What makes up the phloem?
Companion cells, sieve tube elements, fibres and parenchyma are all parts of the plants vascular system called phloem.
Sieve tube members conduct sugars away from the site of photosynthesis ( leaves) to the other plant parts.
Sieve tube members are live cells but lack vital organelles like nucleus and ribosomes.
Companion cells are the life support of the sieve tube members. They carry out all the metabolic functions of the sieve tube members. They provide ATP and nutrients and carry out all the cellular functions of the sieve tubes.
Interconnecting pathways called plasmodesmata allow flow of cytosol and other materials between the companion cells and sieve tube members.