Shown are sections of two sieve tubes with companion cells, mostly in staggered position. Sieve element and companion cell are mostly connected through numerous pore plasmodesm units. P plastids, mitochondria and ER are parietally positioned and evenly distributed. Parietal proteins are locally aggregated. Parietal proteins and ER are sometimes located on the sieve plates or the margins of the sieve pores, but do not impede mass flow. A large spindle-shaped crystalline protein cluster rests close to the sieve plate. The massive protein body is a specialty of the Fabaceae.
C = callose; CC = companion cell; CP = crystalline protein body; CW = cell wall; ER = endoplasmic reticulum; M = mitochondria; N = nucleus; P = Plastids; Pl = P plastids; PP = parietal protein; PPU pore plasmodesm unit; SE = sieve element; SP = sieve plate; V = vacuole.