Plants =)

Bryophytes

Plant scientists recognize two kinds of land plants, namely, bryophytes, or nonvascular land plants and tracheophytes,or vascular land plants. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characters. First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid, gametophyte generation rather than the diploid sporophyte; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin- containing, water-conducting tissue that is found in the sporophytes of all vascular plants. At one time, bryophytes were placed in a single phylum, intermediate in position between algae and vascular plants. Modern studies of cell ultrastructure and molecular biology, however,confirm that bryophytes comprise three separate evolutionary lineages, which are today recognized as mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta) and hornworts (phylum Anthocerotophyta). Following a detailed analysis of land plant relationships, Kenrick and Crane (1998) proposed that the three groups of bryophytes represent a grade or structural level in plant evolution, identified by their "monosporangiate" life cycle. Within this the geologically oldest group, sharing a fossil record with the oldest vascular plants in the Devonian era.



Of the three phyla of bryophytes, greatest species diversity is found in the mosses, with up to 15,000 species recognized. A moss begins its life cycle when haploid spores, which are produced in the sporophyte capsule, land on a moist substrate and begin to germinate. From the one-celled spore, a highly branched system of filaments, called the protonema, develops. Cell specialization occurs within the protonema to form a horizontal system of reddish-brown, anchoring filaments, called caulonemal filaments and upright, green filaments, called chloronemal filaments. Each protonema, which superficially resembles a filamentous alga, can spread over several centimeters to form a fuzzy green film over its substrate. As the protonema grows, some cells of the caulonemal filaments specialize to form leafy buds that will ultimately form the adult gametophyte shoots. Numerous shoots typically develop from each protonema so that, in fact, a single spore can give rise to a whole clump of moss plants. Each leafy shoot continues to grow apically, producing leaves in spiral arrangement on an elongating stem. In many mosses the stem is differentiated into a central strand of thin-walled water-conducting cells, called hydroids, surrounded by a parenchymatous cortex and a thick-walled epidermis. The leaves taper from a broad base to a pointed apex and have lamina that are only one-cell layer thick. A hydroid-containing midvein often extends from the stem into the leaf. Near the base of the shoot, reddish-brown, multicellular rhizoids emerge from the stem to anchor the moss to its substrate. Water and mineral nutrients required for the moss to grow are absorbed, not by the rhizoids,but rather by the thin leaves of the plant as rain water washes through the moss cushion.



As is typical of bryophytes, mosses produce large, multicellular sex organs for reproduction. Many bryophytes are unisexual, or sexually dioicous. In mosses male sex organs, called antheridia, are produced in clusters at the tips of shoots or branches on the male plants and female sex organs, the archegonia, are produced in similar fashion on female plants. Numerous motile sperm are produced by mitosis inside the brightly colored, club-shaped antheridia while a single egg develops in the base of each vase-shaped archegonium. As the sperm mature, the antheridium swells and bursts open. Drops of rain water falling into the cluster of open antheridia splash the sperm to near-by females. Beating their two whiplash flagellae, the sperm are able to move short distances in the water film that covers the plants to the open necks of the archegonia. Slimey mucilage secretions in the archegonial neck help pull the sperm downward to the egg. The closely packed arrangement of the individual moss plants greatly facilitates fertilization. Rain forest bryophytes that hang in long festoons from the trees rely on torrential winds with the rain to transport their sperm from tree to tree, while the small pygmy mosses of exposed, ephemeral habitats depend on the drops of morning dew to move their sperm.Regardless of where they grow, all bryophytes require water for sperm dispersal and subsequent fertilization.


pteridophytes

In this course we are going to use the term Pteridophyte to refer to non-seed vascular plants, i.e. plants with xylem and phloem whose dispersal relies on spores not seeds. (Be aware that some people use this term just to refer to ferns.)
The Bryophytes are a group of simple land plants which in many ways are an evolutionary dead end. They probably evolved from the green algae at about the same time as the first vascular plants. Like them they are "Embryophytes" with multicellular sex organs but they do not have vascular tissue like Rhynia and its relatives.

The first vascular plants rapidly diversified to cover the earth. The sporophyte was the dominant phase of the life cycle of these early Pteridophytes. Unlike the Bryophytes, these plants are in the mainstream of land plant evolution and eventually gave rise to the seed plants.

Let's look at the main groups of Pteridophytes!

They do not form a monophyletic group but consist of several groups, the Lycopodiophyta (club mosses, spike mosses, and quillworts), the Equisetophyta (horsetails), the Psilotophyta (whisk ferns), the Ophioglossophyta (adder's tongues and grape ferns), and the Pteridophyta (true ferns) and are monocots most of the time.

In addition to these living groups of pteridophytes are several groups now extinct and known only from fossils. These groups include the Rhyniophyta, Zosterophyllophyta, Trimerophytophyta, and the progymnosperms.

Modern studies of the land plants agree that all the pteridophytes share a single common ancestor. However, they are not a clade (monophyletic group) because the seed plants are also descended from within this group -- probably close relatives of the progymnosperms.


Gymnosperms

Gymnosperm (Gymnospermae) are a group of spermatophyte seed-bearing plants with ovules on the edge or blade of an open sporophyll, which are usually arranged in cone-like structures. The other major group of seed-bearing plants, the angiosperms, [from the Greek, 'angion' - container] have ovules enclosed in a carpel, a sporophyll with fused margins. A carpel consists of a stigma, style and the ovary. The term gymnosperm comes from the Greek word gumnospermos (γυμνόσπερμος), meaning "naked seeds" and referring to the unenclosed condition of the seeds, as when they are produced they are found naked on the scales of a cone or similar structure. There are between 700 and 900 species of Gymnosperm. It is widely accepted that the gymnosperms originated in the late Carboniferous period of the Paleozoic era within the Phanerozoic eon. Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 380 million years ago. Often gymnosperms are used for economical uses and as folk medicines. Some common uses for them are soap, varnish, lumber, paint, edible plants, and perfumes. Conifers are by far the most abundant gymnosperms with around 600 species. Cycads are the next most abundant group with about 130 species. Approximately 75 - 80 species of Gnetales exist and only one species of Ginkgo remains today.

Gymnosperms are heterosporous, producing microspores from microsporangia that develop into pollen grains and megaspores from megasporangia that are retained in an ovule. Gymnosperm gametophytes are retained in cones of two types. Microscopic male gametophytes are stored in pollen cones and microscopic female gametophytes are kept in ovulate cones. A male gametophyte develops into a pollen grain which is stored within a pollen wall. A female gamteophyte develops from a megaspore and produces one or more eggs. After pollination and fertilization (joining of the micro- and megaspore), the resulting embryo, along with other cells comprising the ovule, develops into a seed. The seed is a sporophyte resting stage. Gymnosperms have a sporophyte dominant life cycle as with all other vascular plants. The seed consists of an embryo, a nutrient rich endosperm food supply and the seed coat or integument. Reproduction in gymnosperms varies greatly. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, while conifers and gnetophytes have sperm with no flagella that are conveyed to the egg along a pollen tube which grows through the micropyle which is a microscopic gap in the integument of the ovule tissue.

Angiosperms


The angiosperms, or flowering plants, are one of the major groups of extant seed plants and arguably the most diverse major extant plant group on the planet, with at least 260,000 living species classified in 453 families (Judd et al., 2002; APG II, 2003; Soltis et al., 2005). They occupy every habitat on Earth except extreme environments such as the highest mountaintops, the regions immediately surrounding the poles, and the deepest oceans. They live as epiphytes (i.e., living on other plants), as floating and rooted aquatics in both freshwater and marine habitats, and as terrestrial plants that vary tremendously in size, longevity, and overall form. They can be small herbs, parasitic plants, shrubs, vines, lianas, or giant trees. There is a huge amount of diversity in chemistry (often as a defense against herbivores), reproductive morphology, and genome size and organization that is unparalleled in other members of the Plant Kingdom. Furthermore, angiosperms are crucial for human existence; the vast majority of the world's crops are angiosperms, as are most natural clothing fibers. Angiosperms are also sources for other important resources such as medicine and timber.

Characteristics
Despite their diversity, angiosperms are clearly united by a suite of synapomorphies (i.e., shared, derived features) including 1) ovules that are enclosed within a carpel, that is, a structure that is made up of an ovary, which encloses the ovules, and the stigma, a structure where pollen germination takes place, 2) double fertilization, which leads to the formation of an endosperm (a nutritive tissue within the seed that feeds the developing plant embryo), 3) stamens with two pairs of pollen sacs, 4) features of gametophyte structure and development, and 5) phloem tissue composed of sieve tubes and companion cells (see Doyle and Donoghue, 1986; Judd et al. 2002; P. Soltis et al., 2004; and D. Soltis et al., 2005, for further discussion). All available evidence strongly rejects hypotheses of more than one evolutionary origin of extant angiosperms