Phloem Tissue Structure and Sap Translocation Mechanisms

Phloem Tissue: The Elaborated Sap Driver

Regarding the elaborated sap driver: The difference between xylem elements and conductors in their living, functional state is that phloem may also contain laticifers and idioblasts. The phloem procambium resulted in secondary phloem produced by the phloem cambium, possessing an axial or radial parenchymatic system. This structure was noted in 1858, related to Nägeli's ploidy and the cortex (Greek term).

Cellular Types of Phloem

• Axial Conductive System

  Sieve tube elements (sieve tube cells).

• Support (Sostén)

  Fibers and sclereids.

• Parenchymatic System

  Storehouse and translocation of substances.

• Radial System

  Parenchymatic cells (phloem rays) observed in secondary growth.

Sieve Cells

Sieve cells are long, thin elements with tapered ends that overlap others. They do not possess sieve plates, only dispersed sieve areas located on all cell surfaces. These sieve areas provide pores through which the protoplasts of the sieve elements connect vertically or laterally. Sieve cells are found in Pteridophytes and Gymnosperms.

Pore Size Variation in Vascular Plants

While Angiosperms typically exhibit narrow, uniform pore sizes, the pore size varies considerably in different walls, even within the same cell, in other vascular plants.

• Pteridophytes

  Sieve cells are long and sharp with poorly differentiated sieve areas.

• Gymnosperms

  Sieve cells are long and thin elements with tapered ends overlapping one over another.

Sieve Tube Members (Tubos Cribosos)

Sieve tube members are a longitudinal series of cylindrical cells, connected by means of simple and compound sieve plates. These plates are formed by more differentiated areas containing comparatively large sieve pores and are located in the end walls. These wall areas have more or less specialized sieve areas, making it difficult to see the communication pathways between the sieve tubes or with adjacent cells.

Maturation and Callose Dynamics

The young sieve elements have all organelles, including vacuoles. A network of endoplasmic reticulum (ER) is irregularly distributed throughout the cytoplasm, where proteinaceous bodies made of P-protein are synthesized. These P-proteins either go to the vacuoles or become associated with sieve plates or areas.

At maturity, the nucleus and other organelles disintegrate. The tonoplast breaks, and the vacuolar content is mixed with the cytoplasm (referred to as mycoplasma). Every pore is covered with Callose, a polymer composed of glucose monomers joined by a β-1,3 link.

As the sieve element ages, callose accumulates until the pore becomes completely obliterated. If the phloem is inactive, the amount of callose may decrease, re-establishing the continuity of the protoplast through the pores during tissue reactivation in spring.

Callose is low or absent in functional cells. It is synthesized rapidly in response to injury. Its accumulation during ontogeny is a response to internal stimuli and not to external influences.