Plants and their structure

A plant has two organ systems: 1) the shoot system, and 2) the root system. The shoot system is above ground and includes the organs such as leaves, buds, stems, flowers (if the plant has any), and fruits (if the plant has any).
The root system includes those parts of the plant below ground, such as the roots, tubers, and rhizomes
Plant cells are formed at meristems, and then develop into cell types which are grouped into tissues. ·Plants have only three tissue types: 1) Dermal; 2) Ground; and 3) Vascular.
Dermal tissue covers the outer surface of herbaceous plants. Dermal tissue is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss.
The ground tissue. comprises the bulk of the primary plant body. Parenchyma. collenchyma, and sclerenchyma cells are common in the ground tissue.
Vascular tissue transports food, water, hormones and minerals within the plant. Vascular tissue includes xylem, phloem, parenchyma, and cambium cells.
Plant cell types rise by mitosis from a meristem. A meristem may be defined as a region of localized mitosis. Meristems may be at the tip of the shoot or root (a type known as the apical meristem) or lateral, occurring in cylinders extending nearly the length of the plant. A cambium is a lateral meristem that produces (usually) secondary growth. Secondary growth produces both wood and cork (although from separate secondary meristems
Parenchyma
A generalized plant cell type, parenchyma cells are alive at maturity. They function in storage, photosynthesis, and as the bulk of ground and vascular tissues. Palisade parenchyma cells are elogated cells located in many leaves just below the epidermal tissue. Spongy mesophyll cells occur below the one or two layers of palisade cells. Ray parenchyma cells occur in wood rays, the structures that transport materials
laterally within a woody stem. Parenchyma cells also occur within the xylem and phloem of vascular bundles. The largest parenchyma cells occur in the pith region, often, as in corn (Zea ) stems, being larger than the vascular bundles.
Collenchyma
Collenchyma cells support the plant. These cells are charcterized by thickenings of the wall, the are alive at maturity. They tend to occur as part of vascular bundles or on the comers of angular stems.
Sclerenchyma
Sclerenchyma cells support the plant. .They often occur as bundle cap fibers. Sclerenchyma cells are characterized by thickenings in their secondary w'alls. They are dead at maturity.
Xylem
Xylem is a term applied to woody (lignin-impregnated) walls of certain cells of plants. Xylem cells tend to conduct water and minerals from roots to leaves. While parenchyma cells do occur within what is commonly termed the "xylem" the more identifiable cells, tracheids and vessel elements. Tracheids are the more primitive of the two cell types, occurring in the earliest vascular plants. Tracheids are long and apered, with angled end-plates that connect cell to cell. Vessel elements are shorter, much wider, and lack end plates. They occur only in angiosperms, the most recently volved large group of plants.
Phloem
Phloem cells conduct food from leaves to rest of the plant. They are alive at maturity and tend to stain green (with the stain fast green). Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the companion cells and sieve cells. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, as sucrose, flows through the sieve cells.
Epidermis
The epidermal tissue functions in prevention of water loss and acts as a barrier to fungi and other invaders. Thus, epidermal cells are closely packed, with little intercellular space. To further cut down on water loss, many plants have a waxy cuticle layer deposited on top of the epidermal cells.
Guard Cells
To facilitate gas exchange between the inner parts of leaves, stems, and fruits, plants have a series of openings known as stomata (singular stoma). Obviously these openings would allow gas exchange, but at a cost of water loss. Guard cells are beanshaped cells covering the stomata opening. They regulate exchange of water vapor, oxygen and carbon dioxide through the stoma.
Flowering Plant Reproduction
The plant life cycle has mitosis occurring in spores. produced by meiosis, that germinate into the gametophyte phase. Ga'metophyte size ranges from three cells (in pollen) to several million (in a "lower plant" such as moss). Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing" eggs). Within the plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns and "fern allies" to angiosperms.
Angiosperms (flowering plants)
All flowering plants produce flowers and if they are sexually reproductive, they produce a diploid zygote and triploid endospenl1. The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers.
Flowers
Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium
Androecium
The individual units of the androecium are the stamens, which consist of a filament which supports the anther. The anther contains four microsporangia within which microspores (pollen) are produced by meiosis. Stamens are thought to represent modified sporophylls (leaves \vith sporangia on their upper surface).
Pollen
Pollen grains (from the greek palynos for dust or pollen) contain the male gametophyte (microgametophyte) phase of the plant. Pollen grains are produced by meiosis of microspore mother cells that are located along the inner edge of the anther sacs (microsporangia). The outer part of the pollen is the exine, which is composed of a complex polysaccharide, sporopollenin. Inside the pollen are two (or, at most, three) cells that comprise the male gametophyte. The tube cell (also referred to as the tube nucleus) develops into the pollen tube. The germ cell divides by mitosis to produce two sperm cells. Division of the germ cell can occur before or after pollination
Gvnoecium
The gynoecium consists of the stigma, style, and ovary containing one or more ovules. These three structures are often termed a pistil or carpel. In many plants, the pistils will fuse for all or part of their lengthLike the stamen, the carpel is thought to be a modified leaf.
The Stigma and Style
the stigma functions as a receptive surface on which pollen lands and germinates its pollen tube. Corn silk is part stigma, part style. The style serves to move the stigma some distance from the ovary. This distance is species specific.
The Ovary
The ovary contains one or more ovules, which in turn contain one female gametophyte, also referred to in angiosperms as the embryo sac. Some plants, such as cherry, have only a single ovary which produces two ovules. Only one ovule will develop into a seed.
Pollination
The transfer of pollen from the anther to the female stigma is termed pollination. This is accomplished by a variety of methods. Entomophyly is the transfer of pollen by an insect. Anemophyly is the transfer of pollen by wind. Other pollinators include birds, bats, water, and humans. Some flowers (for example garden peas) develop in such a way as to pollinate themselves. Others have mechanisms to ensure pollination with another flower.
Flower color is thought to indicate the nature of pollinator: red petals are thought to attract birds, yellow for bees, and white for moths. Wind pollinated flowers have reduced petals, such as oaks and grasses.
The Gametophytes
The male gametophyte develops inside the pollen grain. The female gametophyte develops inside the ovule. In flowering plants. gametophyte phases are reduced to a few cells dependant for their nutrition on the sporophyte phase. This is the reverse of the pattern seen in the nonvascular plant groups liverworts, mosses, and hornworts (the Bryophyta).
Angiosperm male gametophytes have two haploid nuclei (the germ nucleus and tube nucleus) contained within the exine of the pollen grain (or microspore).
Female gametophytes of flowering plants develop within the ovule (megaspore) contained within an ovary at the base oft:he pistil of the flower. There are usually eight (haploid) cells in the female gametophyte: a) one egg, two synergids flanking the egg (located at the micropyle end of the embryo sac); b) two polar nuclei in the center of the embryo sac; and three antipodal cells (at the opposite end of the embryo sac from the egg).
Double Fertilization
The process of pollination being accomplished, the pollen tube gro\\"s through the stigma and style toward the ovules in the ovary. The germ cell in the pollen grain divides and releases two sperm cells which move down the pollen tube. Once the tip of the tube reaches the. micropyle end of the embryo sac, the tube grows through into the embryo sac through one of the synergids which flank the egg. One sperm cell fuses with the egg, producing the zygote which will later develope into the next-generation sporophyte. The second sperm fuses with the two polar bodies located in the center of the sac, producing the nutritive triploid endosperm tissue that will provide energy for the embryo's growth and development.
Fruit
The ovary wall, after fertilization has occurred, develops into a fruit. Fruits may be fleshy, hard, multiple or single. Seeds germinate, and the embryo grows into the next gemeration sporophyte.
Vegetative Propagation
Many plants also have an asexual method of reproduction. Often some species, such as many orchids, are more frequently propagated vegetatively than via seeds. Tubers are fleshy underground stems, as in the lrish potato. Leaflets are sections of leaf will develop roots and drop off the plant, effectively cloning the plant. Runners are shoots running along or over the surface of the ground that will sproout a plantlet, which upon settling to the ground develop into a new independant plant.