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CHAPTER 35. PLANT STRUCTURE AND GROWTH. Plants are the first link in the food chain through photosynthesis . This affects all animals. I. Concept 35.1: Plant Structure
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CHAPTER 35 PLANT STRUCTURE AND GROWTH
Plants are the first link in the food chain through photosynthesis. This affects all animals. I. Concept 35.1: Plant Structure • Plants, like multicellular animals, have organscomposed of different tissues, which in turn are composed of cells A. Three Basic Plant Organs: Roots, Stems, Leaves • They are organized into a root system and a shoot system • Roots rely on sugar produced by photosynthesis in the shoot system, and shoots rely on water and minerals absorbed by the root system
1. Roots • Are multicellular organs with important functions: • Anchoring the plant • Absorbing minerals and water • Storing organic nutrients • A taproot system consists of one main vertical root that gives rise to lateral roots, or branch roots • Adventitious roots arise from stems or leaves • Seedless vascular plants and monocots have a fibrous root system characterized by thin lateral roots with no main root
In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface area • Many plants have modified roots
2. Stems • A stem is an organ consisting of: • An alternating system of nodes, the points at which leaves are attached b. Internodes, the stem segments between nodes • An axillary bud is a structure that has the potential to form a lateral shoot, or branch • An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot • Apical dominance helps to maintain dormancy in most nonapical buds • Many plants have modified stems
Modified stems include: • Stolons—horizontal stems growing along surface of ground (strawberry) • Rhizomes—horizontal stems growing underground (iris) • Tubers—some end in enlarged tubes where food is stored (potatoes) • Bulbs—vertical, underground shoots with leaves modified for food storage (onion)
3. Leaves a. Are the main photosynthetic organ of most vascular plants b. Leaves generally consist of a flattened bladeand a stalk called the petiole, which joins the leaf to a node of the stem c. Some plant species have evolved modified leaves that serve various functions
B. Tissues: Dermal, Vascular, and Ground • Each plant organ has dermal, vascular, and ground tissues • Each of these three categories forms a tissue system
1. Dermal Tissue System • Outer protective covering that secretes the cuticle • In nonwoody plants consists of the epidermis • In woody plants periderm replaces the epidermis in older regions of stems and roots
2. Vascular Tissue System • Carries out long-distance transport of material between roots and shoots • Two types of Vascular Tissue: a. xylem-transports water and dissolved mineral upward from root into shoots b. phloem-transports organic nutrients from where they are made to where they are needed • Vascular tissue of a stem or root is collectively called the stele • The stele of stems and leaves is divided into vascular bundles (strands of xylem and phloem)
Ground Tissue System • Refers to any tissue not dermal or vascular • Include cells specialized for photosynthesis, storage and support
C. Common Types of Plant Cells • Parenchyma • Collenchyme • Sclerenchyma • Water–conducting cells of the xylem • Sugar—conducting cells of phloem
Protoplast—refers to cell contents exclusive of the cell wall 1. Parenchyma Cells • Most abundant plant tissue • Primary walls are thin and flexible • Lack secondary walls • Store food and water • Protoplast has large central vacuole • Perform most of plant’s metabolic functions such as PS and food storage • Least specialized—typical plant cell • Retains the ability to divide and differentiate into other cell types under special conditions (wound healing)
2. Collenchyma • Living support tissue with unevenly thickened cell walls • No secondary walls (no lignin) • Usually grouped in strands or cylinders for support for young parts of plant • Located just beneath the epidermis and along the midrib of a leaf • Provide flexible support
Sclerenchyma • Sclerenchyma cells are rigid because of thick secondary walls strengthened with lignin • They are dead at functional maturity • There are two types: a. Sclereidsare short and irregular in shape and have thick lignified secondary walls b. Fibersare long and slender and arranged in threads
4. Xylem • Water conducting cells • Dead cells with secondary walls • Stain red • Consists of two cell types: a. Tracheids -long, thin, tapered cells with lignin-hardened secondary walls with pits -water moves from cell to cell through pits -function in support and water transport -more primitive -found in xylem of all vascular plants
b. Vessel elements -wider, shorter, thinner-walled -less tapered -long chains of vessel elements called xylem vessels -more efficient water conductors than tracheids -more advanced -found in most angiosperms and a few gymnosperms
5. Phloem • Sugar-conducting cells • Stain green • Alive at functional maturity • Composed of: a. Sieve tube elements -long narrow cells -lack nucleus, ribosomes, and vacuole, but are alive at maturity -sieve plates (end walls) are perforated as well as side walls b. Companion cells -lie adjacent to sieve tubes and direct their activity -connected to sieve tube by plasmodesmata (thin streams of cytoplasm)
II. Concept 35.2: Meristematic Tissues A. A plant can grow throughout its life; this is called indeterminate growth B. Some plant organs cease to grow at a certain size; this is called determinate growth C. Life span of flowering plants: • Annualscomplete their life cycle in a year or less • Biennialsrequire two growing seasons • Perennialslive for many years
D. Meristemsare perpetually embryonic tissue and allow for indeterminate growth • Apical meristemsare located at the tips of roots and shoots and at the axillary buds of shoots • Apical meristems elongate shoots and roots, a process called primary growth • Lateral meristemsadd thickness to woody plants, a process called secondary growth • There are two lateral meristems: the vascular cambium and the cork cambium • The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem • The cork cambium replaces the epidermis with periderm, which is thicker and tougher
III. Concept 35.3: Primary growth lengthens roots and shoots • Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems • Primary Growth of Roots • The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil • Growth occurs just behind the root tip, in three zones of cells: • Zone of cell division • Near tip and includes the apical meristem • Quiescentcenter is made up of reserve replacement cells
2. Zone of elongation • Cells elongate and push root through the soil 3. Zone of differentiation • Elongated cells complete their differentiation and become functionally mature • Region where root hairs develop • Farthest from root tip
The primary growth of roots produces the epidermis, ground tissue, and vascular tissue • In most roots, the stele is a vascular cylinder • The ground tissue fills the cortex, the region between the vascular cylinder and epidermis • The innermost layer of the cortex is called the endodermis • Lateral roots arise from within thepericycle, the outermost cell layer in the vascular cylinder
B. Primary Growth of Shoots • A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip • Leaves develop from leaf primordiaalong the sides of the apical meristem • Axillary buds develop from meristematic cells left at the bases of leaf primordia
1. Tissue Organization of Stems • Lateral shoots develop from axillary buds on the stem’s surface • In most eudicot stems, the vascular tissue consists of vascular bundles that are arranged in a ring • In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring
