|Hollow cylinder surrounding the pith|
Vascular plants are plants that have tissues called xylem and phloem as conducting tissues. Xylem is tissue composed of vessels, fibers, and tracheids responsible for upward conduction of water and dissolved minerals; it also functions as the supporting tissue of stems.
Phloem is conducting tissue that is responsible for moving food manufactured in the leaves to other parts of the plant, including the roots. The botanical name for the vascular plants is Tracheobionta. This group of plants includes both seedless and seed plants, including the flowering plants (angiosperms).
Fossil forms of Tracheobionta are well represented, because the tough, lignified cell walls of xylem preserve well.Most vascular plants produce seeds and are classified as Spermatophyta, but the ferns and related groups are seedless. Tracheobionts are primarily terrestrial, although some are epiphytic or aquatic.
The life cycle of vascular plants is characterized by alternation of a conspicuous diploid sporophyte generation with a reduced haploid gametophyte generation, termed alternation of generations. The sporophyte exists independently of the gametophyte and typically exhibits indeterminate growth.
That is, a single sporophyte plant can live and continue to grow for many years due to the activity of its apical and lateral meristems. The gametophyte, although free-living and independent in the more primitive vascular plants, is dependent on the sporophyte in seed plants.
The spores produced by the sporophyte frequently have a tough, well-defined
wall hardened by the wall material sporopollenin. Some seedless vascular plants, including most ferns, are homosporous; they produce a single type of spore. The spore germinates and grows out of the spore wall to form an exosporic gametophyte.
This free-living haploid plant produces both antheridia and archegonia. Antheridia are saclike sex organs with a thin jacket layer of cells surrounding a mass of sperm cells. In seedless plants, and even in some of the seed plants, the sperm are flagellated and motile.
All of the seed plants, and some ferns and fern allies, are heterosporous; that is, they produce both small microspores and large megaspores.
Microspores form male gametophytes that produce sperm, whereas megaspores form female gametophytes that produce eggs. In seedless plants, the microgametophytes produce typical antheridia; in seed plants, the microgametophytes are pollen grains.
In all but flowering plants, the megagametophyte produces archegonia. In flowering plants, the megagametophyte is an embryo sac. Both the megaand microgametophytes are endosporic; that is, they develop within the original spore wall.
The vascular tissues of stems and roots are called a stele. All roots have a simple stele with xylem forming a core in the center of the root, surrounded by phloem, either in a single sheath or in separate strands.
In woody plants,multiple layers of xylem form in succession, pushing phloem and any external tissues outward. These layers of xylem are wood. Members of all divisions of vascular plants except psilotophytes form roots.
The stems of the simplest vascular plants have a stele similar to that of roots with a solid core of xylem in the center surrounded by phloem.
In more specialized seedless plants, the pattern of xylem may be divided into separate bands surrounded by phloem. In stems of the most complex seedless plants, discrete vascular bundles form a ring around a core of parenchyma, the pith.
In each bundle a core of xylem is surrounded by phloem; such stems are termed amphiphloic. Seed plants and horsetails have a similar arrangement, except that phloem is found only to the outside of xylem in each bundle; the stems are ectophloic.
In woody plants, the ring of vascular bundles become connected by a vascular cambium that produces a succession of layers of new xylem, the wood. In monocot stems, the vascular bundles are distributed throughout the stem, rather than in a single ring surrounding a pith.
Leaves are characteristic of all divisions of vascular plants except some psilotophytes. The simplest leaves are small and scale-like and have a single vascular bundle forming a midrib. The vascular bundle in the leaf is a direct offshoot of the single vascular bundle in the stem. Such leaves are called microphylls.
In plants with stems containing a pith, the leaves are supplied by one or more vascular bundles, which branch within the broadened blade of the leaf to form many veins. A gap is left in the stele where a leaf trace exits to supply the leaf. With a more extensive network of vascular tissue, these leaves are typically large and are termed megaphylls.
Sporangia are specific regions of the sporophyte body where meiosis occurs to produce spores. A single sporophyte can produce tens of millions of spores per year. The position, structure, and development of sporangia provide useful criteria for distinguishing the major groups of vascular plants.
In psilotophytes, two or three fused sporangia are located in the axils of leaves or leaflike append ages. In horsetails (Sphenophyta), sporangia terminate the main stemaxis, forming a cone or strobilus. Lycophytes typically produce sporangia on the upper surface of leaves near their attachment to the stem.
Depending on the group, these sporophylls may resemble vegetative leaves or may be concentrated into terminal cones. In ferns, sporangia also are typically localized on sporophylls. Frequently these sporangia are clustered in groups, called sori (singular “sorus”), on the edge or underside of the leaf.
In gymnosperms, sporangia are concentrated in strobili, which can be quite large. Both the megasporangiate and microsporangiate cones of some cycads may approach a meter in length. In flowering plants, sporangia are localized in the flowers.
Among seedless vascular plants, two distinct sporangium types can be recognized based on structure and development. In the more primitive groups, a superficial layer of cells divides to form two layers.
The outer layer forms the sporangium wall or jacket, while the inner layer becomes sporogenous tissue that undergoesmeiosis to formspores. Eusporangia formed in this way have multiple layered walls and frequently are at least partially embedded in vegetative tissue.
In contrast, leptosporangia begin development froma single superficial cell. Following cell division, the inner of the two daughter cells forms a stalk that raises the sporangium above the vegetative tissue.
The outer daughter cell eventually forms a single-layered jacket or capsule around a mass of sporogenous cells. Certain cells of the jacket layer differentiate to form an annulus, a specialized structure to open the sporangium and aid in spore dispersal.
Antheridia and archegonia are gametangia produced by the gametophyte. In vascular plants they are complex structures consisting of a sterile jacket layer of cells surrounding and protecting the gametes.
Antheridia produce as few as four sperm (in Isoetes) to several thousand sperm in some eusporangiate ferns. All archegonia produce a single egg within a venter, the swollen base of the organ, which is usually embedded in gametophyte tissue.
Extending out from the venter is an elongate neck, which becomes tubular when the neck canal cells degenerate to form an opening through which spermcan swim.At the base of the neck canal, adjacent to the egg, is a ventral canal cell.
The gametangia of seed plants are much reduced. The microgametophyte is a pollen grain, consisting of only two to four cells when it is released from the microsporangium.
Depending on the group, zero, one, or two prothallial cells form, which represent the vegetative male gametophyte. The tube cell and generative cell may be interpreted as an antheridium. The generative cell divides to form two sperm.
Themegagametophyte of gymnosperms consists of a few thousand cells that typically form several archegonia, each consisting of four neck cells, a ventral canal cell, and a large egg. In flowering plants the entire megagametophyte, the embryo sac, typically contains only seven cells, one of which is the egg.
In all plants with an archegonium, fertilization and the early stages of embryo development occur within the venter. This helps to establish a polarity with the first cell division.
In all but the leptosporangiate ferns, the first cell division forms a new cellwall parallel to the surface of the gametophyte; one daughter cell faces inward, while the second faces out through the neck canal. In the horsetails and a few ferns, the apical cell, which will form the new sporophyte body, faces outward, a condition known as exoscopic.
However, in most plants the apical cell faces inward (is endoscopic), and initially the embryo grows into the gametophyte tissue. As the embryo continues to grow, a shoot apex with leaf primordia forms at the apical end, and a root apex forms at the opposite pole.
Some general evolutionary trends within the Tracheobionta are a reduction in the size and independence of the gametophyte generation and increasing dominance of the sporophyte.
In the ferns and fern allies, the gametophytemay be photosynthetic and free-living, often resembling a very small liverwort in size and general shape. These are inconspicuous, however, compared to the large, leafy sporophytes.
In seed plants, the larger megagametophyte is reduced to a countable number of cells, from a few thousand in gymnosperms to as few as seven in flowering plants. This gametophyte is retained within the sporangium and derives water and nutrients from the supporting sporophyte.
This reduction is most dramatic in the microgametophyte, where the pollen grain consists of fewer than a handful of cells. Although the pollen of a few seed plants produces swimming sperm, in most cases a pollen tube grows to deliver sperm to the egg and free water is no longer required for fertilization.
In contrast to the reduction of the gametophyte, the sporophyte becomes increasingly larger and complex with development of the vascular tissues.
Specialized organs, roots, stems, and leaves evolved for absorption of water and nutrients, to provide aerial support, and to increase photosynthetic surface area in a terrestrial environment. Development of vascular tissue provided the physical and physiological support required for the evolution of these structures.