BIOLOGICAL DIVERSITY: SEED PLANTS

Table of Contents

Gymnosperms are Seed Plants | Cycads | Ginkgos | Conifers | Gnetales | Angiosperms are FloweringPlants | Flowers | Trends in Plant Evolution | Learning Objectives | Terms | Review Questions | Links

Gymnosperms are Seed Plants | Back to Top

Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm meansseed: thus the term gymnosperm = naked seeds. Gymnosperms developed during thePaleozoic Era and became the dominant seed plant group during the early Mesozoic Era,as shown in Figure 1. The ancestors of gymnosperms were some now-extinct type ofheterosporous fern or related group. There are 700 living species of gymnospermsplaced into four divisions: conifers (such as pines and spruce), cycads (such as the sagopalm, Cycas revoluta), ginkgos (the maidenhair tree, Ginkgo biloba), and gnetophytes(such as Mormon tea, Ephedra).

Figure 1. The fossil records of some protist and plant groups. The width of the shaded space isan indicator of the number of species. Image from Purves et al., Life: The Science of Biology,4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman(www.whfreeman.com), used with permission.

Gymnosperms are undoubtedly the group from which the angiosperms developed, although, asCharles Darwin noted in Origin of Species, which group "remains an abominable mystery".Numerous gymnosperm groups have been proposed as flowering plant ancestors over the pastcentury.

Cycads | Back to Top

Cycads are placed in the Division Cycadophyta. They retain several fern-like features, notablypinnate leaves and circinate vernation. However, they usually produce cones ofnonphotosynthetic reproductive structures, a distinctively unfernlike feature. Cycads, like allseed plants, are also heterosporous, unlike the ferns which are all homosporous. Cycad cones areunisexual, in fact the plants producing them are dioecious, having separate male and femaleplants. Cycads also produce free-swimming sperm (a feature found only in ginkgoes amongliving seed plant groups).

Cycads were much more prominent in the forests of the Mesozoic than they are today.Presently, they are restricted to the tropics. Zamia floridana is the only cycad occurring nativelyin the continental United States. Several species of Cycas, notably C. revoluta (shown in Figure2), are commonly encountered cultivated plants in warm, moist areas. Cycas revoluta leaves areoften used in Palm Sunday services in some churches, both for their feathery appearance andease of obtaining from local greenhouses.

Figure 2. Cycas revoluta from Hawaii, Top: female cones bearing large, orange seeds. Imagefrom http://www.botany.hawaii.edu/faculty/carr/images/cyc_rev_f.jpg; Bottom: male conebearing numerous pollen-producing sporangia. Image from

http://www.botany.hawaii.edu/faculty/carr/images/cyc_rev_m.jpg.

Ginkgos | Back to Top

The ginkgoes also were a much more prominent group in the past than they are today. The solesurvivor of this once robust and diverse group is Ginkgo biloba, the maidenhair tree shown inFigure 3. Extensively used as an ornamental plant, Ginkgo was thought extinct in the wild untilit was discovered growing natively in a remote area of China. Ginkos are dioecious, withseparate male and female plants. The males are more commonly planted since the femalesproduce seeds that have a nasty odor. Pollination is by wind. Recently, Ginkgo has become thecurrent herbal rave, although scientific studies have debunked the claim that the herbalsupplement made from ginkgoes improves memory. I think.

Figure 3. Ginkgo biloba, close-up of young shoot. Image fromhttp://www.dinoworld.net/ginkgo.JPG.

Precise systematic placement of the ginkgoes has yet to bet determined. Ginkgoes have motile(swimming) sperm, a rarity among living seed plants (only ginkgoes and cycads have thisfeature today), although the vegetative anatomy of ginkgoes is more conifer-like (long shoot andshort shoot morphology discussed below; structure of their wood). Ginkgoes, like the cycads,are dioecious, and also have similar seed features to cycads.

Plants possibly allied to the modern ginkgoes have been found in Permian-aged and later rocks.These plants have been classified in the leaf-genera Ginkgoites (shown in Figure 4) and Baiera,although recent studies suggest these genera are really morphological variants and that themodern genus Ginkgo should be used to include these fossils. During the Mesozoic ginkgoeswere worldwide in their distribution and important elements in the gymnosperm forests thatdominated the land.

Figure 4. Ginkgoites leaf from the Triassic of North Korea. Although this fossil is not thetypical fan shape of most modern ginkgo leaves, modern ginkgoes do produce leaves of thisgeneral shape. Image from http://www.dges.tohoku.ac.jp/museum/fosgal19.html.

Conifers | Back to Top

The conifers remain a major group of gymnosperms that include the pines, spruce, fir, baldcypress and Norfolk Island Pine (Araucaria). The division Pinophyta contains approximately550 species of conifers. The conifers are cone producing trees and shrubs that usually haveevergreen needle-like leaves. Needles have a thick cuticle, sunken stomates, and a reducedsurface area. The conifers, as a group, are well adapted to withstand extremes in climate andoccur in nearly all habitats from the equator to the subpolar regions. The taiga biome consistslargely of various conifer species.

Auracarias

Members of this group of conifers have numerous small, scale-like leaves spiraling around theirstems. Araucaria, a major genus that gives its name to the group, is a common ornamental

because of the symmetry and beauty of its growth form. The monkey puzzle tree, shown inFigure 5, is a species of Araucaria.

Figure 5. Image of Araucaria sp. Note the large female comes at the tips of branches. Imagefrom http://www.botany.hawaii.edu/faculty/carr/images/araucar_sp3.jpg.

The fossil record of Auracarias and similar plants is quite good. The fossil genusAuracarioxylon that grew in Arizona during the Triassic Period comprises the largest group ofpetrified wood in the Petrified Forest National Park of Arizona.

Taxodiaceae: Sequoias and more

Members of this group include some of the largest trees, and have been significant members ofthe forests of the world since the Mesozoic. Sequoia, shown in Figure 6, and Sequoiadendronare major genera in this group.

Figure 6. Top: Habit photograph of Sequoia sempervirens. Note the tall, woody stem withleaves borne at the top of the stem. Image fromhttp://www.botany.hawaii.edu/faculty/carr/images/seq_sem_hab.jpg. Bottom: Close-up ofSequoia sempervirens leaf and cone. Image fromhttp://www.botany.hawaii.edu/faculty/carr/images/seq_sem_cu.jpg.

The Pine Life Cycle

Pines have an interesting life cycle, shown in Figure 7, that takes two years to complete. Not allseed plants have such a long time span to complete their life history: some flowering plantsmanage to do it in as little as a few weeks.

The sporophyte, as in all other vascular plant groups, is the dominant, photosynthetic part of thelife cycle: when you are holding pine needles in your hand you are holding sporophyte parts.Pines have specialized reproductive structures in which meiosis occurs: pine cones. Pollengrains are produced in the male cones, and contain the male gametophyte (which consists ofonly a very few cells). Pollen released from the male cones is carried by wind to the femalecones, where it lands. The cones close and the next year the pollen grain germinates to producea pollen tube that grows into the female gametophyte. The sperm cell (from the pollen grain)

and egg cell fuse, forming the next generation sporophyte. The sporophyte develops into anembryo encased within a seed. The seed is later released to be transported by the wind to where(hopefully) it lands and germinates. If you have seen a large pine tree you realize there arehundreds or more female cones on such a tree. Pine pollen has been noted to travel greatdistances from the plant that produced it, if the wind is strong enough. To aid this transport pinepollen has two air sacs, and thus is quite distinctive, as shown in Figure 8.

Figure 7. The pine life cycle. Note: to view these four segments in correct sequence you will need to increaseyour browser window width as much as possible. Images from Purves et al., Life: The Science of Biology, 4thEdition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used withpermission.

Figure 8. Pine pollen from 38,000 year old sediments in the White Mountains of easternArizona. Image from http://www.smu.edu/geology/palynolab.htm.

Gnetales | Back to Top

The Gnetales, shown in Figure 9, are an odd group: they have some angiosperm-like featuresbut are not themselves angiosperms. Cladistic analyses support placement of the gnetales (orsome portion of them) as outgroups for the flowering plants. Three distinctive genera comprisethis group: Welwitschia, Gnetum, and Ephedra. Ephedra occurs in the western United Stateswhere it has the common name "Mormon tea". It is a natural source for the chemical ephedrine,although there is no evidence the Mormons in Utah (where the plant is extremely common) everused it for tea. Welwitschia is limited to coastal deserts in South Africa, although fossil leaf,cuticle and pollen evidence indicates plants of this type were widespread during the MesozoicEra. Welwitschia is noted for its two long, prominent leaves. Gnetum has leaves that lookremarkably like those in angiosperms, as well as vessels in the xylem, generally considered anangiosperm characteristic.

Figure 9. Representative gnetalean plants. Top: Ephedra female plant. Image fromhttp://www.botany.hawaii.edu/faculty/carr/images/eph_sp_f.jpg. Middle: Welwitschia mirabilishabit shot of a cultivated specimen. Image fromhttp://www.botany.hawaii.edu/faculty/carr/images/wel_mir.jpg. Bottom: Gnetum gnemon habitshot showing pinnate leaves bearing female strobili. Image fromhttp://www.botany.hawaii.edu/faculty/carr/images/gne_gne.jpg.

Among the gnetalean plants, Ephedra is perhaps the best known. One folkloric name for theplant is "Mormon tea". This is a misnomer as there appears little or no evidence that membersof a religion that bans stimulants such as caffeine ever brewed a tea from the plant. However,the plant does produce the drug ephedrine, a stimulant lately linked to deaths of athletes.

Welwistchia is a very bizarre plant natively growing only in the coastal deserts of South Africa.The plant produces two long leaves and a crown of reproductive cones rimming a brown, centralbody. Pollen resembling Welwitschia has been found in many parts of the world, indicating aformerly more widespread distribution of this enigmatic plant.

Angiosperms are Flowering Plants | Back to Top

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearingduring the later part of the of the Age of Dinosaurs (the beginning of the Cretaceous, 140million years ago). All flowering plants produce flowers. Within the female parts of the flowerangiosperms produce a diploid zygote and triploid endosperm. Fertilization is accomplished by avariety of pollinators, including wind, animals, and water. Two sperm are released into thefemale gametophyte: one fuses with the egg to produce the zygote, the other helps form thenutritive tissue known as endosperm.

The angiosperms (angios = hidden) produce modified leaves grouped into flowers that in turndevelop fruits and seeds. There are presently 235,000 known living species. Most angiospermsalso have larger xylem cells known as vessels that improve the efficiency of their vascularsystems.

Whence came the angiosperms? This was Darwin's "abominable mystery". Clearly angiospermsare descended from some group of Mesozoic-aged gymnosperm seed plant....but which one?Click here to view an online lab exercise in phylogeny and try to figure things out!

The classical view of flowering plant evolution suggests early angiosperms were evergreen treesthat produced large Magnolia-like flowers. Click here to view an illustration of suggested pathsof floral evolution. However, this view has recently been contradicted by the oldest fossil yetfound, a 140 million year old plant found by David Dilcher and his associates.

The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most partescaped the major extinctions at the end of the Cretaceous.

Flowers | Back to Top

Flowers are collections of reproductive and sterile tissue arranged in a tight whorled arrayhaving very short internodes. Sterile parts of flowers are the sepals and petals. When these aresimilar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen(male, collectively termed the androecium) and carpel (often the carpel is referred to as thepistil, the female parts collectively termed the gynoecium). Lily flowers (shown in Figure 10)demonstrates these concepts.

Figure 10. Flower of lily (Lilium) illustrating a complete flower. The image is reduced fromgopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Angiosperm/Lilium/Flower_dissection/Flower.Follow that link to view a larger image of this flower.

Flowers may be complete, where all parts of the flower are present and functional, orincomplete, where one or more parts of the flower are absent. Many angiosperms produce asingle flower on the tip of a shoot (like the lily pictured in Figure 10, or tulips). Other plantsproduce a stalk bearing numerous flowers, termed an inflorescence, such as is seen in manyorchids. Many flowers show adaptations for insect pollination, bearing numerous white oryellow petals. Others, like the grasses, oaks, and elms, are wind pollinated and have their petalsreduced and often inconspicuous.

Angiosperm Life Cycle

Flowering plants also exhibit the typical plant alternation of generations, shown in Figure 11.The dominant phase is the sporophyte, with the gametophyte being much reduced in size andwholly dependant on the sporophyte for nutrition. The is not a unique angiosperm condition, butoccurs in all seed plants as well. What makes the angiosperms unique is their flowers and the"double fertilization" that occurs. Technically this is not double fertilization, but rather a singleegg-sperm fusion (fertilization proper) plus a fusion of the second of two sperm cells with twohaploid cells in the female gametophyte to p[produce triploid (3n) endosperm, a nutritive tissuefor the developing embryo. More details on this aspect of the flowering plants are available inthe FLOWERING PLANT REPRODUCTION: Fertilization and Fruits chapter.

Figure 11. Life cycle of corn, a typical monocot angiosperm. Note the formation of endospermby "double fertilization". Images from Purves et al., Life: The Science of Biology, 4th Edition,by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used withpermission.

Angiosperm Systematics | Back to Top

The flowering plants, the division Magnoliophyta, contain more than 235,000 species, six timesthe number of species of all other plants combined. The flowering plants divide into two largegroups, informally named the monocots and the dicots. The techjnical names for these groupsare the class Magnoliopsida for dicots and the class Liliopsida for monocots.

The dicotyledons are in the class Magnoliopsida and have these features: either woody orherbaceous, flower parts usually in fours and fives, leaves usually net-veined, vascular bundlesarranged in a circle within the stem, and produce two cotyledons (seed leaves) at germination.Prominent dicot families include the mustards, maples, cacti, peas and roses. Several dicotfamilies are noteworthy because of the illegal drugs (shown in Figure 12) derived from them:the Cannabinaceae (marijuana) and Papaveraceae (poppies from which opium and heroin arederived). Erythroxylum coca (in the dicot family Erythroxylaceae) is the plant from which theillegal drug cocaine is extracted.

Figure 12. Some dicot plants bused for making illegal drugs. Top: Erythroxylum coca, leftimage, the plant that is the source of cocaine, fromhttp://www.drugs.indiana.edu/graphics/photographs/cocaine1.gif, right image, cocaine, fromhttp://www.drugs.indiana.edu/graphics/photographs/cocaine3.gif; Bottom left, Cannabissativa, the marijuana plant; image fromhttp://www.drugs.indiana.edu/graphics/photographs/marij6.gif, right: Papaver somniferum,the opium poppy being harvested; image fromhttp://www.drugs.indiana.edu/graphics/photographs/opium7.gif.

Not all dicot plants are misused to produce illegal drugs. Notable dicot families with legitimateuses include the pea family, which includes the crop plants beans, clover, and peas as well asmany ornamental landscape plants such as Acacias. Beans are an excellent source of nonanimalprotein as well as fiber. Another dicot of enormous use is cotton, Gossypium, shown in Figure13. Chocolate and cola are products of the plant family Sterculiaceae. Coffee is produced fromCoffea arabica, a plant in the family Rubiaceae, while tea comes from Camelia sinensis(Theaceae), a plant native to China.

Figure 13. Economic dicots. Top: Gossypium sp., cotton., image fromhttp://botit.botany.wisc.edu/images/401/Magnoliophyta/Magnoliopsida/Dilleniidae/Malvaceae/Gossypium/Cotton_boll_leaves_MC.html;Bottom: Capsicum annuum, the chili pepper; Image from http://www.botany.hawaii.edu/faculty/carr/images/cap_ann_233.jpg.

The class Liliopsida has plants that are herbaceous (a majority are, only palms and bamboostand out as monocot trees), flower parts are in threes, leaves are usually parallel-veined,vascular bundles are scattered within the stem, and produce one cotyledon (seed leaf) atgermination. Monocot families include lilies, palms, orchids, irises, and grasses.

The monocot family Poaceae (known previously as the Gramineae) includes the grasses such ascorn, oats, wheat, rye, and rice that are staple food products as well as ornamental plants such ascrabgrass and tiff grass. The importance of this plant family to modern civilization cannot beoverstated, as the first six plants mentioned in the previous sentence provide 75% of our food,either directly as food we eat or indirectly as food for animals we eat.

Trends in Plant Evolution | Back to TopSeveral evolutionary trends within the plant kingdom have been noted. The monophyleticnateure of this kingdom is not in dispute, with the first major division being between vascularand nonvascular plants. Wihin the vascular plants we see increasing changes in the relationshipbetween sporophyte and gametophyte, culminating in flowering plants.

Developing from green algal ancestors, plants show a trend for reduction of the complexity,size, and dominance of the gametophyte generation. In nonvascular plants the gametophyte isthe conspicuous, photosynthetic, free-living phase of the life cycle. Conversely, the angiospermgametophyte is reduced to between three and eight cells (hence it is very inconspicuous) and isdependent on the free-living, photosynthetic sporophyte for its nutrition.

Plants also developed and refined the root-shoot-leaf axis with its specialized conducting cellsof the xylem and phloem. The earliest vascular plants, such as Cooksonia and Rhynia, were littlemore than naked (unleafed) photosynthetic stems. Some plants later developed secondarygrowth that produced wood. Numerous leaf modifications are known, including "carnivorous"plants such as the Venus flytrap, as well as plants that have reduced or lost leaves, such asPsilotum and the cacti.

A third trend is the development of the seed to promote the dormancy of the embryo. The seedallows the plant to wait out harsh environmental conditions. With the development of the seedduring the Paleozoic era plants became less prone to mass extinctions.

The fourth trend in plant evolution is the encasing of a seed within a fruit. The only plant groupthat produces true fruit is the flowering plants, the angiosperms. Fruits serve to protect the seed,as well as aid in seed dispersal.

Table 1. Plant classification.

Division Characteristics Examples and Uses

Bryophyta nonvascular plants, gametophytedominanted life cycle mosses, liverworts, hornworts

Psilophyta naked photosynthetic stems, noleaves, free-sporing Psilotum nudum, Tmesipteris;no commercial uses

Lycophyta (theclub mosses)

microphyll leaves, sporangia instrobili; more significant in thePaleozoic

Lycopodium, a homosporous plant, Selaginella, aheterosporous plant; no commercial uses except asground cover in some areas

Sphenophyta(=Equisetophyta;the horsetails)

megaphyll leaves, sporangia intight cones; plants with jointedstems; more significant in thePaleozoic

Equisetum, the scouring rush or horsetail; nocommercial applications

Filicophyta(ferns)

megaphyll pinnately compoundleaves, sporangia on underside ofleaves, clumped in sori; leavesarise by circinnate vernation(uncoiling to look initially like acrozier or sheperd's crook)

Ornamental plants such as the Boston fern; tree ferns;commercial applications as ornamentals; ecologicallyimportant plants in some areas of the world

Cycadophyta(the cycads)

long, pinnately compoundmegaphyll leaves with a leatheryfeel arising from soft woode4dstems by circinnate vernation; newleaves arise as a crown or whorl;reproduction by seeds produced infemale cones, pollen produced inmale cones

Cycas revoluta, a common ornamental known as theSago palm;Zamia floridana, only cycad native to the48 contiguous United States; commercial uses asornamental plants

Ginkgophyta(the ginkoes)

fan shaped, usually bilobed leaveson a woody tree with long anmdshort shoot anatomy; seedsproduce a foul odor when mature;pollen elliptical, monoaperturate

Ginkgo biloba, the maidenhair tree; commercial usesas ornamental plants and allegedly as a memory aid inherbal form

Coniferophyta

needle-like or scale-like leaveswith thick cuticles and sunkenstomata; soft wooded plants; seedslacking fruit, dispersede fromhardened cones; pollen can bebisaccate (Pinus) or lacking sacs(Taxodium)

Pinus(wood, resin, pine straw, pine nuts, paper); Taxus(yew) a natural source for the anticancer drug taxol;Douglas fir (wood); ornamental plants

Gnetophyta

living plant group most closelyrelated to flowering plants; vesselsin some members of group;reproductive structures close toflowers; pollen elliptical eithusually one aperture

Ephedra (Mormon tea) source of ephedrine; Gnetum;Welwitschia have no commercial uses

Magnoliophyta

flowering plants (angiosperms)that encase their seeds within aripened ovary wall known as afruit; fruits may be dry or fleshy,single or multiple; two majorclasses the monocots and dicots;xylem may include vessels as wellas tracheids; range fromherbaceous annuals to perienneltrees

Monocots: cereal grains form staple of economy anddiet; palms are used in many places as buildingmaterial; ecologicalimpact as pioneer species indisturbed habitats;

Dicots: food crops such as beans, peas; fibers fromcotton used to make clothing and paper; illegal drugssuch as marijuana, cocaine, heroin; legal products suchas chocolate, tea, and coffee; Nicotiana leaves maketobacco a dangerous legal "drug"; ornamental plants

Learning Objectives | Back to TopBe able to compare and contrast the characteristics of seed, nonseed, and nonvascular plants. To dothis complete the table below.

Characteristic Nonvascular Plants Nonseed Plants Seed PlantsDominant Phase of Plant Life CycleSwimming SpermPollen TubePollen or Spores

Homospory or Heterospory or MixedExamples of Plants in this Group

Be able to list the uses of some common gymnosperm plants, such as pines and ginkgoes.Be able to list some of the uses of flowering plants.It has been stated that the ancestor of flowering plants most likely was a gnetalean plant (or at least aplant closely related to them). What evidence supports this?How might the fact that a species in monoecious or dioecious affect your decision to use it as a cropplant? As a landscape plant?

Terms | Back to Top androecium angiosperms Auracarioxylon carpel cones coniferscycads dioecious endosperm Ephedra fertilization ginkgoesGnetales Gnetum gymnosperms gynoecium homosporous internodesMesozoic Era Paleozoic Era petals pistil pollen grains pollen tubesepals sporophyte stamen Welwitschia zygote

Review Questions | Back to Top1. Endosperm tissue is unique to which of these groups? a) cycads; b) conifers; c) flowering

plants; d) ferns2. Seed plants have ___. a) pollen grains with male gametophytes developing on the inside;

b) female gametophytes developing inside an ovule retained on the sporophyte; c) roots; d)all of these

3. Which of these plants is not a gymnopserm? a) corn; b) pine; c) redwood; d) ginkgo4. Which of these plants is not a flowering plant? a) corn; b) wheat; c) lily; d) douglass fir5. Cycads and ginkgoes were important plants to the world's terrestrial ecosystems during the

____. a) Paleozoic era; b) Cenozoic era; c) Mesozoic era; d) precambrian6. The presence of long shoot/short shoot anaytomy characterizes which of these pants? a)

Ginkgo; b) Cycas; d) Zamia; d) ferns

Answers: 1. c; 2. d; 3. a; 4. d; 5. c; 6. a;

Links | Back to TopNon-Flowering Plant Family Access Page Sorted by family on the non-flowering plants. Thumbnailphotos are linked to larger versions. This site is a great educational resource maintained by Gerald D.Carr.Encyclopedia of Plants Scientific and common names for garden plants.The Botanical Society of America The official website of the plant folks.Cycad Pages This Australian website is a great stepping stone for more information about a favoriteplant group of mine: cycads. This page is maintained by Ken Hill of the Royal Botanic GardensSydney and Dennis Stephenson of the New York Botanical Garden.Ginkgo biloba History, uses, and other information about this living "fossil tree" whose leaves areessentially unchanged since the Tertiary period.The Ginkgo Pages Cor Kwant has produced a great resource on a fascinating plant. A great resourcefor images and information!Gymnosperm Database Christopher J. Earle has developed a nifty resource for digging deeper into thegymnosperms. A series of links from this page take you into images and descriptions of the variousgymnosperm groups.Erythroxylum: The Coca Plant This ethnobotanical page, by April Rottman, describes the systematicsand uses of the plant as well as its product, cocaine.Barley: The Versatile Crop This page, part of a series of ethnobotanical leaflets, is by Bryan Young,and describes the natural history and economic uses of barley, the 4th most important crop.

All text contents ©1995, 1999, 2000, 2001, 2003, 2004, by M.J. Farabee. Use of text for educationalpurposes is encouraged.

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Email: mj.farabee@emcmail.maricopa.edu

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