Stems
In this chapter you will learn about the origin and
development of stems. Items such as the apical meristem and the tissues derived
from it, leaf gaps, cambia, secondary tissues, and lenticels are included. In
addition, you will learn about the distinctions between herbaceous and woody
dicot stems, and monocot stems as well as annual rings, rays, heartwood and
sapwood, resin canals, bark, laticifers, and vascular bundles. Finally you
will learn the economic importance of wood and stems.
At the end of this chapter the successful student will be able to
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- Woody twig consists of an axis with attached leaves.
- Alternately or Oppositely arranged.
- Leaves attached at a node.
- Stem region between nodes is an internode.
- Leaf has a flattened blade and is usually attached to the twig by
a petiole.
- Axil - Angle between a petiole and the stem.
- Axillary Bud located in axil.
- Terminal Bud often found at twig tip.
- Stipules - Paired appendages at the base of a leaf. Often remain
throughout leaf life span.
- Deciduous trees and shrubs have dormant axillary buds with leaf scars
left after leaves fall.
- Bundle scars mark food and water conducting tissue.
-
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- Apical meristem is dormant before the beginning of the growing season.
- Protected by bud scales and by primordia.
- When a bud begins to expand, apical meristem cells undergo mitosis and
three primary meristems develop
- Protoderm - Gives rise to epidermis.
- Procambium - Produces primary xylem and primary phloem cells.
- Ground Meristem - Produces tissues composed of parenchyma cells.
Longitudinal Section of a Coleus Leaf
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- Narrow band of cells between the primary xylem and primary phloem may
become vascular cambium.
- Cells produced by the vascular cambium become components of secondary
xylem and secondary phloem.
- In many plants, a second cambium, cork cambium, arises.
- Produces cork cells and phelloderm cells.
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- Steles
- Stele is made up of primary xylem, primary phloem, and pith (if
present).
- Protostele - Solid core.
- Sphinosteles - Tubular with pith in center.
- Eusteles - Vascular bundles.
- Dicotyledons - Flowering plants that develop from seeds having two seed
leaves.
- Monocotyledons - Flowering plants that develop from seeds with a single
seed leaf.
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- Herbaceous Dicotyledonous Stems
- In general, annuals are green, herbaceous plants.
- Most monocots are annuals, but many dicots are also annuals.
- Herbaceous dicots have discrete vascular bundles composed of patches of
xylem and phloem.
- Procambium produces only primary xylem and phloem, but vascular cambium
arises later and adds secondary phloem and xylem to the vascular bundles.
-
- Woody Dicotyledonous Stems
- Vascular cambium of a typical broadleaf tree produces relatively large
vessel elements of secondary xylem (spring wood).
- Xylem produced next has smaller or fewer elements, and is referred to as
summer wood.
- One year’s growth of xylem is called an annual ring.
- Vascular cambium produces more secondary xylem than phloem, thus bulk of a
tree trunk consists of annual rings of wood.
- Examining rings can determine the age of a tree, and provide some
indications of climatic conditions.
- Vascular Rays consist of parenchyma cells that function in lateral
conductions of nutrients and water.
-
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- The stems of most monocots have neither a vascular cambium nor a cork
cambium and thus produce no secondary vascular tissues or cork.
- Xylem and phloem exist in discrete vascular bundles.
- Secondary meristem produces only parenchyma cells to the outside and
secondary vascular bundles to the inside.
If you go to
http://www.botany.hawaii.edu/faculty/webb/BOT201/CellTissOrgan/CellTissOrgan-10.htm,
you will see the following comparison of a Monocot vs. Dicot Stem. Note
particularly the arrangement of the vascular bundles.
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- Older, darker wood at the center is called heartwood, while the lighter,
still-functioning xylem closest to the cambium is called sapwood.
- Formed at roughly the same rate as heartwood.
- Softwood - Xylem consists primarily of tracheids; no fibers of vessel
elements.
- Bark - Refers to all the tissues outside the cambium, including the
phloem.
- Mature bark may consist of alternating layers of crushed phloem and
cork.
- Secondary growth of stems demonstrated
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- In a living tree, 50% of the wood weight comes from water content.
- Dry weight is composed of 60-75% cellulose and 15-25% lignin.
- Density and Durability are two of the most important characteristics in
commercial wood.
- Sawing
- Radially cut (quartersawed) boards show the annual rings in a side view.
- Tangentially cut (plain-sawed) boards show annual rings as irregular
bands of light and dark streaks.
- Knots
- Bases of lost branches covered by new annual rings produced by the
cambium of the trunk.
- Found in greater concentration in older parts of the log, towards the
center.
- Wood Products
- About half of US and Canadian wood production is used as lumber,
primarily for construction.
- Veneer - Thin sheet of desirable wood glued to cheaper lumber.
- Second most extensive use of wood is pulp.
- In developing countries, approximately half of cut timber is used for
fuel.
- Less than 10% in US and Canada.
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Last modified:
October 08, 2004 by
Cynthia Herbrandson 
© Copyright 1999, Kellogg Community College.
All rights reserved.