PLANTS
The plant kingdom contains multicellular phototrophs that usually live on land. All plant cells have a cell wall containing the carbohydrate cellulose, and often have plastids in their cytoplasm. The plant life cycle has an alternation between haploid (gametophyte) and diploid (sporophyte) generations. There are more than 300,000 living species of plants known, as well as an extensive fossil record.
Plants divide into two groups:
1) plants lacking lignin-impregnated conducting cells (the nonvascular plants). Living groups of nonvascular plants include the bryophytes.
2) plants containing lignin-impregnated conducting cells (the vascular plants). Vascular plants are the more common plants like pines, ferns, corn, and oaks.
Fossil and biochemical evidence indicates plants are descended from multicellular green algae.
Plants have an alternation of generations: the diploid spore-producing plant (sporophyte) alternates with the haploid gamete-producing plant (gametophyte).
Bryophytes are small, nonvascular plants that first evolved approximately 500 million years ago. The earliest land plants were most likely bryophytes. Bryophytes lack vascular tissue and have life cycles dominated by the gametophyte phase. Roots are absent in bryophytes, instead there are root-like structures known as rhizoids. The group includes the hornworts, liverworts, and mosses.
Vascular plants tend to be larger and more complex than bryophytes, and have a life cycle where the sporophyte is more prominent than the gametophyte. Vascular plants also demonstrate increased levels of organization by having organs and organ systems. The earliest vascular plants had no roots, leaves, fruits, or flowers.
Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm means seed: gymnosperm = naked seeds. Gymnosperms developed during the Paleozoic Era and became dominant during the early Mesoszoic Era. There are 700 living species placed into four divisions: conifers, cycads, ginkgos, and gnetales.
The Angiosperms (angios = hidden) produce modified leaves grouped into flowers that in turn develop fruits and seeds. There are presently 235,000 known living species. The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers.
GENERAL PLANT ORGANIZATION
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 cell types rise by mitosis from a meristem. A meristem may be defined as a region of localized mitosis. 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.
1) The Dermal tissue covers the outer surface of herbaceous plants. It is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss, and acts as a barrier to fungi and other invaders.
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 bean-shaped cells covering the stomata opening. They regulate exchange of water vapor, oxygen and carbon dioxide through the stoma.
2) The Ground tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma, and sclerenchyma cells are common in the ground tissue.
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.
Collenchyma cells support the plant. These cells are characterized by thickenings of the wall, the are alive at maturity. They tend to occur as part of vascular bundles or on the corners of angular stems.
Sclerenchyma cells support the plant. They often occur as bundle cap fibers. Sclerenchyma cells are characterized by thickenings in their secondary walls. They are dead at maturity.
3) The Vascular tissue transports food, water, hormones and minerals within the plant. Vascular tissue includes xylem, phloem, parenchyma, and cambium cells.
Xilem 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. The more identifiable cells, tracheids and vessel elements. Tracheids are the more primitive of the two cell types, occurring in the earliest vascular plants. Vessel elements are shorter, much wider, and lack end plates. They occur only in angiosssperms, the most recently evolved large group of plants.
Phloem cells conduct food from leaves to rest of the plant. They are alive at maturity. Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the companion and sieve cells. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, as sucrose, flows through the sieve cells.
Angiosperms, flowering plants, are divided into two groups: monocots and dicots.
Monocot seeds have one "seed leaf" termed a cotyledon (in fact monocot is a shortening of monocotyledon). Dicots have two cotyledons. Both groups, however, have the same basic architecture of nodes, internodes, etc.
Monocot stems have scattered vascular bundles. Dicot stems have their vascular bundles in a ring arrangement. Monocot stems have most of their vascular bundles near the outside edge of the stem.
Monocot roots, interestingly, have their vascular bundles arranged in a ring. Dicot roots have their xylem in the center of the root and phloem outside the xylem. A carrot is an example of a dicot root.
Monocots have their flower parts in threes or multiples of three; example the tulip and lily (Lilium ). Dicots have their flower parts in fours (or multiples) or fives (or multiples). Examples of some common dicot flowers include the geranium, snapdragon, and citrus.
Monocots usually don't have secondary growth. Some, such as bamboo and palm trees, have secondary growth. Monocot secondary growth differs from dicot secondary growth in that new bundles are formed at the edge of the stem. These new bundles are close together, providing support for the stem.
The leaf consists of the (generally) flat blade, one or more leaf veins, a petiole, and usually an axillary bud. The petiole can be long (as in celery and bok-choy) or short (as in cabbage and lettuce). Leaves may be simple or compound. Leaves attach to stems at nodes (internodes are the spaces between nodes).
Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.
A horomone is any chemical produced in one part of the body that has a target elsewhere in the body. Plants have five classes of hormones.
1) Auxins. A group of hormones involved in controlling plant growth and other functions; once thought responsible for phototropism by causing the cells on the shaded side of a plant to elongate, thereby causing the plant to bend toward the light.
2) Gibberellins. A group of hormones that stimulate cell division and elongation in plants.
3) Cytokinins promote cell division. They are produced in growing areas, such as meristems at tip of the shoot. Zeatin is a hormone in this class, and occurs in corn.
4) Abscisic acid promotes seed dormancy by inhibiting cell growth. It is also involved in opening and closing of stomata as leaves wilt.
5) Ethylene is a gas produced by ripe fruits. Ethylene is used to ripen crops at the same time. Sprayed on a field it will cause all fruits to ripen at the same time so they can be harvested.
PLANT NUTRITION
Plants obtain their nutrition from the soil and atmosphere. Using sunlight as an energy source, plants are capable of making all the organic macromolecules they need by modifications of the sugars they form by photosynthesis. However, plants must take up various minerals through their root systems for use.
Carbon, Hydrogen, and Oxygen are considered the essential elements. Nitrogen, Potassium, and Phosphorous are obtained from the soil and are the primary macronutrients. Calcium, Magnesium, and Sulfur are the secondary nutrients needed in lesser quantity. The micronutrients, needed in very small quantities and toxic in large quantities, include Iron, Manganese, Copper, Zinc, Boron, and Chlorine.
Plants use these minerals in:
- Structural components in carbohydrates and proteins
- Organic molecules used in metabolism, such as the Magnesium in chlorophyll and the Phosphorous found in ATP
- Enzyme activators like potassium, which activates possibly fifty enzymes
- Maintaining osmotic balance
Plants need nitrogen for many important biological molecules including nucleotides and proteins. However, the nitrogen in the atmosphere is not in a form that plants can utilize. Many plants have a symbiotic relationship with bacteria growing in their roots: organic nitrogen as rent for space to live. These plants tend to have root nodules in which the nitrogen-fixing bacteria live.
Roots have extensions of the root epidemal cells known as root hairs. While root hairs greatly enhance the surface area (hence absorbtion surface), the addition of symbiotic mycorrizae fungi vastly increases the area of the root for absorbing water and minerals from the soil.
Xylem is the water transporting tissue in plants that is dead when it reaches functional maturity. Water is pulled up the xylem by the force of transpiration, water loss from leaves.
Plants make sugar by photosynthesis, usually in their leaves. Some of this sugar is directly used for the metabolism of the plant, some for the synthesis of proteins and lipids, some stored as starch. Other parts of the plant also need energy but are not photosynthetic, such as the roots. Food must therefore be transported in from a source, an action accomplished by the phloem tissue.
One plant response to environmental stimulus involves plant parts moving toward or away from the stimulus, a movement known as a tropism.
Nastic movements are plant movements independent of the direction of the stimulus. Nastic movements, such as nyctinasty, result from several types of stimuli, including light and touch. Legumes turn their leaves in response to day/night conditions (photonasty). Mimosa , also known as the sensitive plant, has its leaves close up when touched (thigmonasty).
Phototropism is the reaction of plants to light in which the plants bend toward the light.
Geotropism or Gravitropism is a plants' response to gravity: roots grow downward, showing positive geotropism, while shoots grow upward in a negative response.
Thigmotropism is plant response to contact with a solid object. Tendrils of vines warp around objects, allowing the vine to grow upward.
Photoperiodism is the ability of certain plants to sense the relative amounts of light and dark in a 24-hour period; controls the onset of flowering in many plants. Phytochrome is a plant pigment in the leaves of plants that detects the day length and generates a response.
Heliotropism Sunflowers turn to face the sun throughout the day. (Solar tracking).
The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into the gametophyte phase. Gametophyte 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 or pollen) and/or archegonium (producing eggs or ova). 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 are flowering plants, were the last of the seed plant groups to evolve, appearing over 140 million years ago. All flowering plants produce flowers. Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm. Fertilization is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: one fuses with the egg to produce the zygote, the other helps form the nutritive tissue known as endosperm.
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. 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).
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.
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 length.
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 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 generation sporophyte.