to dry conditions. They share a close common ancestor withflowering plants. Each genera has some species that produce nectar, andattract insects. It was recently discovered that double fertilization,a trait we thought was unique to flowering plants, also occurs in Ephedra,one of the three surviving genera of gnetophytes. Ephedra,incidentallyis the natural source of the alkaloid ephedrin, used to treat hayfever,sinus headaches, and asthma. Its medicinal properties have been knownforat least 5,000 years!
Most gnetophytes are stem plants, like Ephedra, branchedphotosyntheticstems with no leaves. Gnetum has leaves like those of modernflowers.But the third genus, Welwitschia, is one of the strangestplantson earth.Welwitschiareally looks like something out a sciencefiction novel. It grows in the deserts of southwestern Africa. Most ofthe plant is deep underground, with a root stretching down to the watertable. The top appears above the soil as a squat cup- shaped stem withtwo strap-shaped leaves. These are the only leaves the plant will evergrow, and they may live a hundred years or more and reach severalmeters,usually torn into strips. Male or female strobili grow from the marginsof the upper stem.
Division Coniferophyta – (550 sp. in 50 genera, fr. Gr.conus=cone,ferre=to bear) – conifers
The conifers are the largest and most successful group of livinggymnosperms.Many of our familiar forest trees are conifers, including pines,spruces,firs, hemlocks, yews, redwoods and cypress trees. They are an ancientgroup,dating back 290 mya. They evolved during the Permian, toward the end ofthe Paleozoic, at a time when the climate was very cool and dry. Theirspecial water conducting cells, called tracheids, allowed them tothrivein these climates and these same adaptations let them continue todominatein colder and dryer environments today, such as northern latitudes,mountainslopes, and sandy soils. Because they are superior competitors in suchhabitats even today, they are the only Division of gymnosperms tosuccessfullycompete with the flowering plants.
Most conifers are evergreens, with the larch and the bald cypressbeingnotable exceptions. Their needle-shaped leaves are also an adaptationtoconserve water. Needles usually occur in small bundles, each bundleemergingfrom a base that is actually a greatly truncated branch. Conifers havetremendous economic importance, as a source of timber and forbyproductssuchas pitch, tar, turpentine, and amber and other resins. Millions aresoldeach year as Christmas trees.
Pine Life Cycle
All conifers produce cone shaped strobili, both male cones (oftencalledpollen cones) and female cones (often called seed cones or ovulatecones).Both male and female cones are usually produced on the same tree, butnotat the same time, so the trees do not fertilize themselves. Femaleconesare large and conspicuous, with thick woody scales. Seed cones canpersiston the tree for several years after fertilization. Male cones are smalland puny looking, and usually don’t last long on the tree. A fewspecies,like junipers and the locally common podocarpus (front of Richardson),have seeds that are covered with a fleshy coating, and resemble smallberries.(not real fruit – Incidentally, all parts of the podocarpus arepoisonous.)
The sporangia produced by the sporophytes are located at the basesofthe sporophylls, and collected in the strobilus we call a pine cone.Themicrospore mother cell in the microsporangia produces the haploidpollengrains. Each scale or sporophyll in the male cone has twomicrosporangiaon its lower surface. Each pollen grain consists of only four cells.Whenthe immature pollen grain finally reaches the seed cone, the megasporemother cell in the megasporangium produces four haploid megaspores.Threeof these megaspores degenerate, and only the fourth germinates into thefemale gametophyte.
The female gametophyte consists of two or more archegonia, with asingleegg in each one. All eggs are usually fertilized. Female cones are alittlemore complicated than male cones (wouldn’t you know). Each visiblescalein the seed cone is really a much reduced lateral branch in itself. Soeach scale is homologous with the entire male cone. The megasporangium,which is called a nucellus in seed plants, is covered with a layer ofprotectivecells called an integument, which is open at one end. This tinyopening,the micropyle, marks the point where the male pollen tube will growintothe megasporangium. The megasporangium, together with its integument,makesup the ovule. Seeds develop from ovules. Each scale in the seed conehastwo ovules on the upper surface of the scale, and so will ultimatelybeartwo seeds side by side.
The pollen grains formed in the microsporangia of pines have tinywingon either side. (Why? Because they are wind-pollinated?Maybe…butwe’ve recently found that it helps them to float up through themicropyleto the egg, like tiny water wings.). The ovulate cones open toreceivepollen, then may close again to protect the developing embryos.
When pollen grains land on the ovulate cones, they grow a longpollentube. By the time this tube reaches the archegonia, about 15 monthsafterpollination, the male gametophyte is fully mature. The pollen tubeentersthrough the micropyle. The sperm nucleus divides in two, and the pollentube discharges two sperm. One sperm nucleus degenerates, the otherfertilizesthe egg. It takes the female gametophyte about 15 months to mature, andabout the same time for the pollen tube of the male gametophyte toreachit.
The seed develops within the megasporangium. The seed is thestructurecontaining the embryonic plant and the stored nutrition to support it.A section of the surface of the scale usually detaches along with theseed,giving the seed a little wing to help disperse it farther from thetree.
Conifer seeds are very complex little structures, containing cellsfromthree generations of the tree. The nutritive tissues inside the seedareactually the haploid body cells of the female gametophyte. The seedalsocontains the developing diploid sporophyte, the little embryonicconifer.The outer wrapping of the seed, the tough and protective seed coat, isformed from the diploid cells of the parent sporophyte. Pine seeds,alongwith acorns, are the most important source of plant food for NorthAmericanwildlife.
To Do and View
Examine the cycads and cycad frond on display. How do theleavesof cycads differ from those of angiosperms? Cycad leaves are full ofpotentneurotoxins, carcinogens, and other toxic chemicals? Cycads areprotectedin another way, as you know if you”ve bumped into one of the manycycadson campus. Why evolve such potent defenses?
Examine the ginkgo leaves and seeds. You might detect a faintodor, a reminder of the very nasty smell these seeds make when theirfleshycovering starts to rot. The delicate appearance of the leaves gives theginkgo its common name, the maidenhair tree. Where can we find thesetreeson campus?
Note the difference between the fleshy-covered seeds ofGinkgoand Podocarpus, and the dry seeds of Pinus. Whatfunctionwould this fleshy covering have served? The answer to this question mayalso explain why ginkgo seeds really stink.
Compare Ephedra to the other gymnosperms. Until recently, wethought that this curious “stem plant” was closely related to floweringplants. Ephedraundergoes double fertilization, a fundamentaltrait of flowering plants. Recent evidence, however, suggests thatGnetophytesare more closely related to pines than to angiosperms.
Examine the Podocarpus branch. This plant is related to theyew.Depending on the season, the plant may have one or more purplishfleshy-coveredseeds, smaller versions of the ginkgo seeds. The seeds are verytemptingto small children, but the seeds, as well as the leaves and other partsof the plant, are toxic. You can find this tree growing all over campusand throughout the city.
Note the difference between the broad leaves of theangiospermson display, and compare them to the needle-shaped leaves of pines.Needlesare an adaptation to conserve water in cold, dry environments. They arealso an excellent shape for species like pines that rely on windpollination(why?).
Review the stages in the pine life cycle, using the slides andothermaterial on display.
Examine slides of the megaspore mother cell. Observethestructure of the strobilus (female pine cone) and note the megasporophyllsand megasporangia.
You will need to look at several sporangia, and possibly morethan one slide, to actually find the megaspore mother cell.Noticethat the sporangia sitting on the sporophylls are directly exposed totheoutside air. Gymnosperm means “naked seed”.
Examine slides of the male strobilus (pine cone).Notethe microsporangia and the microsporophylls. You canswitchto high power and observe the pollen grains in the sporangia or switchto the pollen grain slide. Notice the two large wings (looks likeMickeyMouse). These wings were presumed to aid in wind pollination, butrecentevidence suggests they help the pollen grain float up through themicropyleto the egg.
Examine the pine cones on display. The smaller male cones areonly on the tree for a short time. The larger female cones may persistfor years (conifer = to bear cones).
Things to Remember
Know the life cycle of the pine. Be able to identify thevariousstages.
Ecological, Evolutionary, and Economic Importance
Ephedra is the natural source of the drug ephedrin,whichis used to treat hay fever, sinus headaches, and asthma (eg. sudafedtablets).
Zamia floridana is the only cycad native to the U.S., and wasused by the Seminoles as a source of food.
Conifers are used for resin, pitch, turpentine, lumber, paper, andChristmastrees.
Pine seeds are a critical source of food for wildlife.
Cycads are important for landscaping, and add nitrogen to the soilforother plants.
Cycad stems are ground for use as sago flour in India, Japan, andothereastern nations.
Ginkgos are used for bonsai, as a source of herbal medicine, and aspopular urban shade trees (because of their yellow autumn foliage andtheirresistance to air pollution).
Why do conifers have an adaptive advantage in cool, dryenvironments?
Conifer seeds are very complex structures, containing cells fromthreegenerations of the tree. Can you figure out which tissues come fromwhichgeneration of the conifer?
Introduction to Angiosperms
Just as Gymnosperms forced non-seed plants into the ecologicalbackground,the evolution of Angiosperms, sometime during the Cretaceous, forcedgymnospermsinto restricted habitats. Wherever the earth was cold or dry,gymnospermscould prevail. But in all other habitats, flowering plants rapidlybecamethe dominant plant life.
Flowering plants are able to survive in a greater variety ofhabitatsthan gymnosperms. Flowering plants mature more quickly thangymnosperms,and produce greater numbers of seeds. The woody tissues of angiospermsare also more complex and specialized. Their seeds are enclosed in a fruitfor easy dispersal by wind, water, or animals. The leaves ofangiospermsare mostly thin, extended blades, with an amazing diversity of shapes,sizes, and types.
The surface of the pollen grain has a complex three-dimensionalstructure.This structure is unique for each species, like a floral thumbprint.Thisis one of the ways that female plants can “recognize” pollen grains ofthe right species. It also means that pollen grains, which are abundantin the fossil record, allow us to reconstruct ancient plantcommunities,and these communities in turn tells us about ancient climates.
All angiosperms produce flowers, reproductive structuresthatare formed from four whorls of modified leaves. Most flowers have showypetals to attract pollinators, bribing insects and other animals withnectar,to get them to carry the male gametophyte through the air to anotherflower.Animalpollination is common in angiosperms, in contrast to the mostly wind-pollinatedgymnosperms.
The ovules in angiosperms are encased in an ovary, notexposedon the sporophylls of a strobilus, as they are in gymnosperms.Angiospermmeans “covered seed”. The ovules develop into seeds, and thewallof the ovary forms a fruit to contain those seeds. Fruits attractanimalsto disperse the seeds.
Flowers consist of four whorls of modified leaves on a shortenedstem:sepals,petals,stamens(an anther atop a slender filament), and one or morecarpels.Imagine a broad leaf with sporangia fastened along the edges of theleaf.(Some ferns actually look like this.) Now fold that leave over alongthemidrib, and you”ve enclosed the sporangia in a protected chamber.Congratulations!You”ve just made a carpel.
The carpels are fused together to form apistil, which consistsof astigma (upper surface), a style (long, slender neck),and an ovary (round inner chamber at the bottom) containing oneor moreovules. The flower is analogous to the strobilus of pinesand more primitive plants, except that only the inner two whorls(stamensand carpels) actually bear sporangia. The base of the flower is calledthe receptacle, and the tiny stalk that holds it is the pedicel.Thelife cycle of flowering plants is described in more detail below.
Kingdom Plantae – Angiosperms
Division Anthophyta – flowering plants (= Magnoliophyta,Angiospermophyta)
Class Monocotyledonae – monocots (Zea, Lilium)
Class Dicotyledonae – dicots (Helianthus, Tilia)
Terms sepal petal stamen anther filament carpel pistil stigma style ovary ovule integument pollen grain embryo sac micropyle pollen tube double fertilization endosperm seed seed coat cotyledons (seed leaves) dicot monocot simple fruit simple dry fruit simple fleshy fruit compound fruit multiple fruit aggregate fruit animal pollination wind pollinationFlowering Plant Life Cycle
Let’s start with the male plants, which are a little lesscomplicated…Microsporesdevelop in microsporangia in the anthers, at the tip ofthestamen.Each anther has four microsporangia. Microspores develops by meiosisfromthe microspore mother cell. These microspores develop into pollengrains.
Pollen grains are the male gametophytes in flowering plants.Inside the pollen grain, the microspore divides to form two cells, atubecell and a cell that will act as the sperm. Cross walls break downbetweeneach pair of microsporangia, forming two large pollen sacs. Thesegraduallydry out and split open to release the pollen.
Meanwhile, inside the ovary, at the base of the carpel, the ovules,are developing, attached to the wall of the ovary by a short stalk. Themegasporangia is covered by an integument, protective tissuesthatare actually part of the parent sporophyte. The nucellus andintegumentstogether make up the ovule ( —-> seed).
The megaspore mother cell divides by meiosis to produce four haploidmegaspores. Three of these megaspores degenerate, and the survivingfourthmegaspore divides by mitosis. Each of the daughter nuclei dividesagain,making four nuclei, and these divide a third time, making a grand totalof eight haploid nuclei. This large cell with eight nuclei is theembryosac. This embryo sac is the female gametophyte in flowering plants.
One nucleus from each group of four migrates to the center. Thesearecalled the polar nuclei. The remaining three nuclei of each groupmigratesto opposite ends of the cell. Cell walls form around each group ofthreenuclei. The mature female gametophyte thus consists of only sevencells,three at the top, three at the bottom, and a large cell in the middlewithtwo nuclei. One cell of the bottom three cells will act as the egg.
When the pollen grain reaches the stigma of the carpel, itgerminatesto form a pollen tube. This pollen tube will grow through the neck orstyle,all the way down to the bottom of the carpel, to a small opening calledthe micropyle.
The male gametophyte has two cells. One is the tube cell, the otherwill act as a sperm. As the pollen tube grows closer to the embryo sac,the sperm nucleus divides in two, so the mature male gametophyte hasthreehaploid nuclei.
While the pollen tube is entering the ovule, the two polar nuclei inthe female gametophyte fuse together, making one diploid nucleus. Thetwosperm nuclei enter the embryo sac. One sperm nucleus fuses with the eggnucleus to form a diploid zygote. The other sperm nucleus fuses withthefused polar nuclei to make a triploid cell.
This 3N cell will divide repeatedly to form the endosperm, thestorednutritive material inside the seed. This double fertilization occursonlyinangiosperms and in Ephedra, the gnetophytes (though Ephedra doesn’tformendosperm).
The integuments develop into the tough outer seed coat, which willprotectthe developing embryo from mechanical harm or dessication. Thus theovule,the integuments and the megasporangium they enclose, develops into theseed. The walls of the ovary then develop into the fruit. Allangiospermsproduce fruit, although we might not recognize many of these structuresas “fruits”. (No such thing as “vegetables”, a convenient way to referto a combination of fruits and leafy plant parts).
Seeds and Fruits
There is an incredible diversity of flower structure, not only inthenumber of sepals, petals, stamens, and carpels, but also in the waythesemodified leaves are attached with respect to the ovary. Linnaeus usedthesevery characteristics to sort out the different related groups offloweringplants in his invention of binomial nomenclature, genus and species.Allof these differences can affect the final physical appearance of thefruit.The ovary wall has three layers, each of which can develop into adifferentpart of the fruit.
Simple fruits are fruits that develop from a single ovary.Theycan be either dry, like grains, nuts and legumes, or fleshy,like apples, tomatoes and cucumbers. Compound fruits developfroma group of ovaries. They can be either multiple fruits or aggregatefruits.In multiple fruits, like the pineapple, the group of ovariescomefrom separate flowers. Each flower makes a fruit, and these fruit fusetogether. In aggregate fruits, like strawberries andblackberries,the fruit develops from a flower with many carpels. Each of thesecarpelsdevelops as a separate fruitlet, that fuse together to form thecompoundfruit.
Seeds all bear the plant version of the belly button. They have acrescent-shapedscar called a hilum, where the ovule was attached to the wall ofthe ovary. Right above the hilum, if you look very carefully, you canalsosee a little pinprick scar that is a vestige of the micropyle.
Inside the seed, the tiny sporophyte embryo develops. When it isnearlyready to germinate, the seed contains one or two thick embryonicleaves.These seed leaves, or cotyledons, will support the tender babyplantwhile it establishes its roots and starts to grow its regular leaves.
Most angiosperms, like roses, marigolds, and maple trees, aremembersof the Class Dicotyledones, the dicots (170,000 sp.). Theseflowershave seeds with two seed leaves (di – cotyledon). Some angiosperms,likelilies, onions, and corn , are in the Class Monocotyledones, the monocots(65,000 sp.). The seeds of monocots have only one seed leaf (mono -cot..).There are several other differences between these two groups, which wesummarized in the last lab (plant structure). There are seed leaveseverywherein Spring, and its impossible to tell what they will become just bylookingat them.
To Do and View
Examine slides of Lilium mature anthers. Observe the microsporangia,with all the developing pollen grains inside. Microspores areformedby meiosis, and these haploid cells develop into pollen grains, the malegametophyte in flowering plants. Find the anthers on the real andmodelflowers.
Examine slides of Lilium pollen tubes. You will see pollengrains in every stage of germination, many with a long pollentubeattached.
Examine slides of Lilium embryo sac (8 nucleate stage). Onlowpower, you can see the overall structure of the ovules veryclearly.Try to identify the protective integuments and the tiny openingor micropyle where the pollen tube will enter. You may have tohuntthrough the slide to find the embryo sac. The material has tobesliced just right to pass through the embryo sac. (That”s why there areso many sections on each slide.)
The embryo sac is the female gametophyte of flowering plants.The pollen tubes grow down through the style and up into the ovarythroughthe micropyle. One male nucleus fertilize the egg nucleus, the otherfuseswith two other embryo sac nuclei to form a 3N cell that develops intothestored food or endosperm. This process is called doublefertilization.The ovules, each with a fertilized egg, will develop into seeds,with the integuments forming the seed coat.
Examine the fruits on display. Be able to distinguish betweensimpledry fruit (rice, corn, oats, peanuts), simple fleshy fruit(tomatoes,cucumbers, peppers), and the two types of compound fruit,multiplefruit (pineapples) and aggregate fruit (strawberries,blackberries,or raspberries). Try to visualize, from cross sections of these fruits,how the carpels and ovules were arranged in the flowers that made thesefruits.
Examine the biomounts of dicot and monocot seeds and seedlings,and any other angiosperm seeds on display. Notice that the seeds ofcornand other monocots send up a single cotyledon or seedleaf (hence mono-cots). The seeds of beans and other dicotssend up two seed leaves (hence di-cots). These plump leaves carry thephotosyntheticload while the young seedling establishes its roots, stem andfirsttrue leaves.
Things to Remember
Know the life cycle of flowering plants.
Understand the functions of flowers, seeds, and fruit.
Be able to distinguish monocots from dicots.
Economic, Ecological, and Evolutionary Importance
Most of our agricultural crops are angiosperms.
Commercial fruits and flowers are multi-billion dollar industries.
Angiosperms are the dominant planetary vegetation.
Why are angiosperms better competitors than gymnosperms in mosthabitats?
The evolutionary innovation of the seed is analogous to theevolutionof the amniotic egg in reptiles. Both allowed a large group oforganismsto become fully terrestrial. How does the seed give angiosperms anevolutionaryadvantage over more primitive plants?
The competitive success of angiosperms is partly due to animalpollination,which allowed angiosperms to exist as small scattered populations. Thewind pollinated gymnosperms needed large contiguous populations foreffectivepollination. The coevolution of angiosperms and their pollinators hasgreatlyincreased the diversity of angiosperms.
Links to Explore
The Gymnosperm Database Home Page offers a wealth of information onindividual species of gymnosperms, including copious links, at:
http://www.conifers.org/ One stop shopping for info on cycads, courtesy of Sidney”s RoyalBotanicalGardens: http://plantnet.rbgsyd.gov.au/PlantNet/cycad/index.html The Virtual Encyclopedia of cycads is – well – virtually encyclopedic! http://www.plantapalm.com/vce/vce_index.htm Hey, don”t badmouth those plants, some of those little fellows canreallygrow on you. Don”t believe me? Check out the Parasitic PlantConnection:http://www.science.siu.edu/parasitic-plants/Find out what plants are good for at Plants for a Future. The siteincludesa database of over 7,000 plants that are good to eat or useful in otherways:http://metalab.unc.edu/pfaf/You”ll find an entire course of plant systematics served up still warmand online, courtesy of the University of Maryland:http://www.inform.umd.edu:8080/PBIO/pb250/index.htmlOooh, pretty pictures of angiosperms are waiting at:http://www.phy.duke.edu/~fortney/vg/vg.htmlGet the scoop on how to grow and use herbs, including lots of herballinks,at: http://metalab.unc.edu/herbmed/culiherb.html That ain”t all – check out the herbs at:http://ibiblio.org/herbmed/index.htmlHerbs are good for the body and the mind – stay naturally healthy,courtesy of the Herbal Information Center: http://www.healthy.net/clinic/therapy/herbal/herbic/index.html Mary, Mary, quite contrary, how does your garden grow? Find out atGarden Web, the gateway to gardening online: http://www.gardenweb.com/ Back to top