Phloem Structure and Function

The development of vascular tissueswas prerequisite to the evolution of land plants, allowing for the division of labour between different organs. Instead of the entire plant body, the roots alone became specialized for the uptake of water and nutrients. The aerial organs diversified into stems, leaves andflowers. Photosynthesis shifted increasingly into leaves, expanding the light-exposed area with large twodimensional blades. The shoot axis served not only as mechanical support for the aerial organs, but also provided functional links between the organs by vascular tissues: the xylem supplied the surface parts of the plantwithwater and nutrients, and the phloem supplied the heterotrophic organs – roots, flowers, fruits and buds – with photoassimilates. Both vascular tissues are continuous into the farthest ends of the organs. The internal structure of the conducting cells within the vascular bundles is optimized for low-resistance transport. Low resistance is achieved by apoptosis and total autophagy in the case of xylem elements, and by only selective autophagy in the case of phloem elements that are living when functional.