end show slide 1 of 36 copyright pearson prentice hall biology

Copyright Pearson Prentice Hall

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Biology


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23–3 Stems


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Stem Structure and Function


What are the three main functions of stems?


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Stems have three important functions:

• they produce leaves, branches and flowers

• they hold leaves up to the sunlight

• they transport substances between roots and leaves


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Stems make up an essential part of the water and mineral transport systems of the plant.

Xylem and phloem form continuous tubes from the roots through the stems to the leaves.

This allows water and nutrients to be carried throughout the plant.


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Stems are surrounded by a layer of epidermal cells that have thick cell walls and a waxy protective coating.


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Leaves attach to the stem at structures called nodes.

The regions of stem between the nodes are internodes.

Small buds are found where leaves attach to nodes.

Node

Internode

Bud

Node


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Buds contain undeveloped tissue that can produce new stems and leaves.

In larger plants, stems develop woody tissue that helps support leaves and flowers.

Bud


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Monocot and Dicot Stems


The arrangemnet of tissues in a stem differs among seed plants.

How do monocot and dicot stems differ?


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In monocots, vascular bundles are scattered throughout the stem. In dicots and most gymnosperms, vascular bundles are arranged in a ringlike pattern.


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Monocot Stems

Monocot stems have a distinct epidermis, which encloses vascular bundles.

Each vascular bundle contains xylem and phloem tissue.


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Vascular bundles are scattered throughout the ground tissue.

Ground tissue consists mainly of parenchyma cells.

Monocot

Vascular bundles

Epidermis

Ground tissue


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Dicot Stems

Dicot stems have vascular bundles arranged in a ringlike pattern.

The parenchyma cells inside the vascular tissue are known as pith.

Dicot

Cortex

Pith

Vascular bundles Epidermis


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The parenchyma cells outside of the vascular tissue form the cortex of the stem.

Dicot

Cortex

Pith

Vascular bundles Epidermis


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Primary Growth in Stems

How do primary growth and secondary growth occur in stems?


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Primary Growth of Stems


All seed plants undergo primary growth, which is an increase in length.

For the entire life of the plant, new cells are produced at the tips of roots and shoots.

Year 1Year 2Year 3

Primary growth

Apical meristem

Primary growth

Leaf scar


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Primary growth of stems is produced by cell divisions in the apical meristem. It takes place in all seed plants.

Apical meristem


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Secondary Growth of Stems


The method of growth in which stems increase in width is called secondary growth.

In conifers and dicots, secondary growth takes place in the vascular cambium and cork cambium.


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Vascular cambium produces vascular tissues and increases the thickness of stems over time.

Cork cambium produces the outer covering of stems.

The addition of new tissue in these cambium layers increases the thickness of the stem.


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Formation of the Vascular Cambium

Once secondary growth begins, the vascular cambium appears as a thin layer between the xylem and phloem of each vascular bundle.

Vascular cambium


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The vascular cambium divides to produce xylem cells toward the center of the stem and phloem cells toward the outside.

Secondary xylem

Secondary phloem


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These different tissues form the bark and wood of a mature stem.


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Formation of Wood

Wood is actually layers of xylem. These cells build up year after year.

Wood Bark


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As woody stems grow thicker, older xylem cells near the center of the stem no longer conduct water.

This is called heartwood. Heartwood supports the tree.

Xylem: Heartwood


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Heartwood is surrounded by sapwood.

Sapwood is active in water and mineral transport.

Xylem: Sapwood


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Formation of Bark

On most trees, bark includes all of the tissues outside the vascular cambium — phloem, the cork cambium and cork.

Bark


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The vascular cambium produces new xylem and phloem, which increase the width of the stem.

Vascular cambium


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The phloem transports sugars produced by photosynthesis.

Phloem


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The cork cambium produces a protective layer of cork.

Cork cambium


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The cork contains old, nonfunctioning phloem that protects the tree.

Cork


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23–3


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23–3

Structures on a stem that can produce new stems and leaves are called

a. nodes.

b. internodes.

c. buds.

d. branches.


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The vascular bundles in a monocot stem

a. form a cylinder, or ring.

b. are scattered throughout the stem.

c. form concentric rings.

d. separate into xylem bundles and phloem bundles.


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23–3

The outermost layer of a tree that contains old, nonfunctioning phloem is

a. bark.

b. cork.

c. pith.

d. apical meristem.


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23–3

Xylem and phloem are contained in

a. the epidermis.

b. vascular bundles.

c. the pith.

d. cork cambium.


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23–3

In stems, secondary growth results in

a. growth at the tips of roots.

b. growth at the tips of shoots.

c. an increase in the width of stems.

d. an increase in the length of stems.

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end show slide 1 of 36 copyright pearson prentice hall biology

Documents

function stems

leaves slide

new stems

dicot stems vascular

bud slide

main functions of stems

vascular tissue

node internode bud node