Primary conductive tissues: xylem and phloem

Angiosperms have developed vascular tissues for the long-distance transport of water, organic and inorganic substances between aerial parts (leaves, stems) and undergrounds (roots) parts of the plant. With the development of cuticle and lignification it consists in another element of the conquest of terrestrial environment.

The primary vascular tissues are xylem and phloem. Xylem is the transport system for water and minerals and phloem is the transport system of photosynthates (sugars produced through photosynthesis). There are complex tissues consisting of different types, conducting and no conducting. Xylem and phloem are located to each other within vascular bundles. They are described here after and illustrated on another page with light micrographs of longitudinal sections. Their distribution relative to each other varies according to the organ, root or stem, and also differs in monocot and dicots as illustrated by micrographs of cross section in young different roots and stems. There are also differences in the same organ (for instance, leaves) of different species.

Xylem conducting tissue of raw sap.

Conductive elements of xylem are tracheids or tracheary elements and vessels. – Tracheary elements are elongated fusiform cells, ~ 10 µm wide and they constitute the main conducting cells in gymnosperms and ferns. They are relatively rare in angiosperms. – Vessels are generally less fusiform and shorter than tracheid’s but they are larger (100 µm) and much more efficient in transport of water and minerals (higher speed and higher amounts). They are the main conducting cells of Angiosperms, and they are superposed and joined longitudinally to one other to form tubes whose length may vary from a few centimeters to several meters in trees. Walls of tracheids and vessels are characterized by lignified walls, secondary walls with lignin a complex phenolic material filling the spaces in the cell wall between cellulose, hemicellulose and pectins. Lignin forms a 3D network and provides rigidity and impermeability to tissues enabling long distance water transport of water in xylem.

Based on lignification levels and pits, there are different types of xylem conducting cells: – Tracheids: are the fundamental cell types in the xylem. They are dead at maturity and their walls contain pits. The most common patterns of secondary lignified walls are annular, spiral, scalariform, reticulate and pitted, the most advanced type.

 -Vessels are the second category of xylem conducting elements and they are also dead at maturity. The most common patterns of secondary cell wall thickenings are annular, spiral, reticular and pitted. Vessel elements develop openings in their end walls that lack both primary and secondary cell walls. These openings, called perforation plates, can be of several types. Two or more vessel elements join end to end to form a vessel.


Phloem the conducting tissue of photosynthates (elaborated sieve)

This tissue as xylem is a complex tissue consisting of several cell types: conducting and non-conducting. Conducting elements are sieve cells and sieve tubes members (elements). A sieve tube is formed by superposition end to end of sieve tube members. Sieve tube elements are separated by sieve plates which have open pores to facilitate the movement of photosynthates along the sieve tube. At maturity the conducting cells/elements have cellulosic walls and are alive: they lack a nucleus; their plasma membrane is preserved, and they contain plastids and mitochondria. Various types of parenchyma cells among which specialized cells called companion cells are associated to phloem sieve tubes as well as fibers as support for this tissue. Companion cells are linked via plasmodesmata to phloem sieve These parenchymatous cells are smaller and narrower than sieve tubes. They have dense cytoplasm, large well differentiated nuclei, small vacuoles, and they are related to adjacent sieve tubes by multiple plasmodesmata tubes (1, 2 or 3 cc /phloem sieve element). The function of companion cells is to shift sugar and amino acids in and out of the sieve elements. They are also essential in case of damaging of sieve tubes. Sieve tubes are essential to distribute nutriments from source organs site of food production (leaves) toward sink organs in which food I stored for later use, for coordination of development via hormones and for long distance transport of biotic (virus, pathogens) and abiotic (external factors such as heat, cold, salinity) signals.cond