GACE Agriculture (6–12) (709)

Plants are classified using a hierarchical system: Kingdom, Phylum, Class, Order, Family, Genus, and Species. Agricultural plants fall into two major groups: monocotyledons (monocots) and dicotyledons (dicots).

Monocots have one seed leaf (cotyledon), parallel leaf venation, fibrous root systems, and flower parts in multiples of three. Examples include corn, wheat, rice, grasses, and onions.

Dicots have two seed leaves, netted leaf venation, taproot systems, and flower parts in multiples of four or five. Examples include soybeans, cotton, tomatoes, beans, and most broadleaf plants.

Plants are also classified by life cycle. Annuals complete their life cycle in one growing season (corn, wheat, soybeans). Biennials require two growing seasons to complete their cycle, producing vegetative growth the first year and flowers/seeds the second year (carrots, cabbage, onions). Perennials live for more than two years and may produce seeds multiple times (alfalfa, fruit trees, bermudagrass).

Root Systems: Roots anchor plants, absorb water and nutrients, and store carbohydrates. The root cap protects the growing tip. The zone of cell division contains meristematic tissue. The zone of elongation is where cells lengthen. The zone of maturation is where root hairs develop. Root hairs dramatically increase surface area for absorption.

Taproot systems have one dominant root growing downward with smaller lateral roots. Fibrous root systems have many similarly sized roots spreading outward. Adventitious roots arise from stems or leaves rather than from other roots.

Stem Structure: Stems support leaves and flowers, transport water and nutrients, and store food. The epidermis is the outer protective layer. The cortex lies beneath the epidermis and contains parenchyma cells. Vascular bundles contain xylem (water transport upward) and phloem (sugar transport throughout plant). The pith is the central tissue in many stems.

Herbaceous stems are soft and green. Woody stems develop bark and secondary growth. Nodes are points where leaves attach. Internodes are stem sections between nodes.

Leaf Structure: Leaves are the primary site of photosynthesis. The blade is the flat, expanded portion. The petiole is the stalk connecting blade to stem. The midrib is the central vein. Leaf margins may be entire (smooth), serrate (saw-toothed), lobed, or other patterns.

Internal leaf anatomy includes the upper epidermis with waxy cuticle, palisade mesophyll (tightly packed cells for photosynthesis), spongy mesophyll (loosely arranged with air spaces), lower epidermis with stomata, and guard cells that control stomatal opening.

Flower Structure: Complete flowers have four main parts: sepals (calyx) protect the bud, petals (corolla) attract pollinators, stamens (male parts) consist of anther and filament, and pistil (female part) consists of stigma, style, and ovary.

Perfect flowers contain both male and female parts. Imperfect flowers have only stamens (staminate) or pistils (pistillate). Monoecious plants have separate male and female flowers on the same plant (corn). Dioecious plants have male and female flowers on separate plants (holly).

Photosynthesis: The process converting light energy to chemical energy. The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.

Light reactions occur in thylakoid membranes and produce ATP and NADPH. The Calvin cycle occurs in the stroma and produces glucose. Chlorophyll a and b absorb primarily red and blue light, reflecting green.

C3 plants (most crops) fix CO₂ directly into a 3-carbon compound. C4 plants (corn, sorghum, sugarcane) have specialized anatomy and fix CO₂ into a 4-carbon compound first, making them more efficient in hot, dry conditions. CAM plants (cacti, pineapple) open stomata at night to conserve water.

Cellular Respiration: The process releasing energy from glucose. The equation is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP.

Glycolysis occurs in the cytoplasm. The Krebs cycle occurs in mitochondria. The electron transport chain produces most ATP. Aerobic respiration requires oxygen and yields approximately 36-38 ATP per glucose. Anaerobic respiration (fermentation) occurs without oxygen and yields only 2 ATP.

Transpiration: Water loss through stomata. Transpiration creates a pull that moves water upward through xylem. Factors affecting transpiration include temperature, humidity, wind, light, and soil moisture. Transpiration cools plants and delivers minerals throughout the plant.

Translocation: Movement of sugars through phloem from sources (leaves) to sinks (roots, fruits, growing tips). Translocation moves in multiple directions unlike xylem transport.

Asexual Reproduction: Produces genetically identical offspring (clones). Methods include cuttings (stem, leaf, root), layering (air layering, tip layering), division, grafting, budding, tissue culture, and runners/stolons.

Sexual Reproduction: Involves fusion of gametes and produces genetic variation. Pollination transfers pollen from anther to stigma. Self-pollination occurs within the same flower or plant. Cross-pollination occurs between different plants.

Fertilization occurs when sperm from pollen reaches the egg in the ovule. Double fertilization in angiosperms produces both embryo and endosperm. Seed development includes embryo, endosperm (food storage), and seed coat. Fruit development occurs from the ovary wall.