STEM STRUCTURE DIAGRAMS and VASCULAR BUNDELS

VASCULAR SYSTEMS

• Vascular tissues or vascular bundles consist of an internal system of tubes that transport water, dissolved minerals and dissolved food throughout the plant.
• The vascular bundles in a stem arc continuous, tube-like strands connecting the vascular tissue of the root to the vascular tissue in the leaves.
• The transportation of water and dissolved minerals is carried out by specialized tissue called xylem in the vascular bundles.
• The transportation of dissolved food is carried out by specialized tissue called phloem in the vascular bundles.

XYLEM: Structure and Function
Structure:
• There are two types of xylem cells: vessel elements and
tracheids. Vessel elements are only found in angiosperms.
• Both vessel elements and tracheids have "pits" which arc
thin regions in the cell walls.
• The ends of these cells are either absent or perforated such that when these cells are connected end-to-end tubes or pipes are created.
• Cellulose and lignin are deposited on the exterior surfaces of xylem cells to form rigid cell walls. «When the cell walls are complete, the cytoplasm dies, leaving dead, hollow xylem. This results in very thick strong cell walls able to resist great water pressure.
• See page 532 Figure 14.16.


PHLOEM: Structure and Function

Structure:
• The phloem is made up of a companion cell, sieve-tube elements and sieve plates.
• These cells remain alive, unlike the xylem cells.
• Although sieve-tube elements lack a nucleus at maturity, because they have a cytoplasm and all of the other organelles it is still able to function.
• The cytoplasm of the sieve-tube cell is connected by pores to a nucleated companion cell next to it. It appears that the companion cell directs the activity of the sieve-tube cell.
• The sieve-tube elements are arranged end-to-end with sieve
plates separating them. Because of the large pores in the sieve plates, the cytoplasm of extends from one sieve-tube cell to the next.


Function:
• The function of the phloem is to transport food from one region of the plant to another.
• This process is still poorly understood.

CHECK THIS WEB SITE OUT
This will help you understand the parts of a tree trunk, and its specific sructures.
On the diagram on the web site click on the name to get more detail about each structure.

http://www.botany.uwc.ac.za/ecotree/trunk/woodanatomy.htm