VASCULAR PLANTS WITHOUT SEEDS
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VASCULAR PLANTS WITHOUT SEEDSThere are four living divisions of seedless vascular plants ("lower vascular plants") - Psilophyta, Lycophyta, Arthrophytes and Pteridophyta. Extinct taxa have been discussed in PLANT BIODIVERSITY I. The seedless vascular plants differ from the nonvascular plants in the following respects. Vascular system with lignified cells. Sporophyte dominant - usually perennial. Mostly the sporophyte has true stem, leaves and roots. Gametophyte small, inconspicuous - usually annual. Have apical meristems.
Simplest vascular plants. Two living genera. Tmesipteris (8 species) usually epiphytic, relatively large, flattened lanceolate leaves, humid forests NZ and Australia. Psilotum (whisk fern, 2 species) is more widespread, found throughout the tropics and subtropics extending into temperate regions eg. New Zealand, Arizona and in Europe in S. Spain. Habitat ranges from open soil to forests and cliff edges. Psilotum nudum is the commonest.
Psilotum is unique among vascular plants, having no roots or leaves. Branched rhizomatous system, short multicellular rhizoids. Prostrate rhizome has conspicuous cuticle, mycorrhizal association - main absorptive surface of rhizome. Erect chlorophyllose shoots from rhizome, 15-20 cms, branch dichotomously, further branching sometimes in 3's in a different plane. Shoot has longitudinal grooves with stomata abundant in the grooves infequent. on ridges - visible as whitish spots. No leaves, small enations. Sporangia at ends of branches, subtended by a bifid (= forked appendage). Spore bearing organ is called a synangium, formed by fusion of 3 sporangia. Sporangia walls are several layers of cells thick (multistratose) and are of multicellular origin - eusporangiate - very common in vascular plants. Spores are all of same size - homosporous. Spores germinate to produce gametophyte -small - subterranean looks like sporophyte rhizome -almost immediately invaded by mycorrhiza -no chlorophyll, entirely dependent on mycorrhiza for nutrition. Unusual - Simple lignified vascular tissue in gametophyte. Monoecious. Antheridia release ~ 250 sperm, multiflagellate,- swim through film of water to archegonia. Archegonia embedded in body of gametophyte - not stalked and free like mosses.
Family Lycopodiaceae. Common name - club mosses. 3 species in Wales, Hupersia selago found on uplands in this area. Lycopodium clavatum (creeping) found from arctic to tropics. Many species rare, restricted to tropical rainforests. Sporophyte evergreen, small spiny leaves on shoots - microphylls (evolved from enations). Strobili (area of shoot where sporangia are formed) at end of shoots - sporangia subtended by sporophylls (usually different shape from vegetative leaves) which dry out to expose eusporangiate sporangia. Thick spore walls - may be several years before germination - can be considered an advanced feature allows dispersal of plant in time as well as space. Gametophyte structure varies, some species thallus, some species carrot shaped. If growing on surface then chlorophyll and sometimes mycorrhiza, if subterranean then mycorrhiza is obligate. Archegonia and antheridia similar to Psilotum - simple in structure. Sperm - biflagellate, fertilization probably facilitated by chemotactic attraction - archegonia produce citric acid.
Family Selaginellaceae. Common name - spike mosses. Resemble Lycopodium in vegetative features. Sporophyte evergreen, spirally arranged leaves. Some species have leaves all alike, in some shoot is dorso-ventrally flattened with leaves in 4 ranks. Most of 700 species in tropics and subtropics. Majority have cushion habit, some have a vine like form. The resurrection plant (tumbleweed in Westerns). A big difference from Lycopodium (in all species of Selaginella) is heterospory. Spores still produced in tetrads, some spores very large - megaspores. Four fill a megasporangium Microsporangium produce ~ 600 microspores. Spores are dispersed, germinate into 2 types of gametophyte, megagametophytes and microgametophytes. The strobilus is similar to Lycopodium but with megasporangia towards the bottom. Also in Selaginella the start of another important trend - endosporic development - cells differentiating within the spore wall. The megagametophyte is exposed when spore wall splits as gametophyte increases in size. Gametophyte remains small with many archegonia embedded in its surface. The microspore becomes the microgametophyte - this is so reduced that it is itself the single antheridium. When the spore wall is ruptured numerous biflagellate sperm are released. Gametophyte phase is negligible. Effects of this are -
1. Gametophyte always dioecious, increasing chance of cross fertilization.
2. Male gametophyte develops entirely within spore wall eliminates problems of adverse environmental conditions which free living gametophytes must endure
3. Dispersal is more ineffective (especially for megaspores - few, heavy) and need a spore of both types for sexual reproduction.
In some species of Selaginella megagametophyte remains within megasporangium until fertilization has occurred - very similar to the maintenance of a seed on a plant - can be considered an advanced feature. Heterospory evolved in Devonian - similar but independent evolutionary events resulting in retention of megaspores eventually led to evolution of the seed in gymnosperms. This group is showing the first stage of the evolution of the seed. Seeds are always heterosporous.
Equisetum (common name - Horsetails) is the only extant (living) genus of this group. Most abundant and diverse about 300 Mya - some species immense woody trees which formed extensive forests. Today 15 species arctic to tropics except Australia and New Zealand. Generally small, herbaceous perennials, largest (a tropical one) up to 8 m tall but still only 2 cm diameter. Evolutionary history parallels lycophytes, some respects more advanced, some respects more primitive. Sporophyte - jointed stem, usually grooved. Leaves and branches in whorls at nodes. Roots also arise at nodes. Sporangia produced on a strobilus at tips of reduced branches. Spores have elators attached to aid dispersal. Some species heterosporous, most homosporous. Gametophyte generation is exosporic - develops outside the spore after germination. Gametophyte usually a small chlorophyllose thallus. Monoecious, requires water for fertilisation so mode of sexual reprodution is basic, similar to the other groups. More advanced in having highly differentiated stem anatomy. Clear specialization of cells and tissues for conduction of water, nutrients, undertaking photosynthesis.
Ferns -largest group of non-seed producing vascular plants. Important element of the world flora with about 10,000 species. Not a relict group, continuing as an actively speciating group. Particularly prevalent and conspicuous in warm, humid regions. Largely herbaceous, some arborescent -look like simple trees. Size from a few cms to 20 m. Adapted to a variety of habitats, 500 species in boreal and arctic latitudes - most in tropical rainforests. Fossil record is very rich but so many species so evolutionary lines between them are not clear. Some families found today are found in the Carboniferous so these are amongst the most ancient of living plants. Leaves (fronds) - usually easily recognized, 90% have compound leaves. Leaves usually evergreen, arranged spirally on small rhizome or stem. Anatomy of roots and leaves similar to the general pattern of seed plants.
Central part of frond is an extended petiole, the rachis. Leaflets on the subdivided blade = pinnae, and may be further dissected into pinnules. Immature frond tightly coiled (= fiddlehead) uncoils as it grows. In most species sporangia are borne on leaves, in some species on the stem. Most homosporous, 2 orders heterosporous. Ferns have a relatively primitive system of reproduction, and evolutionarily advanced vegetative features.
Ferns can be separated into to major groups on the basis of their sporangia. In primitive ferns the sporangia are eusporangiate as they are in all other vascular plants. In most ferns the sporangia is leptosporangiate and these are termed true ferns. Eusporangiate = multicellular in origin, sporangium wall several layers of cell thick. Leptosporangiate =- single cell origin, sporangium wall - 1 cell thick, contains no more than 64 spores - allows plant to produce many sporangia very quickly eg. Helypteris up to 50 million spores per plant per season. Sporangia usually clumped in masses called sori on abaxial surface (underside) of the frond. Often sori protected by a flap of tissue called the indusium - positioning of sori and the nature of the indusium if present are diagnostic features which help in identification of species. Spore dispersal is by a ring or annulus of differentially thickened walled cells in the wall of the sporangium. As water evaporates from these cells tension is released and annulus snaps back to throw spores into the air. Spores germinate if conditions are light and humid. Light essential for germination of many fern spores - some seem to have a phytochrome system - red light promote germination, far red inhibits. Germinated filaments require blue light to change develop from a filamentous to thallus structure.
Prothallus - (analagous to protonema in bryophytes) usually heart-shaped. Consists of 2 flattened lateral "wings", an apical "notch", unicellular rhizoids. Antheridia and archegonia formed on underside of the thallus. Each antheridia produces 32 multiflagellate sperm. Chemotactic, water required for fertilization. Zygote divides and differentiates into a shoot and a root, become photosynthetic and independent of the gametophyte growing out of gametophyte tissue like mosses but with no dependency.
These groups evolved in parallel - but we can identify advanced features.
Heterospory - preliminary to the evolution of the seed.
General changes in structure of sporangia.
All require water for sexual reproduction
Advanced vegetative features.
GYMNOSPERMS Seed-bearing flowerless plants. From the Greek "naked seed". It is with this group that we see a major step in the advancement of the reproductive systems of plants. - the evolution of the seed. A number of very distinct classes within the gymnosperms, they are found in a great range of habitats and show great variety in their structure. In the same group because they have similar reproductive patterns.
Gymnosperms have a common ancestry among a group of Devonian plants known as the progymnosperms. Progymnosperms are an extinct group of plants which show the link between the lower and higher groups of vascular plants as they represent the earliest stages of leaf development and the development of the internal anatomy which permitted continuing lateral growth. In terms of their reproduction, many progymnosperms were still primitive in that they produced spores all of equal size - homosporous. But it is thought that a number were heterosporous and produced seeds. The earliest fossil seeds were found in sediments from the Upper Devonian - 35 Myrs after the first vascular plant appeared. It seems that in the Lower and Middle Devonian there was an increasing variation in the sizes of spores produced by homosporous plants. The key change occurred when large spores started to produce the female gametophyte and the small spores produced male gametophyte. In other words, when the difference in spore size was also accompanied by a separation of the sexes - heterospory. The next stage in the evolution of the seed was the development of the integumentary lobes, from the tissues surrounding the megasporangium. Such lobes are seen in the earliest seed from the Upper Devonian and they appear to have been produced by the reduction and fusion of series of branches which surround the megasporangium. This theory is supported by the fact that seeds have been found with different degrees of integumentary enclosure i.e. it seems that the megasporangium has been progressively enveloped by the sterile material. That gives us the ovule, which consists of the megagametophyte (endosporic development), the nucellus (megasporangium), and integuments. Once fertilized = seed.
The pollen grain is really the microspore of seed plants. Spores from the Devonian gradually evolved into an intermediate stage known as "pre-pollen" in the Carboniferous, and then to true pollen in the Mesozoic. This evolutionary series involved the reduction of the male gametophyte and its increased retention inside the protective spore wall, and finally, germination by pollen tubes then appeared. Seeds evolved in the Devonian, pollen evolved much later. So why did seeds and pollen evolve?
Essentially, seeds and pollen represent a very reduced and enclosed gametophytic phase. This reduction and enclosure seems to be partly in response to the development of the tree habit, which was evolving at the same time.
1. It takes energy and nutrients to produce eggs and sperm. Maintaining the small gametophyte on the large sporophytic plant, protected and hidden from the elements and from predators - gives far greater efficiency of multiplication and dispersal of the organism.
2. Due to the evolution of pollen the whole gametophyte plant carrying the sperm cell can now travel, in air currents, to the reduced female gametophyte so we've lost the need for water to be involved in the process.
3. After fertilization the sporophyte can provide extra nutrition and continues to protect the embryo. This is an intermixing of the gametophyte and sporophyte phases.
4. Finally, the whole young sporophyte can still be dispersed - in all other cases the young sporophyte has just grown out of a gametophyte sitting on or in the ground, but now the whole structure is small and protected enough to be dispersed and if held on a tree, dispersal by wind currents would be relatively wide.
Using Pinus as an example. Pines are evergreen, and their needle like leaves persist for 2-14 years, but they show seasonal growth activities. Pines are monoecious - the male and female strobili appear separately but on the same plant. The strobili that contain the microspores are called the pollen cones and these appear in spring (year 1). Strobili consist of sporophylls (adapted leaves) with 2 microsporangia attached to the underside of each. The sporophylls are arranged spirally on a central axis. The microsporangia (pollen sacs) contain the microspores (pollen) which is formed like all microspores in tetrads by meiosis.
Within the spore/pollen wall the gametophyte develops - endosporic. The microspore nucleus divides resulting into a number of prothallial cells and an antheridial initial. The antheridial initial then divides into a generative cell and tube nucleus. It is at this stage that the pollen is usually shed. (As soon as the first division occurs it has become the gametophyte plant rather than the microspore - can't see any difference from the outside, and distinguishing would complicate matters, so the whole structure is known as pollen.) The pollen is produced in great abundance, released as the strobilus moves in the wind or is knocked - clearly visible as sulphur coloured dust. In pine it also looks rather different from the average microspore because the pollen grains have wings- these are small air filled sacs which are extensions of the exine - essentially an aid to dispersal, making the pollen grains more buoyant. (Exine = outer layer of pollen grain wall). The winged pollen is seen in all pines and some other conifers.
The ovules are also found in cones. these consist of spirally arranged bracts which are hard and woody, in the axils of which we find ovuliferous scales (equivalent to the sporophylls in the male strobilus). Each scale has two ovules on the upper surface. Each ovule has a single integument and a massive megasporangium or nucellus. A single megaspore mother cell undergoes meiosis to form a tetrad of 4 megaspores. - 3 degenerate. The megasporangium contains just one megaspore surrounded by the nucellus which is in turn surrounded by a sterile integument(s) and the whole structure = ovule. The ovule has a micropyle at one end - this is a hole or a passageway between the lobes of the integument. The other end is called the chalaza.
Again the development of the megaspore into the gametophyte generation is not visible - the whole structure is known as the ovule at all stages of development (confusing when getting to grips with alternation of generations - but everything inside the ovule is haploid until fertilization). The single remaining megaspore develops into the megagametophyte and this happens endosporically within the megasporangium. This usually begins with a free-nuclear phase where the nucleus divides inside the cytoplasm and only later are cell walls formed. (In Pinus may be 6 months later.) After cellularization, some of the cells at the micropyle end become archegonial initials and form archegonia. Although several different archegonia may be fertilized only one will reach maturity.
If a pollen grain lands on a micropyle and conditions are favourable, development of the male gametophyte continues. (though it may be 6 months between pollination and fertilization. The tube nucleus migrates towards the tip of the extending pollen tube and the generative cell divides to form a sterile cell and a spermatogenous cell. As the tube lengthens, the sterile cell loses its wall and together with the spermatogenous cell, begins to move down the pollen tube. Just before fertilization, the spermatogenous cell divides to form 2 non-motile gametes, only one of which fuses with an egg cell to form the diploid zygote (back to sporophyte stage).
After fertilization, there is again likely to be a period of free-nuclear division. As cell walls form, suspensor cells are seen to attach the embryo at the chalazal end. Eventually, the embryo differentiates a radicle (= primary root) and a plumule (=primary shoot) with a hypocotyl bearing cotyledons. A year may pass before the embryos are fully mature. Other changes also take place at fertilization so the ovule becomes a seed. As well as containing the embryo, there are the remains of the megagametophyte ( the nucellus) which provides the developing embryo with its nutrition, and a hard seed coat develops from the integuments. When conditions are satisfactory, the cone containing the seeds opens up to allow them to be dispersed. This may happen immediately, or in some species, it may only happen when intense heat or a forest fire causes the cones to open (Monterey Pine). In some cases, a strip off the surface of the ovuliferous scale abscises with the seed to provide it with a wing and aid wind dispersal. The seed will then germinate to provide the adult phase of the sporophyte generation. The full cycle can take two years in pines.
An extinct group - the "seed ferns". These had frond-like compound leaves so looked like ferns but bore seeds and pollen-producing organs, appear to be a precursor to some of the gymnosperms.
There are 4 divisions of gymnosperms still living - Cycads, Ginkgo, Gnetophytes and Conifers. All gymnosperms are heterosporous, producing ovules, seeds and pollen grains. Conifers are the most prolific. Conifers are always trees, of moderate to gigantic size, e.g. giant redwoods reach up to 90m in height and 10m diameter. Leaves are always simple, either needles or scales.
There are over 500 species of conifer alive today. There are only 1/20th as many species of conifer as there are ferns. Despite this, conifers are very conspicuous are among the dominant forest trees of the world - covering extensive areas in boreal forests. A feature of the pollen in many conifers during evolution of pollen, the line which became conifers evolved 2 bladders - sacci - formed from an expansion of the outer exine from the inner wall. The intine this undoubtedly acting as a floating apparatus, assisting in wind dispersal.
The relatively limited geographical range of the 9 genera which are still living, suggests that many may be heading towards extinction - there are about 160species left. They are often talked about as living fossils and are restricted to tropical and sub-tropical climates. Habitat destruction has made them particularly rare in natural vegetation. Cycas is the only species that is widely distributed, found in India, China and Australia. They look rather palms or perhaps tree ferns with thick leathery leaves borne on terminal crowns and a number are cultivated. All species are dioecious, the sexes segregated on different plants. Their reproductive units consist of compact cones usually borne at the apex of the stem and are often very large and heavy. This segregation coupled with the fact they are particularly slow growing, means they are unlikely to survive in the long term, but for human cultivation. A Dioon plant only 2m tall was estimated to be 1000 yrs old. And an Australian species has members which are 5000 yrs old.
Ginkgo biloba is the only living representative of the Ginkgophytes. It is known as the maidenhair tree, and we can trace its ancestors back to the Permian. It is another living fossil - its structure seems unchanged over all these years - and is perhaps the oldest living seed plant. Like Cycads, this species is dioecious, with male trees producing small strobili. In this species the term 'gymnosperm' or 'naked seed' is entirely appropriate as the ovules just hang down in pairs on stalks - entirely unprotected. (when mature are the size and colour of apricots). It does occur in the wild in SE China, and is found cultivated. See New Scientist - 15/2/97- Endangered medicinal plants. G. biloba a "fashionable" remedy for heart ailments and dementia. 2000 tonnes sold every year (1/3 of this to Germany) - trade growing at 25% per annum.
Contains 3 orders, each with a single genera.
The two larger ones Ephedra and Gnetum each contain about 40 species with Ephedra usually being a shrubby plant with small scale like leaves, looking rather like a horsetail. Many Gnetums are lianas - creepers - with cones and also broad leaves which resemble megaphyllous leaves.
The third group is also represented by a single species - Welwitschia mirabilis - a very unusual plant. The form and the proportions of this plant are without parallel in the plant kingdom (weird). The main plant body resembles an inverted cone, only the base of which projects above ground. Just see an elliptical base up to 18" across. The rest of the tap root penetrates to great depth. The surface of an old plant is covered by thick corrugated cork. The top is bowl shaped and other plants will often grow in sand that collects in the hollow. The adult plant bears a single pair of opposite leaves which persist as long as the plant lives. These are thick and leathery in texture- markedly xerophytic. They arise from grooves in the rim surrounding the crown and their growth is entirely basal. As they grow larger, the leaf apex rests on the ground and is gradually destroyed by friction. the constant growth at the proximal end approximately equates to the constant destruction at the distal end, so the leaves tend to remain the same length. However, old plants frequently have leaves 6' long. As the width of the crown grows they also become broader - and frequently turn into narrow ribbons, becoming twisted and tangled. Being gymnosperms of course they have cones - a surprising sight. Found in a narrow strip of arid coastal zone in SW Africa, over an area of 700 miles and 20 miles inland, where fog is a frequent occurrence. This fog seems to be necessary of the germination of the seeds, and perhaps for the persistence of the plant.
Gymnosperms are well adapted to life on land - survived 2 periods of major extinction. One reason is their superior mode of reproduction.
1. The gametophytes can draw on the resources of the mature sporophyte.
2. Environmental water is not necessary for fertilization - opening up larger range of habitats.
3. Major reproductive and dispersal agent is the seed with its greater protection and advantages discussed earlier.