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.

--In the derivation of heterospory and endosporic gametophytes we could say that Marsileales and Salviniales have evolutionary parallelisms with lycophytes such as Selaginella and Isoetes.

Members of Marsileales and Salviniales are aquatic.

Despite sharing habitation of aquatic environment and attributes of heterospory and endosporic gametophyte development some investigators have hypothesized that Marsileales and Salviniales do not form a single evolutionary group.

--Instead many perceive each as being most closely related to different groups of Filicales.

Draw

Marsileales consist of 3 genera: Marsilea, Regnellidium, & Pilularia. We will concentrate on Marsilea.

Marsilea: Pacific coast of U.S. and South Pacific.

Sporophytes of all three genera grow in pools of water or extremely wet soils.

--Marsilea tends to grow in seasonally wet habitats; plants below water until water begins to dry out the plants begin to form sporangia and the resistant dispersal phase of life cycle.

--Plants of the three genera are rooted in substrate.

--When growing in pools of water, only leaves extend above water level.

All three genera form a rhizome that grows along surface of substrate.

--Leaves are produced at intervals along rhizome

--Internodes are relatively elongate.

--Roots most often arise at nodes.

Leaves of Marsilea have four-parted lamina (resemble idea 4-leafed clover).

--Young leaves are circinately vernate.

--Pinnae remain folded together until later development.

Some leaves remain totally submerged below water level and have different appearance than those that are emergent above water level.

--Emergent leaves with lobes perpendicular to petiole and submerged leaves with lobes parallel to petiole are typical of plants that form both submerged and emergent leaves.

--Having submerged leaves with lobes parallel to petiole tends to facilitate water flow over leaf surface and minimizes rippage.

Leaves of Marsilea, Regnellidium, and Pilularia often discussed as evolutionary reduction or simplification series.

Marsilea leaves with four lobes:

Regnellidium leaves with two lobes:

Pilularia leaves unlobed--simple, needle-like form--no lamina expansion.

Sporangia of Marsileales located in structure called sporocarp.

--Sporocarps are hard, ovoid--like a bean seed.

--Borne on a stalk inserted on lower part of petiole of leaf

--Sporocarps very resistant to environmental conditions and may retain viable spores after 20-35 years.

--The homology or structural correspondence of the sporocarp is somewhat unclear.

--May correspond with an entire leaf or correspond with a single pinna

Sporocarp basically consists of two halves along a long axis.

--Set of sori positioned along each half.

--Each sorus said to be covered by indusium and consists of megasporangia and microsporangia.

In each megasporangium, only a single megaspore develops.

In each microsporangium, numerous microspores develop.

Sporangia are not dehiscent--lack annulus or other attributes that lead to opening of sporangium.

In nature sporocarp opens 2 or 3 years after formation.

--When sporocarp splits open, and elongate, gelatinous structure called sorophore projects outward.

--sori are attached to sorophore.

--Sporangia eventually breakdown to release spores.

Inside microspore, spermatids develop within jacket of sterile cells.

--Outer wall of microspore ruptures and inner part of spore with male gametophyte emerges.

--Eventually inner spore wall ruptures, releasing motile sperms.

--Sperm are active for about an hour.

About 5 minutes after a megaspore contacts water, spore wall takes up water.

--This causes expansion of gelatinous matrix around spore.

--Inside gelatinous matrix, at one end of spore, is a space filled with a liquid: called sperm lake.

--Sperm swimming around megaspore get trapped in gelatinous matrix.

--Some manage to move into sperm lake.

During this time, simple female gametophyte develops with archegonium to accomplish fertilization.

Salviniales consist of two genera: Salvinia and Azolla.

--Salvinia primarily a tropical genus with about 10 extant species.

--Azolla more widespread and has about 6 extant species.

Both Salvinia and Azolla are floating aquatic ferns.

--Neither is rooted to substrate; Float on surface of water.

Consider Salvinia first:

--Has rhizome that floats on surface of water.

--Rhizome has elongate internodes.

--At each node a whorl of 3 leaves arises. (i.e., 3 leaves positioned at each node)

Two leaves of whorl float on surface of water.

--Upper surface of leaves somewhat concave and bear numerous hairs with waxy coating.

--If these leaves are pushed below water surface they will hold air bubble among hairs and pop back to surface.

--Thus attributes of leaf help to keep plant on surface and oriented properly.

The third leaf of each whorl is submerged.

--This leaf in not photosynthetic and it is rather finely dissected.

--Looks like root system--but really dissected leaf.

--This leaf may function like root.

--Roots among aquatic plants primarily provide drag (like anchor) to prevent plant from tipping or being washed or blown away.

--Rhizomes of Salvinia are not known to form any roots.

Salvinia bears sporangia in structure called sporocarp.

--Its sporocarp borne at tips of lobes of the submerged leaf.

Salvinia is heterosporous and individual sporocarps bear only microsporangia or megasporangia.

--Although sporocarps with microsporangia and megasporangia are borne on same individual sporophyte bodies.

--Only one megaspore matures in megasporangium.

--Numerous microspores mature in microsporangium.

Gametophyte development is endosporic.

--although gametophytes are simple and consist of few cells.

Azolla, second genus of Salviniales.

--Also floats on surface of water.

--Roots formed by rhizome and hang in water.

--Rhizome with short internodes and crowded leaves.

Leaves are small and have two lobes.

--Upper lobe of leaf is above water surface and is photosynthetic.

--Lower lobe of leaf is below water surface and is often colorless.

--this lobe is thin, tissue-like.

Upper lobe forms pouch-like region over rhizome.

--Space inside of pouch inhabited by filamentous cyanobacterium Anabaena.

--Anabaena fixes atmospheric nitrogen to a form that plant can use.

--Symbiotic relationship in which plant benefits form nitrogen fixation by cyanobacterium.

Azolla often "planted" in rice fields in Asia so that rice crop also benefits from excess nitrogen fixed by Anabaena.

--Azolla with Anabaena like a living fertilizer in field.

--Much more economical and ecologically sound than western practices of using artificial fertilizers that require a lot of energy to produce.

Azolla also forms sporocarps that contain sporangia

--As in Salvinia, sporocarps bear either megasporangia or microsporangia.

Single megaspore forms in each megasporangium

--Simple female gametophyte with endosporic development.

--Gametophyte forms archegonia.

Microspores become encased in break down products of sporangium.

--Some cells still surrounding microspores form hook-like processes.

--These serve to attach clusters of microspores to megaspore.

--Close proximity of microspores hooked to megaspore (with female gametophyte inside) give sperm produced an advantage in swimming to archegonium of female gametophyte.

Evolution of the Seed march 14

The seedless plants were lumped in a group called cryptogams, the seed plants are traditionally called phanerogams

seed plants are

-- by far the largest group of extant plants

- all seed plants are heterosporus like some of the seedless vascular plants.

- the production of seeds is an extreme form of heterospory that has been modified to form an ovule

The seed is a mature ovule that contains an embryo

The immature ovule consists of a megasporangium (with megaspore) surrounded by one or two additional layers of tissue the integuments

several things led to the evolution of the ovule

-retention of megaspores in the megasporangium (nucellus in seed plants)

- reduction to 1 megaspore mother cell in each megasporangium

- survival of only one of the four megaspores (produced by meiosis of the spore mother cell)

- formation of an endosporic megagametophyte

- retained in the megasporangium ( not free living)

- development of the embryo (young sporophyte) within the megagametophyte

- formation of integuments that completely covers megasporangium except for a small hole the micropyle

- modification of the apex of the megasporangium to microspores or pollen grains

note the shift of the unit of dispersal from the megaspore to the seed

evidence from the fossil record

reading the fossil record is like trying to read a book where 90% of the pages are randomly missing.

The oldest ovules or seeds are from the late Devonian 365mya (handout)

-- Elkinsia polymorpha, ovule with 4 or 5 unfused integumentary lobes

- sometimes called "preovules"

- Archaeosperma arnoldii, another step toward seed

-lower part of the integument fused and only the upper part divided into lobes forming a rudimentary micropyle

The integuments of apparently evolved through gradual fusion of the integumentary lobes until the only opening left was the micropyle

Divisions of seed plants:

- Ginkgophyta

- Cycadophyta

- Coniferophyta

- Gnetophyta

- Anthophyta = angiosperms, (250,000 spp.)

The other 4 divisions are gymnosperms (720 spp.)

General characteristics of seed plants:

- vascular

- secondary growth in some spp.

- megaphylls

- all heterosporus

- no antheridia produced

- all have seeds

seed structure:

Draw

In seed plants the megagametophyte develops within the megaspore (like in marsilea and selaginella ) but it doesn't break out of the megaspore as in these ferns.

Some repetition here,

- only one megaspore(3 disintegrate) remains in megasporangium

- megasporangium is fleshy and is called a nucellus in seed plants (also happens in some spp. of Selaginella)

- integuments envelop the megasporangium

- a small opening is left micropyle for entrance of "sperm"

- integuments + nucellus = ovule

after fertilization the whole ovule becomes the seed (seed = mature ovule)

- the integuments become the seed coat

- the embryo (young sporophyte) develops inside the seed

In modern seed plants the embryo develops on the adult sporophyte but in early seed plants the embryo developed after seed dispersal.

microgametophyte = POLLEN GRAIN

-- (essentially a microspore that has undergone 2 divisions and is 3 cells).

Pollen grains are transferred through process called pollination to the ovule.

--Pollen grains have very resistant outer wall which is often elaborately sculptured and characteristic - palynologists study fossil pollen, (pollen is extremely well represented in record) and pollen cores, can be distinguished from spores

Evolution of Seeds and Their Implications

Appearance in the Fossil Record

--360 mya, oldest seed.

--about 50 my later many different types of seed bearing plants present. seed ferns, cordaites, and conifers.

--Angiosperms appear about 125 mya. (we'll discuss angiosperm evolution when we get to them)

Evolutionary Implications of Seeds

-- Survival Value - because sporophyte develops inside gametophyte with a supply of food. Gives sporophyte a head start, which helps in cold and harsh conditions, when photosynthesis is slowed (enzyme activity slows down in the cold) These conditions were around in the Permian, a period of climactic extremes, and may have helped seed plants make it through the period when many other divisions were lost or greatly reduced