Here's a breakdown:

* Phloem: The vascular tissue in plants responsible for transporting sugars (produced during photosynthesis) from the leaves to other parts of the plant.

* Sieve tube elements: Long, thin cells arranged end-to-end to form sieve tubes. They have perforated end walls (sieve plates) that allow the free flow of sugars and other organic compounds.

* Companion cells: Specialized cells that are closely associated with sieve tube elements. They provide metabolic support to the sieve tube elements and help regulate the movement of materials.

It's important to note that sieve tube elements are not living cells in the traditional sense. They lack a nucleus, ribosomes, and other organelles. However, they are connected to living companion cells, which provide them with the necessary support to function.