2023-2024 Lecture 5 - Seedless Vascular Plants -updated 2024 (1).pdf
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Seedless Vascular Plants I 2022-2023 Lecture 5 Seedless Vascular Plants 1. Bryophytes have been limited in their ability to radiate and adapt to novel environments – lack of cuticle – lack of developed conducting tissue (xylem & phloem) 2. 3. 4. Individuals that have access to enough sunlight were a...
Seedless Vascular Plants I 2022-2023 Lecture 5 Seedless Vascular Plants 1. Bryophytes have been limited in their ability to radiate and adapt to novel environments – lack of cuticle – lack of developed conducting tissue (xylem & phloem) 2. 3. 4. Individuals that have access to enough sunlight were able to elevate themselves higher and transfer water throughout their tissues. This elevation of the plant body was achieved to a limited extent in the Bryophyta. (e.g., Polytrichum sp.) Selection pressures are hypothesized to have eventually pushed the evolution of vascular tissue, allowing the plant to be lifted off the ground. Greater access to sunlight More successful spore dispersal Seedless Vascular Plants 5. Tracheophytes – plants with vascular tissue 6. Water would be transported up, and photosynthates down the plant. 7. The water-conducting tissue (xylem) developed lignin – the tough, decay-resistant compound that composes wood. 8. Lignin now allowed for much greater structural support which allowed plants to grow significantly taller. 9. The vascular tissue also lead to the development of organs such as: – roots (water absorption), – leaves (photosynthesis), and – stems (structural support). Seedless Vascular Plants 9. These sporophyte stage now assumed greater responsibility for the survival of these spore-bearing vascular plants. 10. The sporophyte was now the larger, nutritionally independent stage of the life cycle, rather than the gametophyte. 11. Diploidy now offered more opportunity for diversification 12. Diploidy also offered greater development of the plant body for the expression of more alleles – Branching sporophytes offered more sites in which meiosis could take place increased opportunities for variation more options in an increasingly competitive environment. Seedless Vascular Plants Increased physical and nutritional support afforded by the vascular tissue allows for – larger physical size of the plant – increased range as there is less dependence on free water – better adaptation to terrestrial habitats – new methods of spore dispersal now open to the plant due to greater height & range Though possessing vascular tissue there is no development of cambial tissue, hence no true 2º growth Increasing independence of the sporophyte from the gametophyte – autotrophic sporophyte Seedless vascular plants The sporophyte is now the dominant phase of the life cycle and is quite varied in form, being self-supporting – Alternation of generations between a diploid sporophyte and a haploid gametophyte Leaves possess developed cuticle Gamete transfer and fertilization is still carried out by water, producing a new plant (the sporophyte) which grows out of the prothallus as it matures. The spores germinate to form an avascular gametophyte or prothallus which is independent from the sporophyte and is usually much smaller (~few mm or cm). o Prothallus may be monoecious or dioecious Bryophytes cf. Ferns Bryophytes: Pteridophytes: 1. The main plant body is a 1. Main plant body is a haploid gametophyte. 2. Cuticle very poorly developed. 3. True roots and leaves are absent. 4. Vascular tissue is absent. diploid sporophyte. 2. Cuticle is well developed, covering epidermis. 3. True roots and leaves are present. 4. Vascular tissue is present. Bryophytes cf. Ferns Bryophytes: Pteridophytes: 4. 4. 5. Archegonia with long neck having 6 rows of cells having 4 rows of cells vertically. vertically. Sporophyte is a capsule 5. Sporophyte is totally Sporophyte is differentiated into root, stem and leaves. supported by a stalk (seta). 6. Archegonia with short neck 6. Sporophyte is initially dependent on gametophyte dependent on gametophyte (heterotrophic). (autotrophic at maturity). Ferns / Fern-Allies Once collectively referred to as the Pteridophyta or Filicinophyta – Polypodiophyta Range from a few cm through 20m tall – Climbers, herbs, “tree” ferns Most ferns are herbaceous plants First major elaboration of leaf morphology – Petiole and lamina differentiated (cf. higher plants) with all possible variations on compound leaf theme Sporophyte now the dominant form – Larger than the gametophyte (quite small!) Ferns / Fern-Allies taxonomy Four Phyla: 1. Psilophyta: 2. Equisetophyta: 3. Lycopodiophyta: – – – Microsporophylls Varied sporophyte forms Some heterosporous 4. Polypodiophyta: – – – The true ferns Large sporophyte Some homosporous Psilotophyta: Whisk ferns are – – – – – small, green whisk-like plants associated with extinct group of plants called the psilophytes limited to 2 extant genera lack true leaves and roots include the oldest known land plants with vascular tissue Sporophyte has – enations (scale-like microsporophylls) – creeping subterranean rhizome and a cylindrical branching stem with a central vascular strand (protostelic) – Rhizoids emerge along the rhizome Classified with ferns due to similarities in the gametophyte generation homosporous Psilophyta e.g. Isoetes Psilophyta: Synangium Psilotum Equisetophyta: Horsetails & scouring rushes Ribbed stems rough to the feel (SiO2) Whorled, scale-like microsporophylls Sporophyte has – Aerial vertical stems with nodes & internodes (protostelic) with canals – Stem develops from a subterranean rhizome – Numerous stomata – Rhizoids emerge along the rhizome Classified with ferns due to similarities in the gametophyte generation homosporous Equisetophyta Equisetum pratense Equisetophyta e.g. Equisetum sp. Lycopodiophyta Lycopodium sp. (Club Moss) Lycopodiophyta: Selaginella plana Polypodiophyta: Azolla Polypodiophyta: Nephrolepis sp. (Common Fern) Polypodiophyta: Cyathea sp. (Tree fern) Polypodiophyta Well developed sporophyte e.g. Nephrolepis: 1. True roots: adventitious 2. Stems: subterranean rhizome 3. Clusters of leaves or megaphylls termed fronds Arise from the rhizome, showing circinnate vernation Typically begin rolled up as croziers or "fiddleheads" – Some eaten as vegetable, e.g. ostrich fern (Matteuccia struthopteris) Circinnate vernation of croziers Sexual Reproduction Seedless vascular plants carry out their a-o-g in either a homosporous or a heterosporous manner: 1. The homosporous life cycle involves one size of spore. o This spore produces a monoecious gametophyte which produces egg and sperm cells. 2. A heterosporous life cycle results in two sizes of spores, a megaspore (female) and a microspore (male). These spores produce separate gametophytes, producing egg and sperm cells respectively. Homosporous Reproduction Only one type of spore produced by the sporophyte – Sporangia housed in clusters borne on the abaxial surface of the leaves called sori (sing. sorus) – Developing sporangia within the sori are covered by an indusium which degenerates as the sporangia mature Reproduction - Sporangia Tapetal layer – Nourishes the developing spores – Coats the spores, forming surface features ridges spines Thick wall Annulus Stomium / Lip cells Sporangial stalk / sporangiophore Dehiscence liberates spores with the help of wind Types of sori Indusia covering the sporangia Reproduction - sporangia Two types of sporangia – Leptosporangia Arise from divisions of a superficial epidermal cell initial Sporangial wall made up of one cell layer – Eusporangia Arise from divisions of many epidermal cell initials Sporangial wall made up of >1 cell layer Hold many more spores Ø Sporangium develops from Group of Ø Sporangium develops from single superficial cell superficial cell Ø Divides transversely to form an outer Ø Divides periclinally into primary wall layers and inner primary sporogenous and inner cell cells. Ø While inner cell forms the stalk, the Ø The outer wall layers form the wall of entire sporangium develops from the sporangium while inner outer cell. sporogenous cells divide meiotically Ø Sporogenous tissue divides and form haploid spores meiotically to give rise to haploid spores. Reproduction The leptosporangiate ferns are more common Taxonomical characters of spore include: a. b. c. d. Shape Size Surface geometry Wall layers Reproduction - Gametophyte Diminutive surficial thalloid Gametophyte or prothallus: – – – – – Usually < 1cm Variously shaped Most often photosynthetic Monoecious or dioecious Gamete transfer requires water for the flagellate antherozoids/sperm cells – Self-sterility operative Gametophyte Fern gametophyte with young sporophyte developing in notch Fern gametophyte with the sporophyte growing out of the notch in the female gametophyte
