Phloem

Phloem, the plant's food-transport tissue, was likewise named by Nageli in 1858 (its older synonym leptome again comes from Haberlandt, 1914). Like xylem, it has a primary form (from procambium, whose short-lived early protophloem elements get crushed as the later metaphloem develops around them) and a secondary form (from vascular cambium). It is built of four working cell types. Sieve elements, the actual conducting cells, come in two flavours: primitive sieve cells (no companion cells, sieve areas not organised into distinct plates, found in pteridophytes and gymnosperms - where, lacking companion cells of their own, their activity is instead managed by nucleated albuminous cells, the gymnosperm equivalent of a companion cell) and the more advanced sieve tubes (files of sieve-tube elements stacked end to end, connected at well-differentiated sieve plates with larger but fewer sieve pores, each element paired with a nucleated companion cell, found only in angiosperms). Companion cells themselves are living, narrow, nucleated cells connected to their sieve-tube element by plasmodesmata, coordinating the sieve element's transport activity since the sieve element itself loses its nucleus at maturity. Phloem parenchyma is living tissue that stores food …