Plant Stems


Table of Contents
Function
Structure
Herbaceous Stems
Woody Stems
Growth Rings
Cell Types in Vascular Tissue
Stem Specialization
Facts
Glossary
Quiz
References

Function

Stems have several important roles. For one they connect the vascular tissue in the leaves to the vascular tissue in the root.1 This connection permits water and dissolved substances to be carried throughout the plant body. Stems also acts as a support system for the leaves and reproductive organs.1 The raised leaves aides in maximizing sun exposure so that they can photosynthesize more effectively. Raising flowers/cones in ideal position is important for the pollination and fabrication of seeds and fruit.

Structure

Although all species of plants have stems that act as similar functions- there is variation that occurs in some species, herbaceous stems and woody stems. Herbaceous plants have stems that are not comprised of wood, as their stems are fairly flexible, conducts photosynthesis, and has a fine epidermis.1 Woody plants have stems that contain wood, as the stems are comparatively solid, have bark, and generally do not conduct photosynthesis. All gymnosperms contain woody stems and most woody stems are eudicots. However, monocots do not produce wood, though some are comprised of tissue with comparable functions (ex. palms and bamboos).



Herbaceous Stems

The vascular tissue of herbaceous stems are organized in well-defined vascular bundles in ground tissue. Vascular bundles are extended, uninterrupted strands of vascular tissue of xylem and phloem.1 They run unbroken from the root to the leaves. Within each vascular bundle, the xylem is consistently closer to the centre of the stem and the phloem always found closer to the outside of the stem. Having that said, vascular bundles are organized differently in monocot and eudicot stems.
In monocots, image 1, the vascular bundles are located throughout the ground tissue of the stem.
MONOCOTS.png
Image 11

In eudicots, image 2, the vascular bundles form a ring like structure.
EUDICOTS.png
















Image 21

Woody Stems

Woody stems are more complicated than herbaceous, one of the reasons being that they have the ability to grow thicker over many years due to vascular cambium. Vascular cambium is a layer of meristematic cells in the vascular tissue.2 The purpose is to form new xylem and phloem cells. The vascular cambium produces a new layer of xylem and phloem tissue each year. The xylem can be found on the inside of the vascular cambium, and the phloem tissue is located on the outside.
external image stem-parts.jpg
Image 33
Above, image 3, depicts the anatomy of an ordinary wood stem. As shown, bark is located outside of the vascular cambium. The phloem in the bark, transports sugars in the leaves of the plant. The cork cambium is able to produce cork to prevent water loss and dehydration from the stem.3
All the tissue found outside the vascular cambium is bark. It consists of phloem, cork cambium and cork. Phloem in the stem helps to transport sugars produced in the leaves within the plant itself. The cork cambium in the bark is a layer of meristematic tissue.

See Video:
4

Growth Rings

Growth in many plants occur in different times of the year in regions with various temperatures. For many regions, spring is a time for plants' vascular cambium to grow and produce a large number of xylem cells, with fairly thin walls.5 The thin walls appear as a layer of lighter coloured wood, though in summer, less xylem cells are produced and hence appear as a darker coloured wood with its thicker cell walls.5 These two layers put together form a growth ring.

Cell Types in Vascular Tissue

The cell makeup of the xylem and phloem tissue aides the vascular tissue to fulfill it duties.
There are two types of xylem cells, tracheid and vessel elements. The cell walls of a tracheid, long and cylindrical, have small holes that authorizes water and solutes to travel to nearby xylem cells.1 A vessel element, shorter/wider than a tracheid, and also has small holes in its side cell walls like a tracheid's. The other cell walls are considered perforation plates, these are end walls with at least one opening to allow water and solutes to proceed.1
There are three types of phloem cells in vascular plants, sieve cells, sieve tube element, and companion cells. Sieve cells contain narrow pores in their cell walls and all organelles in most other cells (including nucleus), and sieve tube elements contain cytoplasm however lack cell organelles (including nucleus).1 Sieve plates are found at the end walls of the cell, and are responsible for allowing sugar solutions to pass to nearby phloem cells.1 The companion cell, always correlated to the sieve tube element, has a nucleus (and other organelles that the sieve tube lacks) and forms long conducting tubes with the sieve tube element.1



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Image 41

Stem Specialization

All stems fasten the vascular tissue of plants from shoot to root, however, not all types of stems are able to support plants perpendicular. Some plants grow beneath the ground and have been altered to reserve food and water. These 'modified' plants can also give an advantage to a new species of plant. For example, vines are plants that have taken advantage of other objects, due to their modified stems, to raise and brace their leaves.

Facts:

  • some species of plants, stems can able to be modified to store water and carbohydrates
  • many species have stems that have adapted to protect itself from injury and herbivores (ex. thorns on branches that form sharp and piercing bristle that discourage large herbivores).
  • by counting the layers of xylem and phloem tissue created by the vascular cambium, you can count how many years old the plant/tree is based on the number of layers
  • plant stems also have many other purposes such as being used for wood products, food, textiles, chemical, dyes, medication, and feul

Glossary:

herbaceous- a plant with stems that do not contain wood
woody- a plant with stems that does contain wood
vascular bundle- the positioning of vascular tissue consisting of xylem and phloem
vascular cambium- the meristematic cell layer found in vascular tissue
bark- the protective exterior layer of the stem and roots of woody plants; composed of 3 main tissues: phloem, cork cambium, and cork
cork cambium- lauer of meristematic tissue which produces cork in a woody plant
tracheid- an extended, tapered xylem cell containing small hole like pits; tracheids overlap each other to form an unbroken tube from root to shoot
vessel element- a short, flat-ended xylem cell containing small hole like pits; vessel elements stack from end to end to form vessel tubes from root to shoot
perforation plate- perforated end wall of a vessel element





Quiz

*answers found at end of page after references.
1. What is not a function of stems?
a) support
b) water absorbtion
c) food/water storage
d) transpiration

2. What transportes water and other solutes up a plant?
a) xylem
b) phloem
c) vascular cambium
d) tracheid

3. True or False: The more xylem cells present means the thicker the cell walls and the darker the colour.

4. True or False: Sieve cells contain most organelles found in other cells including a nucleus.

5. True or False: Vessel elements have blunt ends that do not contain small holes to allow water and solutes to pass

6. True or False: Companion cells are part of the phloem, although does not conduct sugars.


References


1. Dulson, Jacqueline, Maurice Giuseppe, and Douglas Fraser. "The Importance of Plants." In Nelson biology 11: university preparation, 552-555. Toronto: Nelson Education, 2011.

2. Knee M. Woody Plants. [homepage on the Internet]. 2009 [cited 2012 Jun 17]. Available from: Ohio State University, Horticulture and Crop Web site: http://www.hcs.ohio- state.edu/hcs300/anat2.htm

3. Lamere C. Glossary of Plant Terms. [homepage on the Internet]. 2010 [cited 2012 Jun 17]. Available from: http://grandpacliff.com/Plants/Glossary-Plants.htm

4. 360edmlaurence. Visualizing Meristematic Tissue. [homepage on the Internet]. 2011 [cited 2012 Jun 17]. Available from: http://www.youtube.com/watch?v=h9kCPO7oMf8&feature=related

5. Plant Meristems and Growth. Biology Online [homepage on the Internet]. 2011 [cited 2012 Jun 17]. Available from:, Web site: http://www.biology-online.org/3/7_meristems.htm










Quiz Answers:
1. b
2. a
3. F
4. T
5. T
6. T