Practice question

Angiosperm phloem comprises enucleate sieve tube elements interconnected via sieve plates with P-protein, each ontogenetically associated with companion cell derived from asymmetric division of same mother cell retaining nucleus. Companion cell retains dense cytoplasm, many mitochondria, nucleus and plasmodesmatal connections, sustaining sieve element metabolism, loading sucrose via H+ ATPase sucrose symport and synthesizing proteins transported through pore plasmodesmata units. Gymnosperm equivalent uses sieve cells and Strasburger albuminous cells without direct ontogenetic pairing. No such intimate physiological dependence exists with xylem vessels, tracheids or fibers conducting water or providing mechanical support, emphasizing phloem specialization unique to flowering plants.