Topic 9 Seedless Vascular Plants PDF

UNIVERSITY OF SANTO TOMAS COLLEGE OF SCIENCE MEDICAL BIOLOGY BATCH 2026 BIO422 GENERAL BOTANY LECTURE 1ST SEMESTER 2022-2023 TOPIC 9: SEEDLESS VASCULAR PLANTS...

UNIVERSITY OF SANTO TOMAS COLLEGE OF SCIENCE MEDICAL BIOLOGY BATCH 2026 BIO422 GENERAL BOTANY LECTURE 1ST SEMESTER 2022-2023 TOPIC 9: SEEDLESS VASCULAR PLANTS COURSE OUTLINE INTERPOLATION HYPOTHESIS 1. Overview of Plant Life The interpolation hypothesis postulates that a small 2. Plant Cells and Tissue sporophyte came into existence when a zygote 3. Leaves germinated mitotically instead of meiotically. 4. Roots At a later stage in evolution, the zygote will 5. Woody Structures germinate mitotically and produce a simple 6. Flowers and Reproduction sporophyte that, in the early stages, would have 7. Taxonomy and Systematics consisted of a sporangium and a foot. This would 8. Non-Vascular Plants have resembled a liverwort sporophyte. 9. Seedless Vascular Plants With continued evolution, the sporophyte would have become progressively more elaborate while REFERENCE the gametophytes became simpler. Adapted from: Powerpoint Lecture of Prof. Sabit Book: Mauseth, J.D. (2017) Botany: An Introduction to Plant Biology Sixth Edition. Burlington, Massachusetts: Jones & Bartlett Video Links: TRANSFORMATION THEORY https://drive.google.com/file/d/1-xLlDuZivreaxFXbC2K PKkzutlxxB7ov/view?usp=share_link The transformation theory postulates that after the dibiontic life cycle originated, both gametophyte https://www.youtube.com/watch?v=83KnbZvAzEY&ab and sporophyte became larger, more complex, _channel=Mitochondria and vascularized, in a life cycle with an alternation of isomorphic generations. PDF Links: The transformation theory of the origin of the https://drive.google.com/file/d/11hUitE7n07cipK6V9ZeI vascular plant life cycle postulates that in early land Befnxr3BqYCO/view?usp=share_link plants, the gametophytes are upright structures that are dichotomously branched with an epidermis https://drive.google.com/file/d/1Fwz915mS282dE5EN5 cuticle and vascular tissues, just like the s9fmEY_dakjXGQV/view?usp=share_link sporophyte. With time, the sporophyte became larger and more complex, while the gametophytes became simpler TOPIC OUTLINE 1. Vascular Plants Without Seeds 2. Early Vascular Plants 3. Microphyll Line of Evolution 4. Megaphyll Line of Evolution 5. Ferns VASCULAR PLANTS WITHOUT SEEDS CONCEPTS An important step in the evolution of embryophytes was the conversion of a monobiontic ancestor into dibiontic plants. ○ Monobiontic plants have only one multicellular form EARLY VASCULAR PLANTS ○ Dibiontic plants have multicellular form in RHYNIOPHYTES both the sporophyte and gametophyte The earliest fossils that were definitely vascular land generations plants belong to Cooksonia, a genus of extinct One hypothesis in the evolution is the interpolation plants hypothesis. They had equal dichotomous branching, both An alternative hypothesis is the transformation branches have equal size and vigor. theory. The ends of the branches swollen with large, Currently, one issue that is resolved is that all living multicellular masses of sporogenous tissue and most fossil plants are dibiontic with an surrounded by several layers of sterile cells. alternation of heteromorphic sporophytes and Fossils with these general characteristics are called gametophytes rhyniophytes. Earliest land plants were postulated as having no sporophyte. 1 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE is present in the center. This occurs in the stems of ferns and seed plants. ZOSTEROPHYLLOPHYTES Many features of zosterophyllophytes were similar to those of rhyniophytes, but three characteristics Plants of Cooksonia were homosporous. make us think they were a distinct group. Rhynia and Aglaophyton were other rhyniophytes; ○ Sporangia are lateral early vascular plants similar to Cooksonia ○ Sporangia opened transversely along the ○ In the same rocks with Rhynia and top edge Aglaophyton are fossils of similar plants; ○ Xylem was an exarch protostele. they might have been alternate phases of the same species. Horneophyton, another fossil from the same time, had sporophytes with naked access that branch dichotomously with stomata and terminal sporangia. The similarities between the hornworts Anthoceros, Phaeoceros, and Horneophyton raise the possibility that vascular plants may have arisen from hornwort-like ancestors. Other genera of zosterophyllophytes show that significant morphological changes evolved quickly. Zosterophyllum plants grew as small bunches with cuticle, ordinary epidermal cells, and stomata on upper portions of naked stems. Zosterophyllum plants were similar to rhyniophytes but the ends of fertile branches bore numerous lateral sporangia. ○ Sporangia opened by a suture that passed over the top of the sporangia. XYLEM STRUCTURE XYLEM STRUCTURE Early vascular plants had two types of xylem organization. ○ In both, the center is a solid mass of xylem with no pith called a protostele. ○ In an endarch protostele , the protoxylem is located in the center and the metaxylem differentiates on the outer edge of the xylem mass. ○ In an exarch protostele, the metaxylem is located in the center of the xylem mass and the protoxylem on the edges as MICROPHYLL LINE OF EVOLUTION several groups next to the phloem. LYCOPHYTES Lycophytes represent a distinct line of evolution of the early land plants that resembled zosterophyllophytes. They have lateral sporangia and exarch protosteles, and thus, they may have come from a Zosterophyllum type of ancestor. ○ Several zosterophyllophytes had a smooth surface. ○ Others had outgrowths called enations that ranged from small to long, thin scales. ○ Another type of stele, which did not evolve Enations increase the until later, is the siphonostele; here, pith photosynthetic area of plants. 2 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE The earliest lycophytes were members of the genera Drepanophycus and Baragwanathia. ○ Leaf-like enations were large, up to 4 cm long. ○ Contained a single, well-developed trace of vascular tissue ○ Enations of Lycophyta are called microphylls Isoetes is a genus of small, unusual plants called quillworts; their body consists of a small, corn-like stem that has roots attached below and leaves above. ○ Although Isoetes is a small plant, its short, basal corn-like stem has a small amount of secondary growth. Another important advance was the evolution of true roots. This allowed lycophyte sporophytes to: ○ Anchor firmly ○ Absorb efficiently ○ Grow to tremendous size Many extinct lycophytes such as Lepidodendron, Sigillaria, and Stigmaria had a vascular cambium and secondary growth ○ Wood resembled that of pines having a pith, rays, and elongate tracheids. ○ However, vascular cambium cells could not undergo radial longitudinal division so SIDE NOTES: new fusiform initials could not be Lycophytes represent an ancient line of evolution produced. distinct from seed plants but having convergent ○ Cambial cells became increasingly wider evolution in several characters: leaves, roots, tangentially as the wood increased in secondary growth, and almost seeds circumference. Cambial cells eventually stretched too much to function. The anatomy and morphology preserved in fossil In many lycophytes, sporangia are clustered lycophytes indicate many instances of together in compact groups called cones or strobili convergent evolution with seed plants. for protection. Although many remained homosporous, others became heterosporous, having microspores and MEGAPHYLL LINE OF EVOLUTION megaspores that give rise to distinct EUPHYLLOPHYTES microgametophytes and megagametophytes. TRIMEROPHYTOPHYTA Division Trimerophytophyta was proposed in 1968 for three genera of extinct plants: Trimerophyton, Psilophyton, and Pertica. o Trimerophytes, however, are considered a distinct advancement out of rhyniophytes because of several special features Trimerophytes had an unequal branching in which one stem was more vigorous. This is called overtopping. There are approximately 200 extant genera o Overtopping: on the evolution of unequal Lycopodium species are small herbs with prostrate branching, the ability of one shoot to grow rhizomes that have true roots and short upright for a longer time as compared to other branches; they are homosporous. shoots. Selaginella is heterosporous, and the o Eventually, the branching became more megagametophyte develops inside the megaspore regular and controlled. This existed until wall. the Upper Devonian Period. It came to an ○ Selaginellas can be distinguished from end by evolving into the ancestors of ferns lycopodiums by a small flap of tissue, the and seed plants. ligule, on the upper surface of Selaginella Pertica displays pseudomonopodial branching, leaves. having a single main trunk rather than a series of dichotomies. o Ancestor to the stem structure of seed plants. 3 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE They consist of several genera of extinct plants and one genus, Equisetum, with 15 extant species known as horsetails or scouring rushes. o Their aerial stems have a characteristic jointed structure, with a whorl of fused leaves found at the nodes. o Stems have a pith, and thus, these are siphonosteles Reproductive structures in Equisetum are specialized; sporangia always occur in groups of ORIGINS OF MEGAPHYLLS (EUPHYLLS) 5-10 located on an umbrella-shaped At least three distinct types of homoplasic structures sporangiophore. called leaves occur in plants. Early arthrophytes have true monopodial growth. o Leaves on gametophytes of nonvascular plants o Enations/microphylls of zosterophyllophytes and lycophytes o Megaphylls, leaves that evolved from branch systems, and are present in all seed plants, ferns, and equisetophytes. SIDE NOTE: Although equisetophytes are an ancient clade with only a few surviving species, the plants are by no means primitive, poorly adapted, or rare. Megaphyll evolution is summarized by the telome theory. FERNS o Twigs on the last dichotomy are called telomes. FERNS o Dichotomous branching becomes The rest of the monilophytes have more than 12, overtopping 000 species o All subdivisions of a lateral branch became o Almost all of them are called aligned in one plane (planation) leptosporangiate ferns o Thin sheets of chlorophyll-containing The other groups are called: parenchyma webbing develop between o Marattiales telomes. o Ophioglossales o Psilotales Can be found in almost any habitat FERN VASCULAR CYLINDER If the branch system produces sporangia, the resulting structure would not be a leaf, but a sporophyll. We believe that all megaphyllous plants form a monophyletic clade now, referred to as the euphyllophytes. Euphyllophytes are united by three synapomorphies: o Roots have exarch xylem o Megaphylls o A 30-kilobase inversion in the large single-copy region of their plastid DNA The euphyllophytes contain two sister clades: o Monilophytes (ferns and fern-allies) All ferns are perennial and herbaceous o Lignophytes (woody plants) The vascular system of the stem is an endarch siphonostele EQUISETOPHYTES Equisetophytes have been classified as division Arthrophyta (also called Sphenophyta). 4 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE At each node, a leaf trace is found. LEAF TRACE A segment of the vascular cylinder that diverges from the siphonostele This divergence leaves behind a small segment that is made of parenchyma This new parenchymatous segment is what we call the leaf gap FERN LEAF PRIMORDIA The sorus contains many sporangia Each sporangium consists of a stalk, and the actual sporangium Once the cells of the annulus lose moisture/get Fern leaf primordia have the stake apical cell dehydrated, a crack or opening will be created. As the primordium grows, it curves inwards This opening will expand rapidly to disperse the spores into the air. FIDDLEHEAD/CROZIER The tightly coiled young leaf of a fern Once the fiddlehead matures, the curl unfolds, stretches, and becomes the adult form of the leaf FERN SORI SORI (SORUS - SINGULAR) Clusters of sporangia found on the underside of the adult fern leaf Where meiosis occurs Side note When the fern spores start to germinate, they grow into small, simple heart-shaped or ribbon-shaped photosynthetic gametophytes with rhizoids. EUSPORANGIA AND LEPTOSPORANGIA 5 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE The ferns contain these two types of sporangia and The sorus contains many sporangia. The outer wall they differ in the fundamental aspects of of a sporangium consists of a layer of protective development. jacket cells As a sporangium matures, a row of jacket cells EUSPORANGIUM enlarge to form a band called an annulus. Initiated with several surface cells that undergo On the opposite side of the sporangium, several periclinal divisions. delicate lip cells form. Below the jacket, there are two layers of cells called PERICLINAL DIVISION the tapetum, which nourish the fertile tissue within Cell division where they occur parallel to the tissue/organ surface The fertile tissue consists of sporocyte cells As a result, we get two rows of cells stacked one ○ Each sporocyte cell is diploid, containing over the other two sets of chromosomes, one from each parent ANTICLINAL DIVISION As each cell matures, its nucleus divides twice Perpendicular to the adjacent layer of cell through meiosis As a result, we get two columns of cells instead of Each daughter chromosome now contains one set two rows of chromosomes. The cytoplasm of the sporocyte undergoes cleavage which results in a cluster of LEPTOSPORANGIUM four adhering cells called a tetrad Initiated with a single cell The tapetum now begins to break down, depositing The surface cell divides periclinally and forms a a very tough protective coat of sporopollenin small outward protrusion (See Figure F.) surrounded by this thick resistant wall, and each cell is now called a spore FERN LIFE CYCLE Opening of the sporangia is accomplished by the annulus The annulus begins to dry out as water evaporates from its surface The tension or pull between the remaining water molecules and the wall now increases The thick inner and side walls resist this, but the outer walls are easily pulled inward, causing the annulus to contract Shortening the annulus tears the lip cells apart and makes the spore case open further When too much water is lost, the water molecules are no longer able to hold together, so the pull on the wall is released and the annulus springs forward, scattering out the spores This process is repeated in thousands of sporangia on a leaf so that large quantities of spores are released The familiar fern is known as a sporophyte. If a fern frond is turned over, small circular areas can be seen ○ Each of these areas is called a sorus In some ferns the sorus is unprotected while in many others it’s covered by a cap, called an indusium 6 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE Being close to the soil the small gametophytes are easily flooded when it rains this water plays an important role in fertilization. Water stimulates the cap cell on the antheridium to open, releasing the sperm cells the flagellated sperm are now able to move within the water in search of an egg. Sperm consists of a spiral cell body bearing numerous flagella that move it forward. Water also stimulates the archegonia to open the contents of the neck canal then diffuse into the surrounding water where they act as a sperm attractant. The attractant stimulates a sperm to swim towards the open archegonium. It then swims into the opening and moves up the neck canal towards the egg cell. Fertilization is accomplished when the egg and When spores land on a moist surface, those which sperm nuclei fuse.This creates a single deployed germinate first will form bisexual hermaphroditic cell, the zygote. individuals. A rhizoid emerges and attaches the spore to the soil. This is followed by a sheet of cells which is the young gametophyte or prothallus. A notch forms on the prothallus that contains dividing cells. ○ This is called the notch meristem. Continued growth results in a heartshaped bisexual gametophyte. The lower surface is firmly attached to the soil by numerous rhizoids. Male gametangia called antheridia are formed at the posterior end of the gametophyte. The outer wall of an antheridium consists of ring cells and a cap cell which surround fertile tissue. At the anterior end of the prothallus, close to the notch, there are female gametangia called archegonia. An archegonium consists of a neck containing a The fertilized egg remains attached to the neck canal cell. At the base of the neck there is a gametophyte inside the archegonium. swollen region called the venter that contains an On the lower surface of the gametophyte, the egg cell. zygote begins to divide, forming the embryo's Gametangia of both sexes may be present at the sporophyte plant. same time or at different times. The timing of their The uppermost part of the embryo of the foot appearance will determine if there will be self absorbs nutrients from the surrounding fertilization or cross fertilization. gametophyte tissue. As the embryo grows, the In some ferns, archegonia are formed first and venter tissue stretches and eventually ruptures. when flooded with water, a hormone called A root now grows into the soil for support and to antheridiogen is released. absorb water. This hormone will stimulate adjacent plants to stop Other parts of the embryo form the first leaf and the growing and form antheridia, but no archegonia. future stem the rhizome. A bisexual gametophyte in a female phase may The leaf grows through the notch in the therefore be surrounded by several male gametophyte to reach the sunlight. gametophytes increasing the chances of cross Eventually, the rhizome emerges and provides fertilization. additional leaves and roots. In this way, a new plant is formed that matures into another spore producing fern completing the reproductive cycle. FERNS: PSILOTUM AND TMESIPTERIS PSILOTUM Also known as whisk ferns They are known to be highly derived and highly simplified ferns Do not have the capacity to make roots and leaves Occurs in tropical and subtropical regions 7 I 1MBIO4 BIO421 GENERAL BOTANY LECTURE SYNANGIUM Fused sporangium MSEPTERIS Supported by a bifurcate (“forked”) appendage Limited to Australasia o Primarily Australia and other South Pacific islands VASCULAR CRYPTOGRAMS Have very short gametophytes and have cylinders less than 2 mm in diameter Traditionally, the plants of this chapter have been referred to informally as vascular cryptogams (or often as “ferns and fern allies”). What is most striking about these plants are the many features they lack: seeds, flowers, fruits, and so on. However, we construct cladograms on the basis of shared derived features—synapomorphies—not on a lack of features; therefore, we do not group vascular cryptogams together formally. Dichotomous branching can be observed in Psilotum nudum ENATIONS Structures that resemble small leaves but have no vascular tissue 8 I 1MBIO4