Relation between sieve tubes and companion cells is that they form a sieve element and companion cell complex.
Below are the similarities between the two:
(a) Both are two different types of cells that are found on the phloem of angiosperms.
(b) Transportation of food throughout the plant is done by these two cells.
(c) Both the cells have an enlongated shape.
(d) Both the cells emerge from the meristem.