سه شنبه 12 مهر 1390 11:31 ق.ظ
The Tracheophytes: Vascular Plants In the bryophytes, the haploid gametophyte is the conspicuous, long-lived generation, and the diploid sporophyte is attached to the maternal gametophyte for its entire, relatively short lifespan.
In Tracheophytes, the opposite is true. The gametophyte is short-lived and may be either free-living (the more primitive condition) or attached to the parent sporophyte for its entire, short lifespan. Most of the plants you see in your day-to-day travels are the sporophytes of vascular plants.
Tracheophytes have several evolutionary advances that made them better suited for a fully terrestrial existence than are the Bryophytes.
- xylem and phloem
- lignin (not only structural in and of itself, but allows greater turgor pressure)
- thick waxy cuticle
- fully functional stomates (open and close with turgor pressure)
- profuse branching via apical meristems
- increased young tissues for photosynthesis
- increased areas for sporangia (as opposed to only one sporangium per sporophyte in the Bryophytes)
- highly differentiated plant tissues and organs
The earliest vascular sporophytes were plants consisting only of stems, lacking leaves or roots. They were dichotomously branched, and relatively undifferentiated.
Later, the root system and shoot system evolved, making the plant body parts more versatile, and allowing a division of labor.
Recall the general types of tissue and their functions:
...as well as the difference between primary growth and secondary growth.
Both tracheids and vessel elements are known as tracheary elements, although the two types of cells evidently evolved independently. (Any morphological similarity is due to convergence.)
The earliest vascular plants were little more than an elongate axis with a core of vascular tissue. This axis can be considered the primordialstem, from which the other plant organs evolved.
One can see the progression of complexity in the plant axis reflected in the arrangement of vascular tissues in the cross section of the stem (and now, in the root). The central core of vascular tissue in the plant root or stem is known as the stele, and there are several different types that can be identified by their morphology in cross section.
- haplostele - found in most roots, this is a solid core of xylem surrounded by phloem. It is the most primitive type of protostele.
- actinostele - named for its shape (from the Greek actinos, meaning "ray"), this one is rare. The whisk fern, Psilotum is one extant plant having an actinostele. (You'll meet Psilotum shortly.)
- plectostele - interconnected regions of xylem are collectively surrounded by a mass of phloem. Club mosses (Lycopodiopsida) often exhibit this type of stele arrangement.
- solenostele - the most primitive type of siphonostele, this is a pith surrounded by xylem and phloem. Some primitive ferns show this type of arrangement.
- dictyostele - similar to the solenostele, but with multiple gaps in the xylem/phloem ring surrounding the pith where numerous leaf gaps occur. Found only in ferns.
- eustele - The rings of xylem and phloem are now arranged in bundles, forming a circle around the central pith. This is the most common type of stele in seed plant stems.
Roots evolved from the underground portions of the plant, but retained many primitive characteristics. For example, all roots except those of monocots retain the haplostele configuration.
The first leaves were merely scales that "peeled away" from the ancient plant axis, lacking any trace of vascular tissue. These were known asenations, and are still found in a few living plants (e.g., the Whisk Fern, Psilotum).
Further evolution resulted in somewhat larger scales growing out from the stem, and taking a core of protostele along with it. These early leaves were probably very similar to the microphylls of the living Club Mosses (whom you'll meet shortly). Microphylls contain a single strand of vascular tissue, and do not leave leaf gaps as larger leaves do.
Megaphylls are larger leaves, and are found in plants with siphonosteles or eusteles. Most living plants have megaphylls, though many of these are highly derived and may have lost their original leafy shape (e.g., cactus spines; pine "needles").
The Life Cycle of Seedless Vascular Plants One picture is worth lots of words. Let's locate the Smurfs and Humans in this life cycle of a fern, which is typical of most other seedless tracheophytes.
Evolution of the Seedless Vascular Plants Three phyla of extinct seedless vascular plants are of interest to evolutionary botanists
- Rhyniophyta - named for the Scottish village of Rhynie, where abundant fossils were found.
- Zosterophyllophyta - Named for their resemblance to modern marine anthophytes in the genus Zostera, which superficially resemble grasses.
- Trimerophytophyta - Probably provided the ancestral lineage from which both the pteridophytes and the early gymnosperms arose.
The three extinct phyla above were relatively small in stature, and flourished from the Silurian (about 425 mya) to the middle of the Devonian (about 370 mya).
These were relatively small, undifferentiated plants consisting of upright and underground axes that branched dichotomously, and (when reproducing) ended in a single sporangium that produced homospores. Rhizoids sprouted from the underground axes. The plants had a waxy cuticle and stomata.
The axis anatomy was similar to today's vascular plants: epidermis surrounding a cortex surrounding a stele of central xylem and peripheral phloem. The tracheids were more similar to moss conducting cells than to vascular plant tracheids, but had more lignin in their cell walls.
In the rhyniophytes, xylem cells matured first in the center of the stem, with younger cells found progressively outwards, toward the periphery of the stele. This process is known as centrifugal differentiation.
Also leafless and consisting of dichotomously branching stemlike axes, these small plants had stomates only on the upper surface, suggesting they were nestled in mud or sludge.
Sporangia were borne on small, lateral branches, and like Rhyniophytes, zosterophyllophytes were homosporous.
Unlike the xylem of rhyniophytes, zosterophyllophyte xylem matures first at the periphery of the stele, with younger, less differentiated cells found in the center of the stele. This "opposite" type of maturation from most plants is called centripetal differentiation, and is seen in today's Club Mosses. Because of this similarity and because lycophyte sporangia and sporangia-bearing branches are so similar, Zosterophyllophyta is believed to share a most recent common ancestor with today's Lycophyta.
Trimerophyta specimens are believed to be the sister taxon of Rhyniophyta. They were leafless, but more profusely branched than Rhyniophyta. Reproductive branches terminated in clusters of elongate sporangia, each borne on a smaller, shorter stem. Xylem differentiates centrifugally, as in rhyniophytes.
Still no leaves, but these plants were larger (more than a meter tall) and more complex than the Rhyniophytes and Zosterophyllophytes.
Still homosporous, but reproductive branches ended in clusters of elongate sporangia.
The features of the three extinct phyla give us clues to the orgins of the two extant phyla of seedless tracheophytes
- Lycopodiophyta - The Club "Mosses"
- Pteridophyta - The Ferns and their relatives
Lycopodiophytes and Pteridophytes became dominant from about the late Devonian (about 375 mya) and were dominant through the Carboniferous (about 290 mya).
Seed plants first appear in the fossil record in the late Devonian (about 380 mya), and underwent an explosion in diversity through the Permian and Mesozoic.
When the first flowering plants appeared about 125 mya, it took only about 40 million more years for them to diversify and colonize habitats so successfully that they out-competed other plants in almost every habitat. They retain this dominance today.
Phylum Lycopodiophyta - The Club Mosses Around the mid-Devonian, a major evolutionary split separated the lycophyte lineage from the ancestors of all other vascular plants (euphyllophytes).
Though today's lycophytes are small and herbaceous, ancient ones were the size of large trees, and were the major plant form in the Carboniferous.
Club Mosses are characterized by a stem with a protostele, underground rhizoids, and microphylls.
The more primitive family of extant lycophytes is Lycopodiaceae, the most diverse genus of which is Lycopodium.
The structures you see in two of the species above are called strobili (singular, strobilus. A strobilus is a whorl of sporophylls, and is also sometimes known as a "cone."
The spores are homospores, giving rise to gametophytes that are either (depending on species)
- photosynthetic, lobed masses of cells somewhat reminiscent of liverworts
- underground, non-photosynthetic mycorrhizal symbionts
The genus Selaginella includes familiar species such as the Resurrection Plant (Selaginella lepidophylla), but contains about 750 other species, as well.
Like Lycopodium, the stems of Selaginella bear microphylls, and the sporophylls are arranged in strobili. But this time the plant isheterosporous, producing microspores and megaspores, which develop into male and female gametophytes, respectively.
Megasporangia are borne on megasporophylls, and microsporangia are borne on microsporophylls in the strobilus, which you see in longitudinal section here:
The microgametophyte develops inside the microspore, and at maturity consists of one vegetative cell and an antheridium which produces multiple, biflagellate sperm. This ruptures when environmental conditions are right for sex.
The megagametophyte grows within the megaspore, but ruptures its wall when it reaches a certain size. Only the segment producing archegonia protrudes from the megaspore cell wall, making it available for sperm.
Fertilization takes place once the male and female spores have been shed from the strobilus, and the sporophyte grows inside the archegonium. In some species of Lycopodium and Selaginella, a stalklike suspensor connects the embryo to the gametophyte, and in some of those, the suspensor pushes the sporophyte embryo into the nutrient-rich gametophyte tissues where it feeds, grows, and eventually becomes an independent sporophyte.
Phylum Pteriodiophyta - Ferns and their Relatives This is a very diverse group, with more than 11,000 species, second only to the angiosperms in variety.
The most familiar pteridophytes are the ferns, which can be classified on the basis of the type of sporangia they grow:
Only two orders (Ophioglossales; Marattiales) produce eusporangia; all others produce leptosporangia. Of these, the most widespread and well known are the grape ferns (Botrychium and the Adder's Tongue Ferns (Ophioglossum)
The gametophytes of these genera are non-photosynthetic, subterranean and form symbiotic relationships with fungi that live within their tissues. The gametophytes are homosporous.
Fun Fact for Botanical Trivial Pursuit: Ophioglossum reticulatum has more chromosomes--1260--than any other living organism. And yet how tiny!
The vast majority of ferns, however, produce leptosporangia and are homosporous. The largest taxon is Order Filicales, with at least 10,500 of the 11,000 species of ferns.
Most filicale ferns have stems with a siphonostele, and produce megaphylls. The underground portion of the plant is a rhizome (also with a siphonostele), and this sprouts numerous, true roots with a protostele.
The fern leaf is usually pinnate with a central rachis (the extension of the petiole in pinnate leaves), and young leaves in most species emerged as coiled structures known as fiddleheads
Some make a tasty addition to salads, and others are distasteful or even poisonous (renal toxins are common in ferns!). So be sure you can properly identify a fern before you eat it! Sporophylls are not borne in strobili, but are individual fronds that grow sporangia on their undersides, when mature. The sporangia occur in clusters called sori (singular = sorus), and in some species the sorus is covered with small, protective plate (an outgrowth of the leaf), called an indusium.
Spores grow into a bisexual gametophyte, again reminiscent of a liverwort.
Note the locations of the antheridia and the archegonia. The new sporophyte will sprout from the notch in the "heart," eventually obliterating the gametophyte.
The Psilotales The Whisk Ferns were once believed to be the most primitive tracheophytes because of their superficial resemblance to the Rhyniophytes. Aboveground and belowground parts of the plant contain a protostele, and are considered to be simple stems. However, molecular data now reveal that they are fern relatives that have secondarily lost their fernlike features.
The life cycle is similar to that of other ferns, with a bisexual gametophyte that resembles the Psilotum rhizome. It is subterranean, and forms symbiotic relationship with fungi in the plant tissues.
Equisetales - The Horsetails These ancient fern relatives were once placed in their own phylum, but it is now known that they share a common ancestor with other ferns, and so are subsumed into the Pteridophyta.
The plants consist of hollow stems with rings of dry, scalelike leaves at each node.
The only surviving genus, Equisetum, is essentially unchanged from its Carboniferous fossil relatives, making this perhaps the oldest named genus.
These live in moist areas and bogs. The hollow stems are highly impregnated with silica, which makes them rough and tough. Native Americans are said to have used them to scour pottery, and hence their other common name, "Scouring Rushes."
The strobilus of Equisetum is composed of a whorl of small, sporangium-bearing modified branches called sporangiophores.
The homospores have thickened bands of the cell wall known as elaters wrapped around the spore, and these expand and contract, depending on humidity.
These germinate into bisexual gametophytes that are photosynthetic, free-living, and ranging in size from a few millimeters to about 3.5 millimeters in length.
...and the cycle begins again
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