Vascular Tissues

Vascular tissues are specialized cells that move water & minerals, as well as sugars from photosynthesis throughout the plant

Types of Vascular Tissues

Xylem

  • Specialized cells that conduct water through the plant, and also strengthen plant organs

Phloem

  • Specialized cells that move the products of photosynthesis (sugars) throughout the plant

Xylem

Function

  • Xylem cells are the main "pipes" that conduct water and minerals through the plants

  • These cells also create support in large stems, such as trunks and branches

Features

  • Xylem are hollow, pipe-like cells, oriented longitudinally throughout the plant (=with the axis of the plant)

  • In cross-section, xylem cells have an evenly-thickened (secondary) wall with a fairly large internal space (=lumen)

  • in longitudinal-section, xylem are tapered on both ends, with pits in the walls to regulate the lateral flow of water from cell to cell

  • Thickening on the xylem cell can have different patterns depending on location and maturity

Annular thickenings

  • Unconnected rings of thickened walls

  • These are some of the first xylem cells created during growth (i.e. protoxylem)

  • Allows for minimal strength, but the ability to stretch during growth

Helical thickenings

  • The thickened wall is spiral-shaped

  • Found in earlier formed cells (i.e. protoxylem)

  • Allows for more strength and some ability to stretch during growth

Scalariform thickenings

  • The thickened wall appears ladder-like

  • Found in later formed cells (i.e. metaxylem)

  • Allows for greater strength and less ability to stretch during growth

Pitted walls

  • The xylem wall is completely thickened with scattered openings, called pits

  • Found in latest formed cells and mature tissues

  • Allows for greatest strength but no ability to stretch during growth

Maturation

    • At maturity, xylem cell are dead (devoid of a nucleus and organelles)

    • This creates "hollow pipes" which allow for easy flow of water

Protoxylem

  • The first-formed xylem cells during growth/development

Metaxylem

  • Xylem elements that are larger, and form later during growth/development

Endarch maturation

  • Protoxylem forms in the center of organ, and later formed xylem (metaxylem) is peripheral to it

Exarch maturation

  • Protoxylem forms in the periphery of the organ, and metaxylem forms towards the middle

Patterns

Above: thickenings from a plant vessel

Types of xylem cells

Tracheids

  • Tracheids are long, tapered and hollow cells that transport water and minerals in a longitudinal fashion.

  • They are longer than vessels (see below) but shorter than sclerenchyma fibers (see above)

  • Found in all vascular plants, but prominent in non-flowering plants (e.g. ferns, conifers, etc.)

  • Several types of tracheids found throughout geologic history of plants (e.g S-type, C-type, G-type, and P-type tracheids) See types of tracheids for more information

Vessel elements

  • They are xylem elements that run longitudinally and are usually wider and shorter than tracheids

  • Found mostly in flowering plants, but also Gnetales, Benettitales

Ray cells

  • They are xylem elements that run radially (from center to periphery)

  • Found in the secondary xylem of woody angiosperms

Fibers

  • These sclerenchyma cells are sometimes mixed in with tracheids and vessels

  • They are long thin cells with almost no lumen, providing support

Parenchyma

  • These cells with large vacuoles are embedded within the xylem for storage

Above: secondary xylem cell of pine wood

Phloem

Function

  • Phloem cells are involved in the movement of carbohydrates, which were created through photosynthesis to areas in need of energy due to active growth

Features

  • Unlike xylem, phloem does not have wall thickenings

  • Grate-like openings, called sieve plates, are found on the ends of the phloem cells

Types of phloem cells

Sieve cells

  • These are long, tapered cells that transport sugars in a longitudinal fashion;

  • Mainly found in non-angiosperms

Sieve tube elements

  • Elongated cells in flowering plants

  • When stacked they create sieve tubes

Companion cells

  • Specialized form of parenchyma

  • Controls all of the cellular functions of a sieve element

Parenchyma

  • These cells with large vacuoles are embedded within the phloem

  • Cells used for storage

Above: phloem cells in a bundle; larger cells are sieve tubes; smaller cells companion cells

Above: longitudinal view of phloem sieve tube members

Additional Resources