Within the kingdom Plantae, divisions are grouped into the following categories:
Non-vascular plants - no true vascular system (xylem, phloem), no true leaves, stems,roots; alternation of generations with the haploid gametophyte stage dominant.
Vascular plants - true xylem and phloem; leaves, stems, roots, alternation of generation with diploid sporophyte stage dominant.
Non-seed plants - dispersal is through spores; fertilization requires water; tiny, but independent haploid gametophyte stage.
Seed plants - dispersal is through seeds; fertilization occurs through pollination and water is not required; gametophyte stage is much reduced and depends on sporophyte for nourishment.
Gymnosperms ("naked" seed plants) - non-flowering plants that don''t produce fruit.
Angiosperms ("vessel" seed plants) - flowering plants, ovary walls develop into fruits.
The vascular plants are those plants that have specialized cells for conducting water and sap within their tissues, including the flowering plants, conifers and other gymnosperms, but not mosses, algae, and the like (nonvascular plants). They are set apart in two important ways:
Vascular plants have water-carrying tissues, termed tracheids, in their tissues, enabling the plants to evolve larger and more elaborate structures, while non-vascular plants lack these. In vascular plants, the principal generation phase is the sporophyte, which is diploid with two sets of chromosomes per cell. In non-vascular plants, the principal generation phase is often the gametophyte, which is haploid with one set of chromosomes per cell.
Psilotophyta division contains only two genera: Psilotum, a small shrubby plant of the dry tropics, and Tmesipteris, an epiphyte found in Australia, New Zealand, and New Caledonia. There has long been controversy about the relationships of the Psilotophyta, with some claiming that they are ferns (Pterophyta), and others maintaining that they are descendants of the first vascular plants. Recent evidence from DNA suggests a closer affinity to the ferns. All Psilotophyta share a few characteristics. Psilotophyta are all vascular plants. They lack leaves, instead having small outgrowths called enations. The enations are not considered true leaves because there is only a vascular bundle just underneath them, but not inside, as in leaves. Psilotophyta also don''t have true roots. They are anchored by rhizoids. Absorption is aided by fungi called mycorrhizae. Three sporangia are united into a synangium, which is considered to be a very reduced series of branches. There is a thick tapetum to nourish the developing spores, as is typical of eusporangiate plants. The gametophyte looks like a small piece of subterranean stem, but produces antheridia and archegonia.
The Division Lycophyta is a tracheophyte subdivision of the Kingdom Plantae that includes some of the most "primitive" of extant (living) vascular plants. These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. All have simple leaves. There are three main groups within the Lycophyta, sometimes separated at the level of order and sometimes at the level of class.
These are subdivided at the class level here:
· Lycopsida – clubmosses and firmosses
· Selaginellopsida – spikemosses
· Isootopsida – quillworts
Fern or pteridophyte, is any one of a group of some twenty thousand species of plants classified in the Division Pterophyta or Filicophyta. A fern is defined as a vascular plant that reproduces by shedding spores to initiate an alternationgenerations. New fronds arise by circinate vernation (unrolling leaf formation). The life cycle of a typical fern consists of two distinct stages and proceeds as follows:
· Sporophyte produces spores
· Spores develops into a prothallus (gametophyte)
· Prothallus produces gametes
· Male gamete fertilizes female gamete
· The fertilized gamete (embryo) grows into a sporophyte (the "fern")
· A sporophytic fern consists of:
· Rhizome: Creeping stem, sometimes underground, absorbs nutrients, anchors plant
· Frond ("fern leaf"): green, photosynthesizes
· Spores develop on surface (usually underside)
· Petiole: stem-like part of leaf
· When young, it is curled into a fiddlehead
A gametophytic fern contains:
· Thallus: green, photosynthesizes
· Rhizoids: root-like stems
The gametophyte generation begins with a spore produced by meiosis. The spore is haploid, and all the cells derived from it (by mitosis) are also haploid. In due course, this multicellular structure produces gametes, by mitosis and sexual reproduction then produces the diploid sporophyte generation.
Haploid cells have only one copy of each chromosome. Within higher organisms, only the reproductive cells are haploid, whereas the somatic (body) cells are diploid (two copies of each chromosome) or polyploid (three or more copies of each chromosome, often found in plants). When reproducing, the haploid sex cells of both parents will generally merge to form a diploid cell the zygote, with unique genetic properties, which will quickly become the embryo.
Diploid cells have two copies of each chromosome, usually one from the mother and one from the father. Most somatic cells (body cells) of higher organisms are diploid or polyploid (three or more copies of each chromosome, often found in plants), whereas their reproductive cells are usually haploid (they have only one copy of each chromosome). When reproducing, haploid sex cells of both parents will generally merge to form a diploid cell, the zygote, with unique genetic properties, which quickly becomes the embryo.