PLANTS

Some Key Plant terms to remember from Bio. Bryophyte: non-vascular plants such as moss, liverworts and hornworts Pteridophytes: “seedless” vascular plants such as ferns Gymnosperms: “unprotected” seeds such as conifers Angiosperms: flowering plants two main classes of angiosperm 1. Monocot: one cotyledon (“seed leaf”) have long narrow leaves with parallel veins. Examples include grass, grains. 2. Dicots: two cotyledons, branched leaves and veins. Examples include most trees, flowers.

Monocot root Dicot root - phloem & xylem in circles - xylem in a cross pattern

Monocot stem Dicot stem - vascular bundles scattered - vascular bundles in a ring throughout ground tissue

Plant Transpiration Xylem is tissue that moves water up a plant. Two main xylem cells are tracheids and vessels. Phloem moves sucrose down the plant by cells called sieve tubes. Next to sieve tube cells are companion cells which contain a nucleus and ribosomes which sieve tubes lack. “Transpirational pull” is due to cohesion of water thanks to the hydrogen bonding and makes it possible to pull water up a column from the roots to the leaves.

Stomates open and close by changing their “turgor” pressure Stomates open and close by changing their “turgor” pressure. This change in turgor pressure is the result of uptake and loss of potassium ions. Stomata open when guard cells accumulate potassium from neighboring epidermal cells. This uptake of solute causes water to enter by osmosis. Stomates close when the potassium leaves the guard cells causing a reversal of osmosis. In general stomates open by day and close at night to prevent needless water loss when photosynthesis is not occurring.

Tissue that is neither dermal nor vascular is called “ground tissue” Tissue that is neither dermal nor vascular is called “ground tissue”. Ground tissue aids storage, photosynthesis and support. Ground tissue inside of the vascular tissue layer is called pith and if outside is called cortex.

There are three basic cell types for ground tissues 1. Parenchyma: “typical” plant cells and perform most of the metabolic functions for a plant. 2. Collenchyma: have thicker cell walls, support young plant shoots 3. Sclerenchyma: thick cell walls strengthened by lignin and are thus very rigid. Includes tracheids and vessels of xylem

Alternation of Generation Two multicellular body forms that alternate in the life cycle of land plants are the gametophyte whose cells are haploid and the sporophyte whose cells are diploid. A spore can grow into a new organism without fusing with another cell (in contrast to gametes). Sporophytes produce spores that develop into gametophytes and gametophytes produce gametes that unite to form the zygotes that develop into sporophytes. In bryophytes the gametophyte generation is dominant while in angiosperms it’s the sporophyte generation that is dominant.

Plant Reproduction Key terms: Carpel (pistil): female, made of stigma, style, and ovary Stamen: male, contains anther and filament One ovule forms within the chambers of the ovary . One cell within the ovule (the megasporocyte) goes through meiosis forming four haploid megaspores. In most angiosperms, only one megaspore survives and it divides by mitosis three times, producing 8 haploid nuclei. Three nuclei form the embryo sac, one egg and two “synergids” near the micropyle end, three more nuclei collect at the opposite end (function unknown) and the remaining two, called polar nuclei form in the middle of the embryo sac.

A pollen grain lands on the stigma causing a pollen tube to form through the style. The sperm will travel down the pollen tube and while doing so will divide once by mitosis to become two sperm. One sperm will fertilize the egg and the other combines with the polar nuclei to form a triploid (3n) nucleus that will develop into the endosperm that will provide nutrients to the developing embryo. Process called “double fertilization”

Plant “growth” only occurs from the meristems Plant “growth” only occurs from the meristems. There are two main types of meristem, Apical located at the tips of roots and in the bud shoots. Apical meristem allows the plant to grow in length (called primary growth). The second type of meristem is called lateral meristem, called secondary growth and causes the plant to grow in thickness.

If a plant grows in length it is “primary growth” If a plant grows in length it is “primary growth”. If a plant grows in thickness it is “secondary growth”. Secondary growth occurs from the cork cambium which produces secondary dermal tissue and from the vascular cambium which produces new xylem and phloem.

Key Plant Hormones Auxin: promotes rapid growth and elongation of cells at the apical meristem and promotes fruit growth. Gibberellin: promotes elongation of stems and flowering Cytokinins: promotes cell division especially in germinating seeds and cell differentiation, produced in the roots. Ethylene: promotes dying and dropping off of floral parts, fruit ripening and leaf abscission Abscisic acid: acts as an inhibitor that keeps growth hormones from acting on buds and leaves. Also closes stomates if there is a water shortage. Florigen: aids in flowering

Photoperiodism (response to light) allows plants to detect seasons by controlling when a plant flowers. Length of night is the key. Short day plants (chrysanthemums, poinsettias, soybeans) need long nights Long day plants (spinach, lettuce, cereal grains) need short nights Some plants (tomatoes, rice, dandelions) are day neutral where night length doesn’t matter

Plants detect night length very precisely, with some where only 1 minute will make a difference. Thus some plants flower on the same day every year. If night is interrupted with dim light a short day plant will not flower

Photoperiodism is controlled by phytochromes responding to light wavelength changes. Pr and Pfr switch back and forth depending upon light exposure. In darkness Pfr (far red) is converted to Pr (red) and in sunlight Pr is converted back to Pfr

Tropisms - Phototropism: response to light. Auxin causes plant to bend toward the light by elongating the cells opposite the light side - thigmotropism: response to touch, allows vines to wind around a support - gravitropism (geotropism): response to gravity, ensures that roots grow into soil and stems grow toward surface. How a plant senses gravity is not well established but is believed to involve “statoliths” which are particles in the cells that are heavy and may fall toward the gravitational pull.