Chapter 35: Plant Structure, Growth, and DevelopmentOverview: No Two Plants Are Alike- Plasticity – an organisms ability to alter or “mold” itself in response to local environmental conditions.- The form of any plant is controlled by environmental and genetic factors. As a result, no two plants are identical. - In addition to plastic structural responses of individual plants to specific environments, entire species have by natural selection accumulated chararcteristics of morphology, or external form, that vary little among plants within the species. - Angiosperms comprise 90% of plant species and are at the base of the food web of nearly every terrestrial ecosystem. Concept 35.1: The plant body has a hierarchy of organs, tissues, and cells- Plants, like multicellular animals, have organs that are composed of different tissues, and tissues are composed of cells. - A tissue is a group of cells with a common structure and function. - An organ consists of several types of tissues that work together to carry out particular functions. The Three Basic Plant Organs: Roots, Stems, and Leaves- The basic morphology of vascular plants reflects their evolutionary history as terrestrial organisms that inhabit and draw resources from two very different environments – above and below groundo Plants obtain water and minerals from the soil (below the ground)o They obtain CO2 and light above ground. - The evolutionary solution to this separation of resources was the development of three basic organs: roots, stems, leaves- They organized into two systems: root system and shoot system (consisting of stems and leaves) - Each system depends on the other. o Non-photosynthetic, roots would starve without the sugar and other organic nutrients imported from the photosynthetic tissues of the shoot system. o Conversely, the shoot system (and its reproductive tissues, flowers) depends on water andminerals absorbed from the soil by the roots. Roots- A root is an organ that anchors a vascular (usually in the soil), absorbs minerals and water, and stores organic nutrients o Most eudicots and gymnosperms have a taproot system, consisting of one main vertical root (the taproot) that produces gives rise to lateral roots.  In angiosperms, taproots often stores organic nutrients that the plant consumes during flowering and fruit production.o A fibrous root system consists of a mat of thin roots that spread out below the soil surface and is usually shallower than a taproot system. o The root system helps anchor a plant. o In both taproot and fibrous root systems, absorption of water and minerals occurs near theroot tips, where vast numbers of tiny root hairs enormously increase the surface area. - Some plants have modified roots, roots arising from the stem are said to be adventitious, a term describing any plant part that grows in an unusual location. - Root hairs are extensions of individual epidermal cells on the root surface. o Absorption of water and minerals is also increased by mutualistic relationships between plant roots and bacteria and fungi. 1o Root hairs are not to be confused with lateral roots, which are multicellular organs- Some plants have modified roots. Some modified roots provide additional support and anchorage, while others store water and nutrients or absorb oxygen or water from the air. Stems- A stem is an organ consisting of alternating nodes (the points at which leaves are attached) and internodes (the stem segments between nodes). - At the angle formed by each leaf and the stem is an axillary bud (a structure that has the potential to form a lateral shoot), commonly called a branch.- Growth of a young shoot is usually concentrated at its apex, where there is a terminal bud with developing leaves and a compact series of nodes and internodes. - The presence of a terminal bud is partly responsible for inhibiting the growth of axillary buds, a phenomenon called apical dominance. o By concentrating resources on growing taller, apical dominance is an evolutionary adaptation that increases the plant’s exposure to light. o In the absence of a terminal bud, the axillary buds break dominance and give rise to a vegetative branch complete with its own terminal bud, leaves, and axillary buds. - Modified stems with diverse functions have evolved in many plants as environmental adaptations.o These stems, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots. Leaves- The leaf is the main photosyn. organ of most plants, although green stems are also photosynthetic. o Leaves vary extensively in form; they generally consist of a flattened blade and a stalk, the petiole, which joins the leaf to a stem node. - Monocots and eudocots differ in the arrangement of veinso Most monocots have parallel major veins that run the length of the blade, while eudicot leaves have a multibranched network of major veins. - Plant taxonomists use floral morphology, leaf shape, spatial arrangement of leaves, and the pattern of veins to help identify and classify plants. - Most leaves are specialized for photosynthesis. o Some plants have leaves that have become adapted for other functions. The Three Tissue Systems: Dermal, Vascular, and Ground.- Each organ of a plant has three tissue systems: dermal, vascular, and ground. - The dermal tissue is the outer covering. - In nonwoody plants, it is a single layer of tightly packed cells, or epidermis, which covers and protects all young parts of the plant. - The epidermis of leaves and most stems secretes a waxy coating, the cuticle, which helpsthe aerial parts of the plant retain water. - Vascular tissue, continuous throughout the plant, is involved in the transport of materials betweenroots and shoots. o Xylem conveys water and dissolved minerals upward from roots into the shoots. o Phloem transports food made in mature leaves to the roots; to nonphotosynthetic parts of the shoot system; and to sites of growth, such as developing leaves and fruits. - In angiosperms, the vascular tissue of the root forms a solid central vascular cylinder, while stems and leaves have vascular bundles, strands consisting of xylem and phloem. 2- Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that isexternal to the vascular tissue is called cortex o The functions of ground tissue include photosynthesis, storage, and support. Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma.-