Plant Vascular Systems: Xylem and Phloem Structure and Function

Plant Vascular Systems: Xylem and Phloem Function

Vascular Tissues: Nutrient Transport Systems

Vascular tissues (or drivers) transport nutrients from the roots to the leaves and vice versa. There are two main types:

Xylem (Woody Tissue)

The primary function of the xylem is to conduct crude sap from the roots to the leaves. It is a complex tissue composed of several cell types:

1. Tracheary Elements

These are the most characteristic features: non-living elements through which water circulates. These cells are elongated and tapered at both ends. They can be hollow, with thickened walls often reinforced with lignin. They are arranged one after the other, forming alignments that run longitudinally along the root and stem.

Tracheids (Pteridophytes and Gymnosperms)

In pteridophytes and gymnosperms, these are the only tracheary elements. In each pair of cells, there are pairs of aligned pores through which water passes from one cell to another. These cells are called tracheids.

Tracheae (Angiosperms)

In angiosperms, these cells, called tracheae, become cylindrical and shortened. Their partitions are punctured or even disappear. The series of cells are so well connected to each other that they form real tubes, known as wood vessels.

2. Xylem Fibers

These cells primarily provide a support function.

3. Parenchyma Cells

These cells are specialized in the accumulation of reserve substances.

Phloem (Liberian Tissue)

The mission of the phloem is to conduct the manufactured sap from the leaves to the non-photosynthetic parts of the plant. Like the xylem, it is a complex tissue consisting of several kinds of cells. The most characteristic components are:

1. Sieve Elements

Sieve elements are composed of sieve cells that form a sieve tube. The walls of sieve cells are coated with non-waterproof substances, allowing them to remain alive. They lack a nucleus and contain only part of the cytoplasm; therefore, their survival depends on the associated companion cells.

The separation walls between adjacent cells are perforated by numerous holes or screens, which form the sieve plates.

The circulation of the sap produced by these vessels slows or even stops during the winter season due to a deposit of a precipitated polysaccharide called callose on the sieve plate pores. In spring, these precipitates are dissolved, and the circulation of the plant sap resumes.

2. Companion Cells

These are parenchyma cells closely associated with the sieve elements, with which they are physiologically and functionally related.

3. Fibers and Parenchyma Cells

These components specialize in the storage of reserve substances.