members of the Lignophyte Clade arealso:

Spermatophytes, also called "Seed Plants", probablyevolved in the Upper (late) Devonian Period. The originof the seed - a specialized form of reproduction in whichthe gametophyte is totally enclosed within tissues of theparent sporophyte - must surely count among the mostimportant events in the evolutionary history of plants.

One of our objectives this week is to understandwhat a seed is and how it works.

Living Spermatophytes may be divided into two groups:

Angiosperms or "Flowering Plants" - a probablymonophyletic groups with specialized reproduction. Wewill consider this group in the next two labs.

Gymnosperms or "Naked Seed Plants" - aparaphyletic group of lignophytes representing a rangeof both primitive and derived forms that have survivedwith modification from primitive spermatophytes ofthe Carboniferous Period. We will look at differentrepresentative of the gymnosperms here.

Lab this week will be divided between looking at fossiland prepared material in lab, and a short field trip toobserve living examples in the Greenhouse and oncampus.

Be prepared with sturdy shoes and appropriatedress to spend some time outside!

For each plant group in our survey we have the followingsuggestions:

In your study of seed architecture, observe, drawand label the different tissue layers in both theovule and the seed. From the text, determinewhether each tissue layer is derived from thesporophyte or the gametophyte.

Be certain you understand how sexualreproduction works in seed plants.

Observe each group presented in lab in turn.

Consult your textbook and lecture notes for adescription of diagnostic features of both externalform and internal anatomy for each group.

Using fresh or prepared material, observe anddraw what you see.

Study differences in the life cycle of each group.

It is very important that you understand all structuresand mode of operation of gymnosperm reproduction. Tohelp you with this, consult your text and the guide below:

The Seed Habit

Angiosperm Reproduction

Consult your textbook for useful descriptions,terminology, and pictures. A guide to material in the labis provided in the links below:

Conifers

Cycads

Weird Gymnosperms

Angiosperms

Spermatophytes or "Seed Plants" probably constitutea monophyletic group of vascular plants possessing thatall-important evolutionary innovation - the seed habit.

It is very important to understand what the seed habitrepresents in terms of the common Sporic Life Cyclecharacteristic of all Embryophytes. Be sure youbecome familiar with all important structures since theyform the basis for understanding the reproductivebiology of all "higher" plant groups.

Although "seed habit" may be used to describe thegeneral situation, in many situations, it is useful todistinguish the following more specific terms dependingon whether or not fertilization has taken place:

ovule is an unfertilized seed.seed is a fertilized ovule.

Often it is difficult to tell the difference between ovulesand seeds since it all depends upon whether sperm nucleihave completed their journey and fertilized the egg.

The seed habit represents an extreme form ofheterospory in which a single megaspore developswithin a megasporangium (nucellus) embeddedentirely within protective tissues of the parentsporophyte. In addition, the female gametophytedevelopes completely enclosed within its megaspore andproduces a limited number of eggs. Fertilization of oneor more eggs is accomplished by pollination - transferof pollen - male gametophytes enclosed withmicrospores and producing sperm cells or nuclei.

All ovules/seeds are basically the same in terms of theencapsulated gametophyte. They differ significantly,however, in the size and complexity of the gametophyte.This week, we will consider the primitive condition amongovules/seeds. In these forms, the female gametophyteconsists of thousands of cells. When mature, the femalegametophyte produces one or more archegonia at oneend, each with an egg.

Study the available views of the ovule/seed in theprepared sections with both dissection and compoundmicroscopes. Be sure you understand which tissues arederived from the female gametophyte (thereforehaploid) versus parent sporophyte (therefore diploid).

Using your text, identify, draw and label the followingstructures:

integumentnucellusmicropylepollen chambermegaspore wallmegagametophyte tissuesarchegoniumegg.

Emphasis is rightly placed on the importance ofovules/seeds in defining the seed habit of spermatophytes- especially the role of the female gametophyte andparental sporophyte in the nutrition of developingembryos. However, the whole process is also dependentupon getting sperm to egg! The male side of seed habitheterospory involves production of microspores withinmicrosporangia, also called "Pollen Sacs", borne onthe parent sporophyte. Unlike megaspores, microsporesare released into the environment when mature.However, even here there is a form of encapsulation! Themale gametophyte develops entirely within themicrospore. It is highly reduced in form - usuallyconsisting of a small number of vegetative (orprothallial) nuclei and typically one generative cellthat develops into two sperm nuclei.

With available prepared material, observe pollen grains- microspores with their encapsulated malegametophytes - under the compound microscope. Drawand label what you see. Before you leave lab, be sure youunderstand the important distinction betweenmicrospore and pollen grain.

In gymnosperms, pollen is typically borne on the wind inprodigious numbers. A lucky few find their way to thepollen chamber of the ovules. The most primitive formof fertilization is exhibited by modern cycads. The pollengrain enters the pollen chamber, attaches itself to thechamber wall, and ruptures to produce two swimmingsperm cells. These cells then proceed to find their wayto one or more exposed archegonia, through the neckcanal of an archegonium, and to the egg.

In more advanced gymnosperms, such as conifers,wind-borne pollen grains are trapped by thepollination droplet exuded by the ovule through themicropyle. After awhile, the pollen droplet is pulledinside the pollen chamber. However the femalegametophyte remains totally encapsulated by sporophyteintegument. The pollen grains then produce a growth

called the pollen tube that grows through the tissues ofthe integument into the archegonium. Sperm nuclei aredirectly transferred without swimming through thepollen tube to the egg.

After fertilization in seed plants, like all goodEmbryophytes the embryo of the next sporophytegeneration is retained within tissues of the femalegametophyte. In seed plants, there is typically a period ofdormancy sometime during the construction orfertilization of the ovule/seed. This allows the process ofsexual reproduction to be synchronized with factors inthe external environment, thus avoiding the cold ofwinter, a dry season, or other unfavorable yearlyconditions. In Pinus (pine), seed production takes twoyears.

Observe available material of seeds. Compare figures inyour text. Draw and label what you see.

Conifers

Cycads

Weird Gymnosperms

A diagnostic feature of angiosperms is the fact thatovules/seeds are borne enclosed in carpels. Asdescribed in lecture and previous labs, encapsulation ofthe megagametophyte and microgametophyte is anoutstanding feature of the history of Spermatophytes(seed plants). Angiosperms have carried encapsulationeven further and have streamlined the alternation ofgenerations inherent in a Sporic Life Cycle almost toa bare minimum. It is important to understand how thislife cycle works and how, compared with Gymnospermsand Pteridophytes, it differs.

For a review of the Seed Habit, see:

In angiosperms, like gymnosperms, there is an extremeform of heterospory in which only a singlemegaspore develops within the megasporangium(nucellus). In all seed plants, the megasporangium isenveloped within sporophyte tissues called theintegument. As described in lecture, the integument inGymnosperms is a single layer perhaps in evolutionaryterms derived by enclosure of the once freemegasporangium by sterile sporophyte shoot tissues suchas the cupule of seed ferns.

By contrast, the flowering plants have a doubleintegument - two more-or-less distinct tissue layersenveloping the nucellus. It is thought that the second,presumably outer, integument represents a secondevolutionary event of envelopment, although directevidence from fossils or developmental studies is mostlylacking.

Observe the prepared slides of angiosperm ovules insectional views. In many but not all angiosperms, ovulesare borne in a reflexed (flopped backward) positionwith the proximal end of the seed termed "chalazalend" or "chalaza" attached to the placenta by meansof a stalk - called the "funiculus". As in gymnosperms,the distal end of the ovule has a micropyle providingaccess to the outside through the integument.

Identify these structures in your examples. Now look forthe nucellus and double integument. Draw and label

what you see.

As in all Spermatophytes, the single functionalmegaspore is permanently retained within the nucellusand the megagametophyte develops completely within it.In gymnosperms, the megagametophyte is variable insize, but can be comprised of up to several thousand cellsbearing archegonia & eggs. In angiosperms, on the otherhand, the megagametophyte is highly reduced,usually consisting of only a few nuclei. At the chalazal endare the antipodal cells - all that's left of the vegetativegametophyte body. In the middle is the central nucleusthat before fertilization is typically diploid as a result offusion of two gametophyte nuclei. At the micropylar endof the ovule is the egg apparatus comprised of an eggcell and two synergids.

Using available sectioned material Look for thestructures named above. Draw and label what you see.

Pollen grains in angiosperms are much like thosealready seen in gymnosperms. The microgametophyte,consisting of only a very few nuclei develop entirelywithin the microspore wall and is dispersed as a unit.Also, like advanced gymnosperms, such as Pine,germination of the pollen grain is by means of a pollentube carrying non-swimming sperm nuclei directly toegg.

A major differerence, however, is in the site of pollengermination. In Angiosperms, pollen grains arecaptured at a distance from developing ovules by thestigma - a specialized portion of the pistil made fromsporophyte tissues. Once germinated, the pollen tubegrows from the stigma through the style and enters thecarpel and on to the ovule and egg cell.

This is quite a journey! Unlike gymnosperms, the ovulesin angiosperms can be fertilized without being exposed tothe outside environment. Presumably this provides a

greater measure of protection. It is also interesting tonote that many angiosperms have well developedhistochemical self incompatibility mechanismslocated within the style or stigma. These permit theflower to only allow growth of pollen tubes with specificchemical markers, and is used to promote outcrossing.

When a pair of sperm nuclei from a pollen tube finallymakes it to egg, another remarkable thing happens inangiosperms - double fertilization!

One sperm fertilizes the egg whereas theother fuses with the central nucleus.

Once fused, the central nucleus (2n) + sperm (n) is calledendosperm (3n). Endosperm develops rapidly as anutrient tissue to supply the zygote and subsequentdeveloping angiosperm embryo.

Using available materialm observe, draw and label whatyou can of this remarkable story!

Coniferophyta or "Conifers" are by far the mostabundant and successful group of gymnosperms,especially in north temperate and boreal environments.There are several distinct groups, typically given familyor higher-level taxonomic names. Observe each groupand note their distinctive features. Draw and label oneexample of each.

The "monkey puzzle" trees represent an ancientlineage of gymnosperms with typical conifer-like cones(strobili). In a strobilus containing seeds or ovules, asingly integumented megasporangium is borne onthe adaxial surface of each cone scale. Leaves are smalland flat or needle like, but usually larger than that seenin other conifer groups. Members of the Araucariaceaelive mostly in tropical or sub-tropical regions.

A new "living fossil" belonging to this group wasdiscovered recently (1994) in Australia. See your text forinteresting details!

"Podocarps" are a distinctly southern group of conifersprobably owing their current geographic distribution tothe breakup of the ancient Gondwana continent in theMesozoic. These plants have distinctive strap-shapedleaves. Find examples of this group in the greenhouse.Look for evidence of strobili.

Members of the Pine Family are primarily north-temperate and include several familiar trees such asPinus (Pine), Picea (Spruce), Abies (Fir), Pseudotsuga(Douglas Fir), and Tsuga (Hemlock). Since these arecommon plants on campus, easily recognized by theirdistinctive features, we ask that you become familiar withthe genera so that you are able to recognize each onsight. To help you remember, Pinus has needle like leavesborne on very small short shoots called fascicles. Theothers bear leaves singly on their stems, but differ in the

nature of the leaves and appearance of the cones.Pseudotsuga is unique in showing clearly the compoundnature of the ovulate strobilus characteristic of theentire family - look for bracts below each cone scale, andfor ovules/seeds borne in pairs on the adaxial suface ofeach cone scale.

This group is also well represented on campus by thefamiliar genera Juniperus (Juniper) and Thuja(Arborvitae). Become familiar with each. Unlikemembers of the Pinaceae, vegetative leaves in theCupressaceae are typically small, tightly adpressed, andscale-like. Be sure to look for the cones. The familyTaxodiaceae contains plants that are close relatives ofthe Cupressaceae. Genera include Taxodium (BaldCypress) in southern swamps and the CaliforniaRedwoods Sequoia and Sequoiadendron. Aninteresting member of the Taxodiaceae is Metasequoia(Dawn Redwood) first described as a fossil in 1941, butlater discovered still living in China in 1948. This genus isdeciduous - see our specimen in the greenhouse.

The genus Taxus (Yew) looks very conifer-like until onenotices its highly modified ovulate reproductivestructures. At maturity, individual seeds are partiallyenclosed in a fleshy and red tissue called the aril.Common in midwinter and Christmas decorations, whatfunction do you think the aril serves on the plant?

The Seed Habit

Cycads

Weird Gymnosperms

This primitive group of seed plants have been aroundsince the Permian period, and may well have served asDinosaur food during the Mesozoic. The groupretains the primitive form of leaf in Spermatophytes.Cycad leaves are typically large, very robust, andpinnately compound.

Cycads sporophytes are characteristically monoeciousmeaning they bear either ovules or pollen sacs on a singleplant - never both. In the living genus Cycas (SegoPalm), ovulate strobili consist of somewhat reduced leafhomologs bearing ovules along the sides of each fertileleaf near the base. This situation is reminiscent ofreproduction observed in extinct fossil seed plants, calledPteridosperms ("Seed Ferns") that bore their seeds onmore-or-less normal leaves.

More derived cycad genera bear pollen and ovules inmore compact cone-like strobili. Observe the fineexamples of cycads in the BU Greenhouse. Look for thetwo different kinds of cones. Identify and draw thefollowing:

strobilar axiscone scalepollen sac (microsporangium)pollenovuleseed

The Seed Habit

Conifers

Weird Gymnosperms

Although some gymnosperm groups, such as thePinaceae, are reasonably diverse, other groups are muchless so. Some genera, including the forms discussed here,show many unusual features. The fossil record indicatesthat these modern plants are relict - living holdoversfrom more prosperous Mesozoic times!

The order Gnetales consists of three very differentgenera: Ephedra, Gnetum and Welwitschia, linked bymolecular evidence as well as some shared features ofwood anatomy and pollen. Relationship of this group toother gymnosperms is unclear. Originally thought to berelated to conifers based primarily on wood anatomy,they have more recently been grouped by someresearchers with the flowering plants into a largerAnthophyte clade. However, even more recentmolecular evidence casts doubt on the monophyleticstatus of Anthophytes, and many researchers are againsuggesting relationship of Ephedrales with the conifers.

Stay tuned! Things may change yet again!

In truth, morphologically and perhaps genetically, thegroup doesn't comfortably fit anywhere!

Ephedra:

Definitely a weird plant, the genus Ephedra (MormonTea) lives in xeric conditions in the Southwestern UnitedStates. Leaves are highly reduced and scale like. Theplant looks mostly like a collection of photosyntheticbranches. If conditions are right, sporophytes willproduce pollen and ovulate strobili. Observe thespecimens on display in the Greenhouse. Unfortunately,we lack examples of Gnetum and Welwitschia.However, nice photographs are presented in the text.

Ginkgo biloba:

Finally, there is the enigmatic gymnosperm speciesGinkgo biloba ("Maidenhair Tree"). Note specimenson campus by the Student Services wing and in theGreenhouse. Ginkgo is unique in bearing largeflabelliform leaves with open-dichotomousvenation on distinct short shoots. Like us (but nodoubt by coincidence!) these trees have an XYchromosome system and therefore sporophytes may bedetermined by the kind of gametophytes they produce tobe either "male" (microsporangiate) or "female"(megasporangiate). "Female" trees bear naked ovulestypically in pairs, and the seed coat gives off a foul odorof rotten eggs.

The history of Ginkgo is very interesting. Similar plantsare well represented in the fossil record. It is a commoncultivated plant, but nowhere today does this plant grownaturally in the wild. It appears to have been saved fromextinction by cultivation in temple gardens by the ancientcivilizations of China and Japan.

In recent years both Ephedra and Ginkgo biloba havebecome infamous in unscientific circles associated withthe trade in herbals. No doubt many plants, possiblyincluding these, have useful medicinal properties onlyincompletely understood by modern science if at all. Folkmedicine may offer important clues for furtherdiscoveries. However, of unsubtantiatedclaims, or anecdotal testimony of peddlers and truebelievers! Some of this activity results in unnecessarydeaths. Recently (2004), selling of "herbal" extracts ofEphedra has been banned in theUSA for just this reason.

The Seed Habit

Conifers

Cycads

First, let's talk about your lab notebook!

Your lab notebook will be graded as anintegral part of work in the laboratory.

Listen to your TA for details.

To begin our investigation of plants, let's take a look at:

The Flowering Plants are without peer in the PlantKingdom! With estimated taxonomic diversity ofabout a quarter of a million species - with new onesdiscovered every year - no other group of plants evencomes close. The Angiosperms are also the group with byfar the greatest morphological disparity. The rangeof morphologies and anatomies seen within differentflowering plants, including large trees, small herbs,parasitic plants, epiphytes, aquatics, and even marineforms, is simply astonishing.

Equally surprising is the fact that Angiosperms haveattained almost complete ecological dominance inmany terrestrial environments - including the mesictemperate forest surrounding us on the BU campus!However, they have accomplished this dominance despitebeing evolutionary newcomers. Although most majorgroups of vascular plants can be traced to origins withinthe Devonian through Permian Periods of thePaleozoic Era, the first convincing fossils of floweringplants date from the Lower (early) Cretaceous(approx. 130 ma).

The late and apparently sudden appearance ofangiosperms in the fossil record is a fact that has caughtthe attention of scientists and non-scientists alike. In the19th Century, none other than Charles Darwin viewedthe origin of angiosperms as an "abominablemystery" and their sudden appearance in the fossilrecord without recognizable ancestors a potential falsifierto his Theory of Evolution.

In the intervening years, there has been much systematicand paleobotanical and research on the problem ofangiosperm origins and their subsequent diversification.Fossil evidence points to a single origin of the groupperhaps on the northern Gondwana or southernLaurasian supercontinents sometime in the earlyCretaceous. However, some researchers concerned withthe exceptionally high disparity of angiosperms based onmorphological or molecular data, argue for a muchearlier origin. They also suggest that there existed aperiod of time during which the group is somehow

completely missing from the fossil record.

After their origin, the fossil record of angiospermdiversification is reasonably clear. The group rapidlyexpanded during the Cretaceous becoming ecologicallyimportant by Upper (late) Cretaceous times. Thisdiversification coincided with a major change in the kindsof dinosaurs present in terrestrial environments, withTriceratops and Hadrosaurs perhaps specialized inpart to take advantage of this new food resource.

Angiosperm diversification continued in the subsequentTertiary Period after dinosaur extinction, and hascontinued to the Present with, except for potentialdisturbance by humans, apparently no end in sight!

We will look at different aspects of Flowering Plantmorphology, anatomy, reproduction, and diversity inthis and the next lab. Weather permitting, we will try tospend time outside looking for members of this groupon campus.

Be prepared to go into the field by wearingappropriate clothing and shoes!

Consult your textbook for useful descriptions,terminology, and pictures. A guide to material in thelab is provided in the links below:

The Flower

The Floral Formula

Inflorescences & Fruits

Family Identification

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