Plant Cell and Tissue Types

Simple tissues are composed of one cell type. 

A)  Parenchyma:

• living cells with thin cell walls
• functions:  storage, photosynthesis, secretion
• make up the bulk of the cortex and pith of stems and roots

B)  Collenchyma:

• living cells with unevenly thickened cell walls
• most often found beneath the epidermis
• main function is support

C) Sclerenchyma:

    2 cell types:

     i.  Fibers:  long, slender cells that occur in strands or bundles.
    ii.  Sclereids:  variable in shape, but much shorter than fibers.

• cells are not living at maturity
• very thick cell walls
• primary function is support

Complex tissues are composed of more than one cell type.  More advanced plants have a vascular system which conducts water and nutrients throughout the plant. These tissues are found in a vascular bundle, which may be surrounded by a bundle sheath (layer of parenchyma cells or sclerenchyma cells or both).  

A)    Xylem—water-conducting tissue

• continuous system throughout the plant, made of up tracheary elements
• have secondary cell wall thickenings that may appear as rings or spirals
• tracheids—elongated cells (dead at maturity); usually only type of xylem cell in most gymnosperms and seedless vascular plants
• vessel elements—may be wider than tracheids; contain perforations (areas lacking in cell walls), usually on the end walls; more specialized cell type
• xylem also has parenchyma associated with it, often found in rays (long strands) & called ray parenchyma

B)  Phloem—food-conducting tissue in vascular plants

• primary cells are called sieve elements (because there are clusters of pores at the ends of cells through which adjacent protoplasts are connected)
• cells are living at maturity, but the nucleus and tonoplast are broken down
• sieve cells have sieve areas with uniform, narrow pores
• sieve-tube elements may have larger pores on some walls than on othersà sieve plate
• companion cells are specialized parenchyma cells associated with sieve-tube elements; may play a role in delivery of substances to sieve-tube elements
• albuminous cells—found in gymnosperms

Think of xylem and phloem as long pipes or tubes that extend from the roots all the way throughout the plant to the top leaves, even in tall trees!   In order for tall trees to prevent wilting, water must be moved continuously from the roots to replace water being lost through transpiration.  

Dermal tissues make up the outer covering of the plant body

1. Most cells of the epidermis are relatively unspecialized.
2. Also includes guard cells (which contain chloroplasts) that surround pores, called stomata.  Guard cells are often associated with subsidiary cells and control transpiration and carbon dioxide intake.
3. Trichomes—hair-like structures that fill a variety of roles
1. prevention of water loss
2. defense
4. Periderm replaces the epidermis in stems and roots having secondary growth. 
1. Cork (non-living); also called phellem
2. Cork cambium; also called phellogen
3. Phelloderm (living parenchyma tissue)

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