Ok i just took plant anatomy.....
Plant phloem has two unique types of cells: sieve elements and companion cells.
Angiosperm phloem is comprised of sieve tube elements. These stack together to form sieve tubes. Sieve tube cells contain all cellular components when young. At maturity, they lose their nucleus, vacuole(s), and the majority of their ribosomes. They retain the endoplasmic reticulum, plastids (chloroplasts etc), and mitochondria which are located along the outer edges of the cell.
So because they lose their nucleus, ribosomes, and vacuole, they need some metabolic help in order to transfer photosynthates throughout the plant. This is where the companion cells come in. Companion cells are attached to sieve tube elements and help them to function by offering some of its nuclear machinery. As companion cells mature, they become very dense, especially with ribosomes, to offer the sieve tube cell. They both die at the same time.
So basically, sieve tube cells are DEFINITELY lacking the required amount of ribosomes to function properly. There are very very little to none.
Knowing that the sieve tube cells lose their nucleus is more important
