Lab 9 - Gymnosperms and Angiosperms
Introduction
When mosses and liverworts first evolved, they dominated the terrestrial environment. But they were soon challenged by the more advanced tracheophytes. The ferns and "fern allies" formed the great planetary forests of the late Paleozoic. By the end of the Paleozoic, a new group of plants was challenging the 150 million-year domination of the ferns and fern allies. The seed plants protected the embryonic sporophyte from drying up by encasing it in a tough waterproof seed coat.
The evolution of the seed is as profound a step as the evolution of the shelled egg in reptiles. Just as the evolution of the amniotic egg enabled reptiles to become the first truly terrestrial vertebrates, to break that final link with their aquatic heritage, so did the evolution of the seed allow plants to escape the limitation of growing in very moist environments. These gymnosperms soon became the dominant plants. 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. They rapidly dominated the more primitive gymnosperms, and are the dominant plants on Earth today. These waves of competition are typical of the history of life. The survivors are relegated to scattered populations in restricted habitats, where they live in the shadows of their successful competitors. Among the gymnosperms, only the conifers are major competitors with flowering plants. Having evolved in a dryer, cooler climate, conifers are better adapted to dry or cool habitats, and dominate forests in northern latitudes, at high elevations, and on sandy soils.
Today we will examine both gymnosperms and angiosperms, and compare their complex life cycles. The trend toward a dominant sporophyte stage is now complete. The gametophytes of seed plants are microscopic. The female gametophyte consists of a handful of cells buried in the tissues of the sporophyte. The male gametophyte, the pollen grain, has a brief free-living stage while it is carried from plant to plant by wind, water, or animals. No longer relying on flagellated sperm, and with their developing embryos protected from desiccation, seed plants break the last link with their aquatic ancestors.
Introduction to Gymnosperms
The first seed plants evolved relatively early on, in the late Devonian. By the end of the Paleozoic they were competitive enough to replace the club mosses, horsetails, and whisk ferns, and become the dominant vegetation of the Mesozoic, the era of the dinosaurs. By the end of the Mesozoic, they too would be swept aside by the newly evolved angiosperms, the flowering plants. There are only 720 living species of gymnosperms, a pale remnant of a once diverse and dominant race.
Living gymnosperms are a diverse group of plants, most of which bear their sporangia in large, prominent strobili or cones. These strobili are similar to those of lycopsids and horsetails. Strobili consist of a shortened stem with several modified leaves (sporophylls) that bear sporangia. Like all seed plants, gymnosperms are heterosporous. The sporangia that generate the male microspores and female megaspores are usually borne on separate cones. Male cones (staminate cones) are typically much smaller than female cones (ovulate cones). Sporophylls that bear microsporangia are called microsporophylls. Sporophylls that bear macrosporangia are called macrosporophylls. The pine life cycle is typical of gymnosperms, and is described in detail below.
Taxonomy
Kingdom Plantae
Gymnosperms
Division Gnetophyta - Ephedra, Gnetum, Welwitschia
Division Cycadophyta - cycads (Cycas revoluta)
Division Ginkgophyta - Ginkgo biloba
Division Coniferophyta - conifers (Pinus)
Division Cycadophyta - (~100 sp., 9 genera, fr. Gr. kyos=palm, phyton=plant) - cycads
Cycads have very thick leaves, that look like very tough versions of fern fronds. These palm-like plants have unbranched stems, with a terminal crown of leaves. These leaves are incredibly well defended with sharp tips and with complex secondary compounds, including potent neurotoxins and carcinogenic compounds. They reached their peak during the Mesozoic, with species reaching from 6-60 feet. The Mesozoic is sometimes called the Age of Cycads. A giant cycad today might reach 9-10 feet max.
They are unisexual or dioecious, having separate male and female plants. Dioecious means two houses, vs. monoecious = one house (bisexual, both sexes in one). Only one genus of cycad (Zamia) is native to North America. The Seminoles ate the starchy roots of Zamia pumila, found in southern Florida. In India, Japan, and Sri Lanka, sago flour is often made from cycad stems (it is also made from real palms, which are angiosperms).
Cycads are widely grown as ornamental landscape plants. Cycads also enrich the fertility of barren soil, because they are symbiotic with nitrogen-fixing cyanobacteria. Cycads are extremely slow growing, and can live 1,000 years or more. They are wind pollinated, a strategy which requires immense amounts of airborne pollen. A few may have been pollinated by beetles attracted to the edible pollen grains. This may be the humble beginnings of the complex animal pollination developed by flowering plants. The pollen sacs and ovules are 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 (maidenhair tree)
Ginkgo trees are commonly seen in cities today. They are attractive shade trees, reaching 100 feet or more, with beautiful yellow foliage in the Fall. They are very resistant to air pollution and insects. You can see these trees right on campus (Richardson and the Gibson Hall “loop”).
That the sole remaining species did not join its brethren in extinction we owe to the ancient Chinese and Japanese, who cultivated it in their temple gardens for centuries. Their may no longer be a single living wild tree. It is a popular tree for bonsai, because the leaves will readily miniaturize, and the branches are easy to shape. The species name biloba comes from the two distinct lobes of its fan-shaped leaves, very different from the straplike or needle shaped leaves of other gymnosperms. The common name maidenhair tree comes from the similarity of ginkgo leaves to fronds of the maidenhair fern.
Ginkgos and cycads show a transitional stage between the primitive ferns and the more advanced conifers and flowering plants. They have flagellated sperm, but the male gametophyte grows a pollen tube, a long filament through which the sperm can safely swim to the egg. The pollen grains of other seed plants grow similar tubes. The megasporangia, which contains the eggs, form tiny female strobili on the tips of special branches on the female tree. The microsporangia, which produce the pollen grains, are in male strobili that hang down like little pine cones on the male tree.
The seed that forms on the female trees is covered with a thick fleshy coat which makes the seed look like a little fruit (which it is technically not). They have an incredible odor when they ripen, which one otherwise stodgy botany text describes as “rotting dog vomit”. So be very careful if you plant one of these wonderful trees and select a male tree!! Although in fairness to the female tree, its seed is prized in China as a source of medicinal drugs.
Division Gnetophyta - (70 sp. in 3 genera), Gnetum, Ephedra, Welwitschia
This odd little group of gymnosperms are mainly xerophytes, plants that are adapted to dry conditions. They share a close common ancestor with flowering plants. Each genera has some species that produce nectar, and attract 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, incidentally is the natural source of the alkaloid ephedrin, used to treat hay fever, sinus headaches, and asthma. Its medicinal properties have been known for at least 5,000 years!
Most gnetophytes are stem plants, like Ephedra, branched photosynthetic stems with no leaves. Gnetum has leaves like those of modern flowers. But the third genus, Welwitschia, is one of the strangest plants on earth. Welwitschia really looks like something out a science fiction novel. It grows in the deserts of southwestern Africa. Most of the plant is deep underground, with a root stretching down to the water table. The top appears above the soil as a squat cup- shaped stem with two strap-shaped leaves. These are the only leaves the plant will ever grow, and they may live a hundred years or more and reach several meters, usually torn into strips. Male or female strobili grow from the margins of 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 living gymnosperms. Many of our familiar forest trees are conifers, including pines, spruces, firs, hemlocks, yews, redwoods and cypress trees. They are an ancient group, dating back 290 mya. They evolved during the Permian, toward the end of the Paleozoic, at a time when the climate was very cool and dry. Their special water conducting cells, called tracheids, allowed them to thrive in these climates and these same adaptations let them continue to dominate in colder and dryer environments today, such as northern latitudes, mountain slopes, and sandy soils. Because they are superior competitors in such habitats even today, they are the only Division of gymnosperms to successfully compete with the flowering plants.
Most conifers are evergreens, with the larch and the bald cypress being notable exceptions. Their needle-shaped leaves are also an adaptation to conserve water. Needles usually occur in small bundles, each bundle emerging from a base that is actually a greatly truncated branch. Conifers have tremendous economic importance, as a source of timber and for byproducts such as pitch, tar, turpentine, and amber and other resins. Millions are sold each year as Christmas trees.
Pine Life Cycle
All conifers produce cone shaped strobili, both male cones (often called pollen cones) and female cones (often called seed cones or ovulate cones). Both male and female cones are usually produced on the same tree, but not at the same time, so the trees do not fertilize themselves. Female cones are large and conspicuous, with thick woody scales. Seed cones can persist on the tree for several years after fertilization. Male cones are small and puny looking, and usually don’t last long on the tree. A few species, like junipers and the locally common podocarpus (front of Richardson), have seeds that are covered with a fleshy coating, and resemble small berries. (not real fruit - Incidentally, all parts of the podocarpus are poisonous.)
The sporangia produced by the sporophytes are located at the bases of the sporophylls, and collected in the strobilus we call a pine cone. The microspore mother cell in the microsporangia produces the haploid pollen grains. Each scale or sporophyll in the male cone has two microsporangia on its lower surface. Each pollen grain consists of only four cells. When the immature pollen grain finally reaches the seed cone, the megaspore mother cell in the megasporangium produces four haploid megaspores. Three of these megaspores degenerate, and only the fourth germinates into the female gametophyte.
The female gametophyte consists of two or more archegonia, with a single egg in each one. All eggs are usually fertilized. Female cones are a little more complicated than male cones (wouldn’t you know). Each visible scale in the seed cone is really a much reduced lateral branch in itself. So each scale is homologous with the entire male cone. The megasporangium, which is called a nucellus in seed plants, is covered with a layer of protective cells called an integument, which is open at one end. This tiny opening, the micropyle, marks the point where the male pollen tube will grow into the megasporangium. The megasporangium, together with its integument, makes up the ovule. Seeds develop from ovules. Each scale in the seed cone has two ovules on the upper surface of the scale, and so will ultimately bear two seeds side by side.
The pollen grains formed in the microsporangia of pines have tiny wing on either side. (Why? Because they are wind-pollinated? Maybe. but we’ve recently found that it helps them to float up through the micropyle to the egg, like tiny water wings.). The ovulate cones open to receive pollen, then may close again to protect the developing embryos.
When pollen grains land on the ovulate cones, they grow a long pollen tube. By the time this tube reaches the archegonia, about 15 months after pollination, the male gametophyte is fully mature. The pollen tube enters through the micropyle. The sperm nucleus divides in two, and the pollen tube discharges two sperm. One sperm nucleus degenerates, the other fertilizes the egg. It takes the female gametophyte about 15 months to mature, and about the same time for the pollen tube of the male gametophyte to reach it.
The seed develops within the megasporangium. The seed is the structure containing the embryonic plant and the stored nutrition to support it. A section of the surface of the scale usually detaches along with the seed, giving the seed a little wing to help disperse it farther from the tree.
Conifer seeds are very complex little structures, containing cells from three generations of the tree. The nutritive tissues inside the seed are actually the haploid body cells of the female gametophyte. The seed also contains the developing diploid sporophyte, the little embryonic conifer. The outer wrapping of the seed, the tough and protective seed coat, is formed from the diploid cells of the parent sporophyte. Pine seeds, along with acorns, are the most important source of plant food for North American wildlife.
To Do and View
Examine the cycads and cycad frond on display. How do the leaves of cycads differ from those of angiosperms? Cycad leaves are full of potent neurotoxins, carcinogens, and other toxic chemicals? Cycads are protected in another way, as you know if you've bumped into one of the many cycads on campus. Why evolve such potent defenses?
Examine the ginkgo leaves and seeds. You might detect a faint odor, a reminder of the very nasty smell these seeds make when their fleshy covering starts to rot. The delicate appearance of the leaves gives the ginkgo its common name, the maidenhair tree. Where can we find these trees on campus?
Note the difference between the fleshy-covered seeds of Ginkgo and Podocarpus, and the dry seeds of Pinus. What function would this fleshy covering have served? The answer to this question may also explain why ginkgo seeds really stink.
Compare Ephedra to the other gymnosperms. Until recently, we thought that this curious "stem plant" was closely related to flowering plants. Ephedra undergoes double fertilization, a fundamental trait of flowering plants. Recent evidence, however, suggests that Gnetophytes are more closely related to pines than to angiosperms.
Examine the Podocarpus branch. This plant is related to the yew. Depending on the season, the plant may have one or more purplish fleshy-covered seeds, smaller versions of the ginkgo seeds. The seeds are very tempting to small children, but the seeds, as well as the leaves and other parts of the plant, are toxic. You can find this tree growing all over campus and throughout the city.
Note the difference between the broad leaves of the angiosperms on display, and compare them to the needle-shaped leaves of pines. Needles are an adaptation to conserve water in cold, dry environments. They are also an excellent shape for species like pines that rely on wind pollination (why?).
Review the stages in the pine life cycle, using the slides and other material on display.
Examine slides of the megaspore mother cell. Observe the structure of the strobilus (female pine cone) and note the megasporophylls and megasporangia.
You will need to look at several sporangia, and possibly more than one slide, to actually find the megaspore mother cell. Notice that the sporangia sitting on the sporophylls are directly exposed to the outside air. Gymnosperm means "naked seed".
Examine slides of the male strobilus (pine cone). Note the microsporangia and the microsporophylls. You can switch to high power and observe the pollen grains in the sporangia or switch to the pollen grain slide. Notice the two large wings (looks like Mickey Mouse). These wings were presumed to aid in wind pollination, but recent evidence suggests they help the pollen grain float up through the micropyle to the egg.
Examine the pine cones on display. The smaller male cones are only on the tree for a short time. The larger female cones may persist for years (conifer = to bear cones).
Things to Remember
Know the life cycle of the pine. Be able to identify the various stages.
Ecological, Evolutionary, and Economic Importance
Ephedra is the natural source of the drug ephedrin, which is used to treat hay fever, sinus headaches, and asthma (eg. sudafed tablets).
Zamia floridana is the only cycad native to the U.S., and was used by the Seminoles as a source of food.
Conifers are used for resin, pitch, turpentine, lumber, paper, and Christmas trees.
Pine seeds are a critical source of food for wildlife.
Cycads are important for landscaping, and add nitrogen to the soil for other plants.
Cycad stems are ground for use as sago flour in India, Japan, and other eastern nations.
Ginkgos are used for bonsai, as a source of herbal medicine, and as popular urban shade trees (because of their yellow autumn foliage and their resistance to air pollution).
Consider This
Why do conifers have an adaptive advantage in cool, dry environments?
Conifer seeds are very complex structures, containing cells from three generations of the tree. Can you figure out which tissues come from which generation of the conifer?
Introduction to Angiosperms
Just as Gymnosperms forced non-seed plants into the ecological background, the evolution of Angiosperms, sometime during the Cretaceous, forced gymnosperms into restricted habitats. Wherever the earth was cold or dry, gymnosperms could prevail. But in all other habitats, flowering plants rapidly became the dominant plant life.
Flowering plants are able to survive in a greater variety of habitats than gymnosperms. Flowering plants mature more quickly than gymnosperms, and produce greater numbers of seeds. The woody tissues of angiosperms are also more complex and specialized. Their seeds are enclosed in a fruit for easy dispersal by wind, water, or animals. The leaves of angiosperms are mostly thin, extended blades, with an amazing diversity of shapes, sizes, and types.
The surface of the pollen grain has a complex three-dimensional structure. This structure is unique for each species, like a floral thumbprint. This is one of the ways that female plants can “recognize” pollen grains of the right species. It also means that pollen grains, which are abundant in the fossil record, allow us to reconstruct ancient plant communities, and these communities in turn tells us about ancient climates.
All angiosperms produce flowers, reproductive structures that are formed from four whorls of modified leaves. Most flowers have showy petals to attract pollinators, bribing insects and other animals with nectar, to get them to carry the male gametophyte through the air to another flower. Animal pollination is common in angiosperms, in contrast to the mostly wind-pollinated gymnosperms.
The ovules in angiosperms are encased in an ovary, not exposed on the sporophylls of a strobilus, as they are in gymnosperms. Angiosperm means "covered seed". The ovules develop into seeds, and the wall of the ovary forms a fruit to contain those seeds. Fruits attract animals to disperse the seeds.
Flowers consist of four whorls of modified leaves on a shortened stem: sepals, petals, stamens (an anther atop a slender filament), and one or more carpels. Imagine a broad leaf with sporangia fastened along the edges of the leaf. (Some ferns actually look like this.) Now fold that leave over along the midrib, and you've enclosed the sporangia in a protected chamber. Congratulations! You've just made a carpel.
The carpels are fused together to form a pistil, which consists of a stigma (upper surface), a style (long, slender neck), and an ovary (round inner chamber at the bottom) containing one or more ovules. The flower is analogous to the strobilus of pines and more primitive plants, except that only the inner two whorls (stamens and carpels) actually bear sporangia. The base of the flower is called the receptacle, and the tiny stalk that holds it is the pedicel. The life cycle of flowering plants is described in more detail below.
Taxonomy
Kingdom Plantae - Angiosperms
Division Anthophyta - flowering plants (= Magnoliophyta, Angiospermophyta)
Class Monocotyledonae - monocots (Zea, Lilium)
Class Dicotyledonae - dicots (Helianthus, Tilia)
Let’s start with the male plants, which are a little less complicated. Microspores develop in microsporangia in the anthers, at the tip of the stamen. Each anther has four microsporangia. Microspores develops by meiosis from the microspore mother cell. These microspores develop into pollen grains.
Pollen grains are the male gametophytes in flowering plants. Inside the pollen grain, the microspore divides to form two cells, a tube cell and a cell that will act as the sperm. Cross walls break down between each pair of microsporangia, forming two large pollen sacs. These gradually dry 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. The megasporangia is covered by an integument, protective tissues that are actually part of the parent sporophyte. The nucellus and integuments together make up the ovule ( ----> seed).
The megaspore mother cell divides by meiosis to produce four haploid megaspores. Three of these megaspores degenerate, and the surviving fourth megaspore divides by mitosis. Each of the daughter nuclei divides again, making four nuclei, and these divide a third time, making a grand total of eight haploid nuclei. This large cell with eight nuclei is the embryo sac. This embryo sac is the female gametophyte in flowering plants.
One nucleus from each group of four migrates to the center. These are called the polar nuclei. The remaining three nuclei of each group migrates to opposite ends of the cell. Cell walls form around each group of three nuclei. The mature female gametophyte thus consists of only seven cells, three at the top, three at the bottom, and a large cell in the middle with two nuclei. One cell of the bottom three cells will act as the egg.
When the pollen grain reaches the stigma of the carpel, it germinates to form a pollen tube. This pollen tube will grow through the neck or style, all the way down to the bottom of the carpel, to a small opening called the micropyle.
The male gametophyte has two cells. One is the tube cell, the other will 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 has three haploid nuclei.
While the pollen tube is entering the ovule, the two polar nuclei in the female gametophyte fuse together, making one diploid nucleus. The two sperm nuclei enter the embryo sac. One sperm nucleus fuses with the egg nucleus to form a diploid zygote. The other sperm nucleus fuses with the fused polar nuclei to make a triploid cell.
This 3N cell will divide repeatedly to form the endosperm, the stored nutritive material inside the seed. This double fertilization occurs only in angiosperms and in Ephedra, the gnetophytes (though Ephedra doesn’t form endosperm).
The integuments develop into the tough outer seed coat, which will protect the developing embryo from mechanical harm or dessication. Thus the ovule, the integuments and the megasporangium they enclose, develops into the seed. The walls of the ovary then develop into the fruit. All angiosperms produce fruit, although we might not recognize many of these structures as “fruits”. (No such thing as “vegetables”, a convenient way to refer to a combination of fruits and leafy plant parts).
Whew.
Seeds and Fruits
There is an incredible diversity of flower structure, not only in the number of sepals, petals, stamens, and carpels, but also in the way these modified leaves are attached with respect to the ovary. Linnaeus used these very characteristics to sort out the different related groups of flowering plants in his invention of binomial nomenclature, genus and species. All of these differences can affect the final physical appearance of the fruit. The ovary wall has three layers, each of which can develop into a different part of the fruit.
Simple fruits are fruits that develop from a single ovary. They can be either dry, like grains, nuts and legumes, or fleshy, like apples, tomatoes and cucumbers. Compound fruits develop from a group of ovaries. They can be either multiple fruits or aggregate fruits. In multiple fruits, like the pineapple, the group of ovaries come from separate flowers. Each flower makes a fruit, and these fruit fuse together. In aggregate fruits, like strawberries and blackberries, the fruit develops from a flower with many carpels. Each of these carpels develops as a separate fruitlet, that fuse together to form the compound fruit.
Seeds all bear the plant version of the belly button. They have a crescent-shaped scar called a hilum, where the ovule was attached to the wall of the ovary. Right above the hilum, if you look very carefully, you can also see a little pinprick scar that is a vestige of the micropyle.
Inside the seed, the tiny sporophyte embryo develops. When it is nearly ready to germinate, the seed contains one or two thick embryonic leaves. These seed leaves, or cotyledons, will support the tender baby plant while it establishes its roots and starts to grow its regular leaves.
Most angiosperms, like roses, marigolds, and maple trees, are members of the Class Dicotyledones, the dicots (170,000 sp.). These flowers have seeds with two seed leaves (di - cotyledon). Some angiosperms, like lilies, 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 we summarized in the last lab (plant structure). There are seed leaves everywhere in Spring, and its impossible to tell what they will become just by looking at them.
To Do and View
Examine slides of Lilium mature anthers. Observe the microsporangia, with all the developing pollen grains inside. Microspores are formed by meiosis, and these haploid cells develop into pollen grains, the male gametophyte in flowering plants. Find the anthers on the real and model flowers.
Examine slides of Lilium pollen tubes. You will see pollen grains in every stage of germination, many with a long pollen tube attached.
Examine slides of Lilium embryo sac (8 nucleate stage). On low power, you can see the overall structure of the ovules very clearly. Try to identify the protective integuments and the tiny opening or micropyle where the pollen tube will enter. You may have to hunt through the slide to find the embryo sac. The material has to be sliced just right to pass through the embryo sac. (That's why there are so 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 ovary through the micropyle. One male nucleus fertilize the egg nucleus, the other fuses with two other embryo sac nuclei to form a 3N cell that develops into the stored food or endosperm. This process is called double fertilization. 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 between simple dry fruit (rice, corn, oats, peanuts), simple fleshy fruit (tomatoes, cucumbers, peppers), and the two types of compound fruit, multiple fruit (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 these fruits.
Examine the biomounts of dicot and monocot seeds and seedlings, and any other angiosperm seeds on display. Notice that the seeds of corn and other monocots send up a single cotyledon or seed leaf (hence mono-cots). The seeds of beans and other dicots send up two seed leaves (hence di-cots). These plump leaves carry the photosynthetic load while the young seedling establishes its roots, stem and first true 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.
Consider This
Why are angiosperms better competitors than gymnosperms in most habitats?
The evolutionary innovation of the seed is analogous to the evolution of the amniotic egg in reptiles. Both allowed a large group of organisms to become fully terrestrial. How does the seed give angiosperms an evolutionary advantage over more primitive plants?
The competitive success of angiosperms is partly due to animal pollination, which allowed angiosperms to exist as small scattered populations. The wind pollinated gymnosperms needed large contiguous populations for effective pollination. The coevolution of angiosperms and their pollinators has greatly increased the diversity of angiosperms.
Links to Explore
The Gymnosperm Database Home Page offers a wealth of information on individual species of gymnosperms, including copious links, at:
http://www.conifers.org/ One stop shopping for info on cycads, courtesy of Sidney's Royal Botanical Gardens: 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 can really grow on you. Don't believe me? Check out the Parasitic Plant Connection:
Find out what plants are good for at Plants for a Future. The site includes a database of over 7,000 plants that are good to eat or useful in other ways:
You'll find an entire course of plant systematics served up still warm and online, courtesy of the University of Maryland:
