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Chapter 22 Plant Structure and Function: Stems
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Plant Stems
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Stem Parts and Functions
Overview of Stems 1. Stem Functions 2. Stem Growth 3. Types of Stem Tissue 4. Parts of the Stem Interior Exterior 5. Modifications of the Stem Above-ground modifications Below-ground modifications
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Overview – Classifications by Life Cycle
Annuals One growing season Biennials Two growing seasons Perennials Many growing seasons
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3 Important Stem Functions
Produce leaves, branches, and flowers Hold leaves up to the sun Transport substances from root to leaf and leaf to root
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STEM GROWTH Two Types of Growth Primary Growth Secondary Growth
They can grow above ground and below ground depending on the type of plant
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Trunk: Primary Growth Primary Growth is primarily vertical.
This kind of growth gives the plant its height.
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Trunk: Secondary Growth Secondary growth is seen in woody plants.
This kind of growth produces stronger, thicker stems from the vascular cambium and cork cambium. Horizontal Growth
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Types of Stem Tissue Two types:
Monocot – has vascular bundles that contain the xylem & phloem scattered randomly Dicot – have vascular bundles that contain the xylem & phloem organized in a ring like pattern
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Monocots and Dicots
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Parts of the Stem
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Inside Stems Vascular cambium Cork cambium
Thin cylinder of tissue that runs length of roots and stems Cork cambium Produces cells that develop tough cell walls Form protective outside layers on stem and roots Outer bark on a tree
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Lateral Meristems
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Parts of the Stem
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Inside Stems Pith Xylem Spongy tissue in the center
Gets compressed and squeezed as tree grows in diameter Xylem Transports substances away from the roots Roots throughout to the leaves Osmosis pressure Sapwood - active Heartwood – inactive
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Inside Stems Cambium Layer of cells between the xylem and phloem
Adds growth to the diameter of the tree Tree rings
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Three years of Secondary Growth
Tilia - basswood Secondary Phloem cambium Secondary Xylem
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Each year the cambium produces a layer of secondary xylem and a layer of secondary phloem.
This photo shows secondary xylem from parts of three years in Pinus strobus (white pine). spring of the next year winter of that year fall of that year mid-summer of one year
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Inside Stems Phloem Transports dissolved sugars and other organic compounds throughout the plant. Transports both ways Leaves to roots Roots to leaves
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Phloem
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Inside Stems Cork Cambium and Cork (outer bark)
These cells fill up with waxy substance Waterproofing layer Die and become outer layer of bark Don’t stretch – split and peel away Protect from disease. Insects and dehydration
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Bark = epidermis + phloem + vascular cambium Wood = secondary xylem only! Pith = a small percentage of tree diameter at maturity
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This tree is Pinus aristata (bristlecone pine).
One individual of this species shows more than 5000 growth rings! Inner wood, harvested by boring, was used to validate carbon-14 dating. Imagine the stories that this California tree could tell…perhaps something of migration of Asian peoples down the western coast of North America! They were contemporaries of Pharaohs!
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Nodes and Internodes Stems care divided into internodes.
At each end of an internode is a node. A node is a structure on a stem at which two or more leaves are attached
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Internodes Located between mature tissues Base of nodes in grass stems
Rice, corn, sugar cane
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Shrubs vs. Trees Shrubs: Branch from or near base (usually multi-stemmed) Trees: Branch from well-defined trunk (usually single-stemmed)
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Above Ground Stem Modifications
Stolons (Runner) Spider plant, strawberry plant Horizontal stem that runs along the surface of soil Asexual reproduction
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Above Ground Stem Modifications
Crowns Very short stem at surface of soil
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Stem modifications rhizome corm tubers bulb 03_Stems_PPT
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Below Ground Stem Modifications
Rhizome Underground horizontal stems Iris, ferns Food storage Asexual reproduction Bulb Shortened compressed stem surrounded by fleshy leaves Narcissus, onion, tulip, tiger lily
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Below Ground Stem Modifications
Corm Mostly stem tissue with some scaly leaves at top Crocus, Food storage Tubor White potato Swollen underground stem
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