Specific cells- Vessels:

* predominate in angiosperms;

* have short, wide, thick secondary cell walls;

* are dead, hollow cells; lack end walls;

* have a large diameter, therefore water movement through them is rapid;

*the wall thickenings in vessels are pitted as you can see from the image on the right

* note the annular thickenings in cells below on left and the helical thickenings in cells below on right which act to help support the walls

Tracheids:

* predominate in conifers;

*long, slender cells with tapered, overlapping ends;

*water moves upward from tracheid to tracheid through pit pairs, thus preventing large gas bubbles from forming and thus no cavitation during freezing/defrosting periods. Water can move through the plant with little seasonal disruption allowing water flow and photosynthesis to occur year round. No need for loss of leaves in the winter as in most angiosperms.

There are two kind of sieve elements:

Sieve cells- occur in nonflowering plants; long with tapered overlapping ends; associated with albuminous cells, which help regulate the sieve cells' activities

Sieve tube members- arranged end to end in sieve tubes; larger pores than sieve cells; concentrated along contacting end walls of adjacent sieve tube members;

Characteristics:

• living protoplasts at maturity; no nuclei; most organelles gone; vacuole disappears; little protoplasm;
• an accessory companion cell= controls metabolic regulation as it contains functional DNA. Note the narrow cell to the left of the one with the arrow.
• contain P-protein along longitudinal walls