CELL TYPE CYTOPLASMIC FEATURES CELL WALL FEATURES 3-D GEOMETRY
Sieve Tube Element 
Series of Sieve Tube Elements are longitudinally joined end to end via sieve plates  to form Sieve Tubes.
Closely associated with parenchyma cells called Companion Cells
No nucleus at maturity
No chloroplasts
No mitochondria
Cytoplasm consists of P2 protein fibils (formally known as slime) and
Convoluted and vesiculated Endoplasmic Reticulum in a parietal position (around the circumference of the cell)

 

Thickened primary cell wall with specialized primary pit field in Sieve Plates (Transverse end walls) 
Cell Wall / Lumen Ratio very small
 
Transverse Isodiametric
Longitudinal  Elongated cylinder.  Longest axis is in vertical direction.