Types of cells in Phloem
The phloem consists of 4 types of cells: i).sieve elements, ii) companion cells, iii) phloem parenchyma and sometimes iv) phloem fibres. The metabolites flow only in sieve elements which resemble hollow pipelines referred to as cellular channels. The sieve tube elements are devoid of nucleus but plastids and mitochondria are present.
Figure: Fine structure of phloem showing a full sieve tube element, its companion cell and other cells of phloem. The sieve plate pores are circular openings in the cell walls of adjacent sieve element (inset).
The cell lumen of the sieve element appears open except for transcellular strands that merge with neighbouring sieve cells at the sieve pore plate via plasmodesmata. The continuity of plasmalemma and cytoplasm makes the sieve tubes a longitudinally extending symplast. Do you remember as to what the xylem vessels form, symplast or apoplast?