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Chapter 36


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Ingo Ensminger

BIO204 Notes Sept.8/2011 Chapter 36: Plant Form and Function Key Concepts  The vascular plant body consists of (1) a root system that anchors the individual and absorbs water and key ions, and (2) a shoot system that absorbs carbon dioxide and sunlight. Both systems are dynamic—they grow and change throughout life.  Because body size and shape varies so much among species and individuals, plants are able to harvest light and other resources in unique ways.  Primary growth occurs when cells located at the tips of each root and shoot divide. Primary growth extends the body and gives rise to three primary tissue systems. These tissue systems are specialized for protection, food production and storage, and transport.  In some species, secondary growth makes roots and shoots widen throughout life. Secondary growth occurs when cells near the perimeter of a root or shoot divide. Secondary growth adds transport tissue and provides additional structural support. 36.1 Plant form: Themes with many variations  For photosynthesis to occur, plants need large amounts of light and carbon dioxide and a small amount of water as an electron source (Figure 36.1).  They also need large amounts of water to fill their cells and maintain them at normal volume and pressure.  Obtain nitrogen, phosphorous, potassium, magnesium, and other nutrients to synthesize the macromolecules needed to build and run cells.  The root system and the shoot system are the two basic systems for acquiring and transporting nutrients (Figure 36.2).  The root system is below ground, anchors the plant, and takes in water and nutrients from the soil.  The shoot system is above ground and harvests light and carbon dioxide from the atmosphere. Both systems connected by vascular tissue, which allows transport between them.  Most root and shoot systems have the same general structures (Figure 36.3). The organization of the vascular tissue is not identical in all plants, and the morphology of root and shoot systems and leaves varies widely. The Root system  Many root systems have a vertical section called a taproot and numerous lateral roots that run more or less horizontally.  The root system functions to anchor the plant in the soil absorb water and ions form the soil, conduct water and selected ions to the shoot, and store material produced in the shoot for later use. Morphological Diversity  Root systems have similar functions in all plants but can be highly diverse is structure. Figure 36.4 shows the diversity of root systems in prairie plants.  The diversity of root systems observed in prairie plants has two important consequences: (1) an array of plants can coexist in the same area with less competition for soil resources; and (2) most individuals can survive intense water stress during drought years. Phenotypic plasticity  Roots show a great deal of phenotypic plasticity, meaning that they are changeable depending on environmental conditions.  Even genetically identical individuals will have very different looking root systems if they are growing in different types of environments.  Phenotypic plasticity is important because plants grow throughout their lives. Root actively grow into areas of soil where resources are abundant, and do not grow or die back in areas where resources are lacking. Modified Roots  Adventitious roots are roots that develop from the shoot system instead of the root system.  Roots can also be modified for functions other than anchoring and absorbing water and ions from the soil. Specialized lateral roots function in gas exchange.  Among individuals, the overall size and shape of the root system is variable and dynamic . The Shoot system  The shoot system consists of one or more stems, which are vertical aboveground structures.  Each stem consists of nodes, where leaves are attached, and internodes—segments between nodes.  A leaf is an appendage that projects from the stem laterally. Leaves usually function as a photosynthetic organ.  The nodes where leaves attach to the stem are the site of axillary buds. The axillary bud may develop into a branch—a lateral extension of the shoot system.  At the tip of each stem is an apical bud, where growth occurs that extends the length of the stem or branch.  Once a leaf, node, or internode forms, it does not increase much in size over time. Instead, plants grow by adding more parts rather than increasing the size of each part. Morphological Diversity  The size and shape of shoot systems can vary greatly among species. Plant form can vary as a function of branch angle and internode length (Figure 36.7).  Variation in size and shape of the shoot system allows plants of different species to harvest light at different locations and thus minimize competition. It also allows them to thrive in a wide array of habitats.  Silverswords diverse in size, shape and growth habit. Can be rosettes, thick woody or form cushions or mats.  The diversity of shoot system a suite of adaptations for harversting light and CO in 2arious env. Phenotypic Plasticity  Shoots respond to variation in light availability.  Because the shoot system continues to grow through the lifetime of the plant, it can respond to changes in environmental conditions. Modified Shoots  Modified stems are common. Examples are the stems of cacti, stolons (stems that run over the soil surface), rhizomes (stems that grow horizontally underground), tubers (rhizomes modified to store carbohydrates), and thorns (stems that protect the plant (Figure 36.10). The leaf  In most plant species, the vast majority of photosynthesis occurs in leaves.  The external anatomy of a typical leaf consists of the blade and the petiole (Figure 36.11a). Morphological Diversity  Leaves usually have an easily recognizable blade, but these may vary in size and shape.  Large SA = more loss of water so leaves adapted  The arrangement of leaves on a stem can also vary.  Plants thus have dramatically different ways to arrange their leaves in space, presumably to maximize the efficiency of light capture. Phenotypic Plasticity  Although leaves do not grow continuously, they do exhibit phenotypic plasticity.  One example is that oak trees have large, broad shade leaves (high SA which max. absorption of rare photons) and smaller sun leaves(reduce water loss where light abundant). Modified Leaves  Not all leaves function primarily in photosynthesis.  Other leaf functions include food or water storage, climbing, attraction of pollinators, trapping prey (for carnivorous plants), and collecting soil (Figure 36.14).  Flowerpot plant used by ants has adventitious roots which absorbs nutrients from ant feces and other material that accumulate inside. 36.2 Primary Growth  Plants grow continuously because they have meristems – populations of undifferentiated cells that retain the ability to undergo mitosis and produce new cells.  Apical meristems located at tip of each root and shoot. As cell # increases in response to cell divison and as cell size increases during differentiation, the meristems at each root and shoot tip extend the plant body outward allowing exploring extra space. Process is called primary growth. It increases length if roots and shoots.  Cell derived from apical meristems form primary plant body How do apical meristems produce the primary plant body?  Apical meristems give rise to three distinct populations of cells that give rise to three major tissue systems of the plant (Figure
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