Phloem | Tree Biology

Phloem is the other transport system in plants (first one being xylem) that moves sugars and organic materials. In angiosperms, the transport cells are called sieve tube elements. They have large holes called a sieve plate, and companion cells that load sugar into the sieve tube elements. In gymnosperms, they are called sieve cells and do NOT have a sieve plate. Movement occurs from source (where photosynthesis occurs or sugar is stored) to sink (where sugars are needed), explained by the "pressure flow hypothesis).