LOWER VASCULAR PLANTS These are the spore bearing plants in which the main part of the plant is diploid (sporophyte). There is a small multicellular haploid (gametophyte) portion of the life cycle, however, which lives separate often for only a short time in the wet season. This is a sporic life cycle then, with emphasis on the sporophyte. Representative phyla include; Psilophyta, Lycophyta, Sphenophyta, Pterophyta

Moss and Fern Life Cycles

Group 1: Seedless, Nonvascular Plants Live in moist environments to reproduce Grow low to ground to retain moisture (nonvascular) Lack true leaves Common pioneer species during succession Gametophyte most common (dominant) Ex: Mosses, liverworts, hornworts

Moss Life Cycle

1)Moss gametophytes grow near the ground (haploid stage) 2) Through water, sperm from the male gametophyte will swim to the female gametophyte to create a diploid zygote 3) Diploid sporophyte will grow from zygote 4) Sporophyte will create and release haploid spores Diploid sporophyte . . . . . zygote egg egg zygote zygote egg egg zygote Haploid gametophytes male male female female female male female male

5) Haploid spores land and grow into new gametophytes 6) The process repeats . . . . . ground Haploid gametophytes

Haploid gametophytes . . . . . sporophyte male male female female zygote egg zygote egg egg zygote egg zygote Haploid gametophytes male male female female female male female male

Psilophyta The first fossil land plants had horizontal and vertical shoots. They were either leafless or had minute leaves. One of these plants is called Rhynia. Reconstruction of Rhynia one of the first fossil land plants. Cross section of a Rhynia Stem: Note the concentric circles of primary tissues. The term Stele is is used to indicate the Xylem and Phloem

The Psilophyta contains a genus Psilotum which closely resembles plants like Rhynia. It is essentially a stem that has two interconnected forms. The underground stem (Rhizome) produces Aerial Stems which are Photosynthetic. The Leaves are minute and contribute little to the plant's nutrition. The Rhizome produces Rhizoids which act like root hairs. Psilotum Aerial Stems: These have an Apical Cell and they Branch Dichotomously.

One terminal branch from Psilotum One terminal branch from Psilotum. The Leaves originate at the Shoot Apical Meristem. They have a Spiral arrangement (Alternate). The stem has ridges and valleys. The stomata (white spots) occur in the depressions. This provides a small amount of protection from excess water loss when the Stomata are open. Dichotomous Branching occurs at the Apex by the formation of a second Apical Cell. In this case the branches are the same size. This is called Isotomous (Iso = Same).

Psilotum Underground Rhizome: Some Rhizome tips start to grow upwards and are converted into photosynthetic shoots. No Roots are produced. The Rhizomes serve as roots and they produce Rhizoids which act like root hairs.

The Psilotum Rhizome has the most simple kind of tissue organization with Xylem at the center, surrounded by Phloem, Endodermis, Ground Tissue and Epidermis. Cross Section of the Stele from Psilotum Rhizome: Note the Thick-walled Lignified Xylem which is surrounded by Phloem and the Endodermis.

Psilotum Stele: The Xylem is star-like and is embedded in Phloem Psilotum Stele: The Xylem is star-like and is embedded in Phloem. This is called an Actinostele (Actino = Star). The Xylem of Psilotum contains Tracheids which are similar to those seen in other Vascular Plants. These have characteristic Secondary Wall thickenings, unlike the Hydroids we examined earlier. The Phloem contains Sieve Elements which are similar to those seen with other Vascular Plants, unlike Leptoids which do not have all the features of Sieve Elements.

The anatomy of the Aerial Stem is similar to that of the Rhizome The anatomy of the Aerial Stem is similar to that of the Rhizome. However, the Xylem is star-shaped rather than circular and the Ground Tissue contains Sclerenchyma and Chlorenchyma. Outer part of the Stem showing the three types of Ground Tissue and the Epidermis which has a definite Cuticle and Stomata. There is a debate concerning the Leaves of Psilotum. They do not have Veins although Leaf Traces may diverge from the Stele and approach the leaf bases. I think that they are leaves. If we assume that they had one Vein, they would be Microphylls.

The next step lead to the evolution of distinct Shoots and Roots The next step lead to the evolution of distinct Shoots and Roots. The shoots were specialized for Photosynthesis and the Roots were specialized for absorption and anchorage. Root anatomy would not differ significantly from the cylindrical Rhizome we saw earlier. Furthermore, Root Anatomy is very constant in Land Plants. Branching occurred in the roots and produced a complex root system. Upright stems were able to overtop flat thalli and became dominant organisms. The production of photosynthetic branches further increased the surface area available for photosynthesis. Branching was initially Dichotomous for stems and roots. Lateral branching developed later in Evolution. The ascent of aerial stems and their branches required the development of superior support tissues. This was achieved with the Psilophyta. Xylem Tracheary Elements have thick walls and provide some structural support. However, Sclerenchyma (Scler = Hard) tissue evolved and Sclerenchyma Fibers provided extra mechanical strength. Fibers are found in the Vascular Tissues or in close association with them. Sclerenchyma and Parenchyma are the two principal Ground Tissues. The final step in the evolution of our theoretical plant is the production of Leaves. These are highly specialized for Photosynthesis in many ways. They usually have a wide thin Blade (Lamina) and a thicker Midrib. They may have only one Vascular Bundle (Vein) or a network of interconnected Veins. They are attached to the stem by a nonphotosynthetic petiole. The first Leaves were Microphylls and had one Vein/leaf. The vein had a central location and was surrounded by Photosynthetic Parenchyma. The major functions of the stem are translocation of water and photosynthate and structural support for the leaves. Our theoretical plant has three distinct Organs (Leaf, Stem & Root). Species in the Lycophyta illustrate simple plants that have Microphylls.

4. Spores (n) are produced in trilobed structures on the tips of short branches called synangia. tapetal plasmodium sporangial wall

Psilophyte spores (n) are kidney-shaped.

Psilophyte spores (n) are slow to develop into monoecious gametophytes (n). archegonia

Psilophyte spores (n) are slow to develop into monoecious gametophytes (n). antheridium

Development from the fertilized egg (zygote 2n) proceeds in the archegonium.

Lycophyta Plants in the Lycophyta have erect stems as well as Stolons and Rhizomes. They are relatively large compared to Hepatophyta, Bryophyta and Psilophyta but they rarely exceed a meter in height. They can be epiphytic and pendant stems can be more than a meter in length. They have Microphylls and Roots. Branching is Dichotomous for both organs. The Apical Meristem has several "Initials" rather than a solitary Apical Cell. Long Section of a Lycopod SAM: Note the presence of several large "Initials" at the summit of the stem. SEM photo of a Lycopod SAM

A composite Lycopodium These plants are most abundant in the tropics but a few can survive cold, dry environments. A species growing in the Arctic/Alpine zone near Jasper Canada. This plant is approx. 10 cm in length A composite Lycopodium

Vertical Lycopodium with Isotomous Branching Vertical Lycopodium with Isotomous Branching. This plant is about 15-20 cm tall. A Pendant Epiphytic Lycopodium from Puerto Rico. note the Dichotomous Branching. This plant is over 1 m in length. A local Lycopodium showing Isotomous Branching and well developed Microphylls Lycopodium lucidulum: Note the Vertical Stem. The Roots are adventitous and originate in the stem. They branch Dichotomously.

Shoot tip of L. lucidulum: Microphylls Galore!!!! Roots from the image above: Note the Dichotomous Branching. Cross section of a Lycopodium Microphyll: Note the uniform (unspecialized) Mesophyll and the Cuticle. Stomata are visible on the upper side of the leaf

The species to the right grows locally on disturbed sites and could be a candidate for soil stabilization research. It is a complex plant which has horizontal Stolons which have Isotomous Branching. These produce the Roots which anchor the plant to the substrate. The Stolons also produce Aerial Shoots which have Anisotomous (unequal) branching. The upright stems are about 1 m in height.

Cross section of a Lycopodium Rhizome: The Stele is more complex than that of the Root above. Bands of Xylem are surrounded by Phloem. Note the extensive Sclerenchyma in the Ground Tissue. Simple Stele from a Lycopod Root.

Stele from a Lycopodium Stem: The Xylem resembles sheets or flaps Stele from a Lycopodium Stem: The Xylem resembles sheets or flaps. Each arm is surrounded by Phloem. This is called a Plectostele (Plecto = Plate). This is more complex that the stele in Psilotum. The Leaf Traces are small Vascular Bundles which diverge from the central stele and connect with the vein in the leaf. This produces an integrated vascular system that include the three major vegetative organs. Cross section of L. lucidulum Aerial Stem:The Cortex contains Parenchyma. The Xylem (stained blue) is prominent in the Stele.

While extant Lycopods are small plants with little ecological significance. Forests of tree-sized lycopods once dominated certain  habitats. The most famous of these is Lepidodendron which reached heights up to 30 meters. They had secondary growth. The stems were coated with leaf bases and there appeared to be little internodal elongation. The latter implies that the trees had slow growth which may partly account for their extinction. Lepidodendron Stem Base

Reproduction of Lycophyta: Lycopodium Sporangia are be located in the axil of leaves. Axil refers to  the angle formed between the upper surface of the leaf and the Stem. The upper surface is called Adaxial. The lower surface is Abaxial. (Ad = Towards; Ab = Away). Adaxial means the surface towards the stem. Abaxial means away from the stem. Your intrepid teacher will give a dramatic demonstration at this point! The Sporophylls may closely resemble Vegetative leaves. Immature Sporangia are Green they turn yellow as they mature and open (below). Sporophylls can be organized into Terminal Cones (Strobili). In this case the sporangia are still adaxial. The sporophylls are green and tightly closed until the sporangia mature. The sporophylls turn yellow, dry & become reflexed upon maturation. The sporangia open like those to the right.

Lycopodium is Homosporous. The Spores contain a high concentration of Oil. They were used as flash powder during the early days of Photography. SEM Photo of Lycopod Spore Lycopodium Spores

Sporophyll from Lycopodium

Microscopic view of Immature & Mature Lycopodium Cones Ventral view of Lycopod  Gametophyte with SEM. Lycopodium Sporangium

The Gametophytes do not have recognizable Organs The Gametophytes do not have recognizable Organs. They produce Antheridia & Archegonia. Rhizoids are also present. SEM image of a collapsed Antheridium and Sperm (red) from Lycopod Gametophyte. The Sperm swim through an opening created by the cell that occupies the apex of the Antheridium. Archegonia (Yellow) on the Gametophyte a. Fertilization occurs when the biflagellate Sperm reaches the egg.

Lycophya-Selaginella-Repro-2 Selaginella is another major Genus in the Lycophyta. It has many similar features with Lycopodium. I want to concentrate on the reproductive traits of Selaginella which are significantly different from Lycopodium. Selaginella is Heterosporous!! The Sporophylls of All Species are assembled into Cones. The Cones occur at the tips of short branches. Selaginella is Heterosporous. It produces Microsporangia which have many small Spores. These resemble the sporangia and spores of Lycopodium which is Homosporous. Selaginella also forms Megaspores in Megasporangia. There are only Four Megaspores in each Megasporangium. The Megaspores are large compared to the Microspores. The four Megaspores contain the same biomass as all of the Microspores. This accounts for the size disparity. Why might this be advantageous for successful sexual reproduction? The Sporangia occur in the leaf axils. There are no obvious differences between the leaves which bear Megasporangia or Microsporangia. There is usually a color difference between Megasporangia and Microsporangia. Due to the size of the Megaspores, the outline of the Megasporangium may be distorted, as well. There are various distribution patterns for Micro & Megasporangia in the cones. They are NOT randomly arranged.

Selaginella Cones are four-sided and rectangular in outline. In this case the Microsporangia are red-brown while the Megasporangia are green-yellow. Selaginella Cones are four-sided and rectangular in outline. Cleared Selaginella Cone showing Mega- & Microsporangia Long Section of a Strobilus with many Megasporangia and some Microsporangia The Megaspores are much larger than the Microspores!

Selaginella Gametophyte Development is Entirely Endosporic!!! Each Microspore produces one Microgametophyte. Each Microgametophyte produces one Antheridium. The Microgametophyte is essentially an Antheridium within the Spore Wall. The Endosporic Development of the Antheridium means that it is protected by the Spore Wall plus its own Jacket until it is mature. This helps insure that viable Sperm are produced by each Antheridium. The Sperm are Biflagellate like those of Lycopodium.

Megagametophyte with Archegonia Meiosis produces four Megaspores in each Megasporangium. Each Megaspore is a Large Cell inside a thick, layered wall. The Megaspore cell contains a lot of Storage Lipids. It has a Triradiate Ridge which has three crests or peaks. Cell Divisions start beneath the crests & nowhere else.  These lead to the formation of a relatively thin cellular pad that lies on top of the storage lipids. Only a small fraction of the Megaspore becomes cellular. Archegonia differentiate in the cellular pad, just beneath the crests of the Spore Wall. This reminds me of embryo development in chicks. The embryo starts out as a small disk of cells that floats on the lipid-rich yolk. Whole Megaspore:Note the Triradiate Ridge Archegonia formed in the cellular region of the Megagametophyte Megagametophyte with Archegonia Sections through a megagametophyte

The Megaspore wall splits open near the Archegonia. Fertilization requires the presence of free water so that the Sperm can swim to the Egg. Embryo development is Endoscopic. It grows into the Megagametophyte and derives nutrition from the Storage Lipids. It is also protected from desiccation and other injuries by the thick spore Wall. The Shoot Apex eventually grows out of the opening in the Megaspore Wall. A Root Apical Meristem forms opposite the Shoot Apical Meristem. The Foot remains like an haustorium in the food reserve and the Megagametophyte remains attached to the young Sporophyte for some time. The Shoot apex eventually responds to gravity and grows out of the Megagametophyte. Sporophyte with discernable Root & Shoot: still attached to the Megagametophyte Embryo Development is Endoscopic

The Overall pattern of Development resembles that of a Seed.

Phylum Sphenophyta (Horsetails) Block 7 April 2009 Phylum Sphenophyta (Horsetails) Jennifer Baldwin – Product Representative Lauren Cox – Text Research and Development Danielle David – Image Research and Development Liz Fredrickson – Technical Support