Chapter 22 Plant Structure and Function: Stems

Plant Stems

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

Overview – Classifications by Life Cycle Annuals One growing season Biennials Two growing seasons Perennials Many growing seasons

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

STEM GROWTH Two Types of Growth Primary Growth Secondary Growth They can grow above ground and below ground depending on the type of plant

Trunk: Primary Growth Primary Growth is primarily vertical. This kind of growth gives the plant its height.

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

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

Monocots and Dicots

Parts of the Stem

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

Lateral Meristems

Parts of the Stem

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

Inside Stems Cambium Layer of cells between the xylem and phloem Adds growth to the diameter of the tree Tree rings

Three years of Secondary Growth Tilia - basswood Secondary Phloem cambium Secondary Xylem

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

Inside Stems Phloem Transports dissolved sugars and other organic compounds throughout the plant. Transports both ways Leaves to roots Roots to leaves

Phloem

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

Bark = epidermis + phloem + vascular cambium Wood = secondary xylem only! Pith = a small percentage of tree diameter at maturity

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!

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

Internodes Located between mature tissues Base of nodes in grass stems Rice, corn, sugar cane

Shrubs vs. Trees Shrubs: Branch from or near base (usually multi-stemmed) Trees: Branch from well-defined trunk (usually single-stemmed)

Above Ground Stem Modifications Stolons (Runner) Spider plant, strawberry plant Horizontal stem that runs along the surface of soil Asexual reproduction

Above Ground Stem Modifications Crowns Very short stem at surface of soil

Stem modifications rhizome corm tubers bulb 03_Stems_PPT

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

Below Ground Stem Modifications Corm Mostly stem tissue with some scaly leaves at top Crocus, Food storage Tubor White potato Swollen underground stem