Life of A Flowering Plant Flashcards
Structural Organization of Flowering Plants
Flowering plants are organized into two primary systems: the root system and the shoot system. The root system typically exists below the ground and is specialized for the acquisition of water and nutrients, the storage of starch, and providing physical anchorage to the substratum. In contrast, the shoot system is primarily located above ground and is designed for collecting light, carrying out photosynthesis, facilitating reproduction, and transporting nutrients between the leaves and the roots. Together, these systems form a continuous pipeline for the movement of fluids and provide the structural framework for the plant's life cycle.
Plant Growth and Developmental Processes
Plant growth occurs at specialized regions called meristems, which contain undifferentiated cells capable of continuous division through mitosis. As these cells divide, they form new cells, many of which eventually differentiate to perform specific functions and lose their ability to divide further. There are two main types of growth: primary and secondary. Primary growth, which happens at the apical meristems located at the tips of roots and shoots, increases the height of the plant and develops specialized structures. Secondary growth, which occurs at lateral meristems, increases the girth or thickness of stems and roots in woody plants. This process is driven by the vascular cambium, a meristematic tissue that produces new xylem and phloem. All such growth and activities, including fruit ripening and flower scent production, are regulated by plant hormones produced in various tissues and transported via the phloem.
Woody vs. Herbaceous Plant Characteristics
Plants are classified into two broad categories based on their growth habits: herbaceous and woody. Herbaceous plants, such as grasses, beans, and lettuce, exhibit only primary growth and possess flexible stems. These plants are typically annuals, meaning they complete their life cycle in a single year. Woody plants, including bushes, shrubs, and deciduous trees (those that lose leaves in the fall), exhibit both primary and secondary growth and are usually perennials that live for many years. Secondary growth in woody plants results in the formation of annual rings. These rings consist of light-colored xylem, which grows quickly during the spring and early summer when water is abundant, and dark-colored xylem, which grows more slowly in late summer and early fall as water becomes scarce. A single year of growth is represented by one light and one dark ring combined. Within the center of a woody stem, older xylem that no longer conducts water is known as heartwood; it provides structural support and stores metabolic wastes like gums, resins, and oils. The younger, functional xylem is called sapwood, and the entire structure is protected by an outer layer of tough cells known as bark.
Tissue Systems and Cellular Components
Plants contain three fundamental tissue types: dermal tissue, ground tissue, and vascular tissue. Dermal tissue acts as the outer protective covering and includes the epidermis (a layer of thin, flat cells) and the peridermis, which replaces dead epidermal cells. Most epidermal cells are coated with a waxy cuticle that is waterproof and prevents excessive water loss. Ground tissue makes up the bulk of the internal body of a young plant and consists of three cell types: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are thin-walled, remain alive at maturity, and are involved in photosynthesis, hormone secretion, and food storage (such as in potatoes). Collenchyma cells are elongated, honeycomb-shaped living cells that provides support for young, growing plants (such as in celery). Sclerenchyma cells possess thick secondary cell walls, are typically dead at maturity, and provide rigid support for adult plants and structures like nut shells or pear fruit fibers. Finally, vascular tissue constitutes the internal plumbing system for the transport of water and nutrients.
Vascular System Structure and Function
The vascular system consists of two specialized tissues: xylem and phloem. Xylem is responsible for conducting water and dissolved minerals upward from the roots. Its structure includes sclerenchyma fibers for support, and two types of conducting cells: tracheids and vessel elements. Tracheids are needle-like dead cells that pass water vertically or horizontally, while vessel elements are wide, dead cells that form continuous tubes for vertical transport. Phloem conducts water, sugars, amino acids, and hormones throughout the plant. It is composed of sieve-tube elements, which are living but not metabolically active cells that pass organic molecules through horizontal disks called sieve plates, and companion cells, which nourish and regulate the sieve-tube elements. These tissues form a continuous network connecting the roots to the shoots.
Leaf Anatomy and Gas Exchange
The leaf is a complex organ designed primarily for photosynthesis. Its outermost layers are the upper and lower epidermis, which are covered by a waxy cuticle to prevent water loss. The interior, known as the mesophyll, consists of the palisade layer and the spongy layer; both are photosynthetic, though the spongy layer is also specialized for gas exchange. Stomata (singular: stoma) are pores, typically found on the underside of the leaf, that allow to enter and and vapor to exit. The opening and closing of these pores are regulated by guard cells based on turgor pressure. When turgor pressure is high, the stomata open; when it is low, they close. The evaporation of water through the stomata is a process called transpiration. Within the leaf, vascular bundles (veins) containing xylem and phloem are surrounded by bundle-sheath cells for protection and support.
Stem and Root Anatomy
Stems are structured to hold the plant upright and facilitate transport. In the center is the pith, comprised of parenchyma cells used for food storage and support. Outside the pith is the cortex, followed by vascular bundles and the protective epidermis. Roots are specialized for absorption and anchorage. Most plants have either a taproot (a single central root) or fibrous roots (a network of similarly-sized roots). Branch roots and root hairs significantly increase the surface area for absorption. The root tip is protected by a root cap, which secretes a lubricant as the root pushes through the soil via mitotic division in the apical meristem. Internally, the root contains a cortex for starch storage, an endodermis that surrounds the vascular cylinder, and a pericycle layer that conducts materials into the xylem and phloem. A critical feature of the root is the Casparian strip, a waxy layer surrounding endodermal cells that forces water to pass through plasma membranes rather than moving freely between cells.
Water Transport and Symbiotic Relationships
Water and mineral transport relies on transpiration, cohesion, and adhesion. Transpiration at the leaves creates a tension that pulls water upward. Cohesion (the attraction between water molecules) and adhesion (the attraction of water to xylem walls) via hydrogen bonds create a continuous "water chain" from the roots to the leaves. At the root level, minerals are actively pumped into the vascular cylinder, and water follows by osmosis. Many plants form symbiotic relationships to enhance nutrient uptake. Fungal mycorrhizae release enzymes that break down rocks to provide minerals, while nitrogen-fixing bacteria in the root nodules of legumes (like peas and beans) convert atmospheric nitrogen into usable forms like ammonium or nitrate ions.
Plant Classification: Monocots and Dicots
Angiosperms are divided into two main groups: monocots and dicots (eudicots). Monocots, such as grasses, lilies, orchids, and palms, possess a single cotyledon (seed leaf), parallel leaf veins, flower parts in multiples of , scattered vascular bundles in the stem, and a fibrous root system. Dicots, such as garden plants, wildflowers, and deciduous trees, possess two cotyledons, netted (branched) leaf veins, flower parts in multiples of or , vascular bundles arranged in a ring, and a taproot system.
Flower Anatomy and Sexual Reproduction
In angiosperms, the dominant form is the sporophyte, while the microscopic gametophyte grows within it. The flower is the reproductive organ, consisting of four main parts which are considered modified leaves: the sepals (support the bud), petals (advertise for pollinators), stamens (male structure), and carpels (female structure). The stamen consists of an anther, where pollen is produced, and a filament. The carpel consists of a stigma where pollen lands, a style, and an ovary where ovules are produced. Complete flowers contain all four parts, while incomplete flowers lack one or more. Perfect flowers have both stamens and carpels, while imperfect flowers (like zucchini) have only one or the other.
Gametogenesis and Double Fertilization
The male gametophyte is the pollen grain. A microspore mother cell (MMC) undergoes meiosis to form haploid microspores, which then divide mitotically to form immature pollen. Upon further division, a generative cell produces sperm cells, making the pollen mature. The female gametophyte is the embryo sac. A megaspore mother cell undergoes meiosis to produce haploid megaspores, of which die. The remaining megaspore undergoes mitotic divisions to produce nuclei within a single cell. Cytokinesis then creates separate cells, one of which is the egg. During pollination, a pollen grain lands on the stigma and grows a tube to the ovary. Double fertilization occurs: one sperm fertilizes the egg to form the embryo, while the other sperm fuses with two polar nuclei to form a triploid () endosperm, which serves as nutrient tissue.
Seed Dispersal and Germination
As the flower develops into a fruit, it facilitates seed dispersal via wind, water, or animals. Fleshy fruits attract animals, which eat the fruit and deposit indigestible seeds elsewhere in nutrient-rich feces. Other seeds have specialized structures for wind transport or for attaching to animal fur. Seed germination requires moisture and specific temperatures. Some seeds require dormant periods, exposure to cold, or the breaking of the seed coat. In monocots, the emerging shoot is protected by a sheath called the coleoptile. In dicots, which lack a coleoptile, the shoot above the cotyledons is the epicotyl, and the part below is the hypocotyl. During germination in dicots, the endosperm is often consumed by the developing cotyledons until the plant can begin photosynthesis with its first true leaves.