We have thus far discussed what are considered to be two major extant groups of tracheophytes;

1. Psilophytes, and 2. Lycophytes.

Consider early land plant evolution in relation to the possible origin of those groups and other extant groups.

Earlier discussed oldest land plants such as Cooksonia.

--Had simple dichotomizing axes with terminal sporangia.

--Other kinds of plants that are similar to Cooksonia include Rhynia and Renalia.

--This group of early, very simple land plants such as Cooksonia, Rhynia, Renalia with dichotomizing axes and terminal sporangia are called Rhyniophytes.

Rhyniophytes are not an evolutionary group, but only the bottom part of a single evolutionary group.

--Such a group that receives special emphasis and its own name is called a grade.

--Grade is part of single evolutionary group that has received emphasis because of the similar appearance of the members.

Lycophyte group probably evolved from Rhyniophytes.

--Lycophytes might be thought of as a distinctive evolutionary group that was derived from early members of the Rhyniophyte grade.

Trimerophytes also probably evolved from early Rhyniophytes.

--All trimerophytes are extinct

--present only for Lower to Upper Devonian

The trimerophytes are another grade group that is discussed as being distinct form Rhyniophytes but differ little from them.

--Trimerophytes have erect aerial axes that are pseudomonopodial with a strong central axis.

--more profusely branched than earliest rhyniopnytes.

Psilophytes (Psilotum and Tmesipteris) discussed earlier may have originated among Rhyniophytes or among Trimerophytes.

--The simple body with enations and the positions of sporangia fit body forms found among either Rhyniophytes or trimerophytes.

--Let's initially divide the rest of the land plants into two groups that are most closely related to trimerophytes: 1. ferns and seed plants as one group, and 2. arthrophytes as second group.

--Arthrophytes sometimes called sphenophytes.

Arthrophytes are like lycophytes in being a group once more diverse than they are now.

--Only extant arthrophyte (jointed plant) genus is Equisetum (horsetails)

Arthrophyte group diversified from trimerophyte grade during Middle Devonian.

--These earliest fossils have long, straight aerial axes arising from underground rhizome.

--Branches near top of aerial axis--these dichotomously branched and had

sporangia at tip.

--Hyenia particularly interesting because branches bearing sporangia at tip are recurved characteristic of most of the arthrophytes..

Sporangial branches simplified in groups after Hyenia.

In extant Equisetum, branches with sporangia called sporangiophores.

--Sporangiophore like an umbrella with sporangia on lower side.

In extant Equisetum the sporangiophores are tightly clustered at terminus of some erect axes.

--Forms a strobilus

--note contrast with strobilus of Lycophytes among which the strobilus was composed of sporophylls; whereas Equisetum has no sporophylls and strobilus is composed of modified branches called sporangiophores.

distribution of vascular tissues in axes.

--Stele is pattern of vascular tissue in stem that results from primary growth.

--remember: primary growth is development resulting from cells that form at shoot apex.

--Not from cells produced via lateral cambium

--Thus pattern of secondary phloem and secondary xylem (wood) is not relevant to stele.

Solid central core of vascular tissue = protostele.

Vascular tissue in solid cylinder around pith = siphonostele.

Distinctly separate vascular bundles = Eustele.

Equisetum have a eustele, and a very distinct stem cross section.

Leaves of extant Equisetum are small scale-like structures.

--Numerous leaves at each node.

--Form a ring of tissue around axis with unified basal portion (leaf sheath).

Leaves of Equisetum are somewhat like both microphylls and megaphylls.

--Like megaphylls because have a gap in vascular tissue of stem where leaf bundle diverges.

--Like microphylls because each leaf has only a single vascular bundle.

Other groups that evolved form trimerophytes also evolved megaphyll attributes.

Leaves of Equisetum are small.

--Most photosynthetic activity of plant occurs in stem.

During Carboniferous Period, group of arthrophytes called Calamites evolved.

--Most went extinct soon after end of Carboniferous.

--Calamites much like Equisetum in structure and form, but larger.

--Calamites arborescent (tree-like)

--like Equisetums that grew to about 10m (30 ft.).

--Calamites formed wood via secondary growth, although no wood in extant Equisetum.

--Calamites grew in swampy areas along with Lepidodendron.

sporangia of Equisetum have been discussed--now want to discuss spores.

--all arthrophytes homosporous.

--Spores of Equisetum with laminated walls (i.e., separate layers).

--Outer layer of wall in bands.

--When spores mature the bands of outer wall separate from rest of wall layers

--although they remain attached to spore at one point.

--These bands like arms spreading from spore--called elater.

--This movement of elaters assists in dispersal of spores from sporangium.

Spores give rise to small thalloid (few more than 1 cm in length) gametophytes.

--gametophyte exists separate form sporophyte as free-living individual.

--Are photosynthetic and live on surface of ground in moist places.

Most gametophytes bear both antheridia and archegonia.

--Sperm require film of water to cover gametophyte to swim to egg in archegonium.

Young sporophyte is anchored in gametophyte tissue but becomes independent and free-living.

--Gametophyte degrades after sporophyte independent.

Ferns

Groups that led to the ferns probably diverged from Trimerophytes in the Lower to Middle Devonian (perhaps 360 mya).

groups considered to be ferns are distinct in the fossil record beginning in the Upper Devonian (about 350 mya) near beginning of Carboniferous.

--Many groups of ferns diversified during Carboniferous and then became extinct by early Permian (little later than 280 mya).

--New groups of ferns began to diversify about 220 mya in the Triassic and Jurassic periods.

Ferns have a set of general features shared by most members of the group.

--Perhaps most distinctive part of most ferns is leaf.

Leaves of ferns are megaphylls.

-- many vascular bundles .

-- vascular bundles of the leaves branch dichotomously.

- stem vasculature shows a leaf gap for each leaf in all but a few ferns.

Leaves of ferns form a crozier or "fiddle head"

--That is, it is coiled as it emerges from the soil and uncoils with spring growth.

--Crozier is typical of ferns, but also found in various other plants in which the leaves arise from underground shoot.

--Circination appears to be useful means adopted by numerous plants to protect developing leaf as it emerges from beneath substrate.

Leaves of most ferns arise from a rhizome.

--Rhizome is either underground or found along surface of substrate. Or as trunks (in tree ferns)

roots are adventitious arise between leaf bases

Sporangia of ferns are grouped in sori found on the underside of leaves or on special fertile fronds.

Draw

mainly homosporus spores germinate and form protonema, then a thalate gametophyte with both antheridia and archegonia

Antheridia formed first then archegonia. What is the advantage?

Ferns often divided into two groups on basis of sporangial appearance.

1. Eusporangiate ferns: relatively large sporangia with thick wall composed of several cell layers; sporangium lacks a stalk or has massive stalk; sporangium has large number of spores.

2. Leptosporangiate ferns: sporangium with thin wall of few cell layers; have a delicate stalk; spores of relatively low number (16-512 in homosporous groups).

3. Eusporangia are more like the sporangia of the more primitive groups such as Psilotum, Lycophytes, and Arthrophytes than are the leptosporangia.

--Leptosporangia appears to be derived among ferns.

Begin by discussing eusporangiate ferns.

--Two groups of eusporangiate:

1. Ophioglossales with three or four genera including Ophioglossum and Botrychium that are relatively common in North America.

2. Marattiales with few genera including Marattia and Angiopteris.

Read about these in the book

read about these groups in the book. We will concentrate on the Leptosporanginate groups in lecture.

Leptosporangiate ferns usually divided into 3 major groups: 1. Filicales, 2. Marsileales, and 3. Salviniales.

Filicales.

The ferns with which you are probably most familiar are Filicales.

--This includes most of those growing in natural areas in North America and in your houses.

The croziers--the circinately vernate tips of young leaves--of some ferns are eaten in some areas.

--For consumption: steam croziers for 5 minutes; serve with butter or lemon juice

--may also be used in salads or fried.

Croziers of bracken fern (pteridium aquilinum)

--Considered great delicacy in Japan--

--However, the plants known to contain carcinogens and consumptions probably not safe.

--e.g., when cattle eat bracken fern it leads to sickness and sometimes death.

Most of the embryophyte groups we have discussed have been small with few extant members.

--Filicales group of ferns is large.

--Most diverse in tropical areas

--e.g., in North America, ferns compose about 1/50 of the tracheophyte species in flora; whereas in some tropical areas they may compose as much as 1/8 of tracheophytes in flora.

Stems of Filicales either rhizomes or erect axes.

--Rhizomes may be short and plant will have appearance of being little more than tuft of leaves.

--Or rhizome may be elongate and leaves will be positioned at intervals along the rhizome.

--Most prominent erect axes among Filicales are those of tree ferns.

--Tree ferns with slender axes a few feet tall.

--Tend to have large pinnate leaves.

--Roots grow down outside of stem and help to support plant body.

New cells at apex of fern stem arise ultimately from tetrahedral apical cell.

Leaves of most Filicales are pinnately lobed or compound.

Leaf lamina is typically dorsiventral:

--upper and lower epidermis have cells with chloroplasts unlike epidermal cells of most other tracheophytes.

--Stomates located in epidermis for gas exchange.

--Mesophyll (internal tissues of leaf) may be mostly:

--homogenous or more heterogenous, showing clear regional differentiation with uppermost cells somewhat more elongate and tightly clustered than lowermost cells which have spaces between them to promote gas movement within leaf.

--(This latter organization like palisade and spongy mesophyll of flowering plant leaves).

In most of the Filicales, leaves serve both photosynthetic function and as sporophylls.

--Only few ferns have special differentiation of both photosynthetic leaves and separate sporophylls.

Among most Filicales, the sporangia are clustered in compact groups in localized areas of sporophylls.

--these localized clusters of sporangia are called sori (sing. sorus).

--sorus may be various shapes and positions on leaf.

Sporangia in sorus may or may not be covered by tissue called indusium.

--Indusium is epidermal outgrowth.

--Sorus without indusium said to be naked.

--Variety of manifestations for indusium.

When sporangia marginal and indusium absent, the leaf margin may curl around the sporangia.

Leptosporangia of filicales have stalk one or more cells thick.

--At maturity, the sporangial wall is only one cell in thickness.

--Sporangium has band of cells called annulus.

--Cells of annulus have thickened walls on 3 sides of cell (outer wall not thickened--is a weak side of cell).

--As sporangium matures, water is lost form cells of annulus via evaporation.

--Strong adhesion between cell walls and water.

--With continued evaporation, thin outer wall drawn inward by adhesion tendency.

--Causes annulus to invert shape and rip open sporangium.

--As more water evaporates from cells of annulus and little adhesion tendency left, tension in sprung walls caused annulus to return to former shape--flinging out the remaining spores in the process.

Filicales are homosporous and gametophytes have exosporic development.

--Exosporic implies that gametophyte body develops outside of spore wall.

--In contrast with endosporic development in some lycophytes such as Selaginella and Isoetes.

Gametophytes of Filicales are photosynthetic and free-living.

--They live on surface of soil or other substrate in case of epiphytes.

--Variety of forms of gametophyte--all basically thalloid , most heart-shaped.

Antheridia and archegonia may be formed on same gametophyte.

--Or gametophyte may be unisexual and bear either male or female gametangia.

--Sometimes gametophytes that develop both antheridia and archegonia develop these gametangia sequentially so mature male and female gametes do not occur on same gametophyte at same time.

--This promotes sexual crossing among gametophytes in population and increases genetic variability.

--When mature antheridium flooded with water its distal "cap cell" is loosened and sperm are released into water.

--Sperm are corkscrew shaped and have flagella that they use to swim toward archegonium.

--Similarly, flooding mature archegonium causes enlargement of cells at distal end of neck and separation of these cells from one another.

--Cells of the neck canal disintegrate and are discharged through opening at top of neck as mucilage.

--This discharge in many ferns includes chemicals to which sperm respond.

--Sperm swim chemical gradient toward archegonia.

--Eventually sperm enter neck of archegonium and one will fertilize the egg.

Zygote forms inside of archegonium.

--Cell divisions of zygote lead to sporophyte that remains anchored in gametophyte during its early development.

--After young sporophyte becomes established the gametophyte dies.

--Sporophyte is independent and free-living at that point.

Last two groups of ferns to discuss are Marsileales and Salviniales.

Along with Filicales, Marsileales and Salviniales are considered to be leptosponrangiate ferns.

As we have discussed Filicales and eusporangiate groups of ferns are homosporous and have exosporic gametophyte development.

Marsileales and Salviniales both have derived attributes of heterospory and endosporic gametophyte development.