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 cells are hollow and pipe-like. They are 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 the longitudinal section, the xylem is tapered on both ends. Sometimes, they are dotted with pits on their walls to allow for lateral flow of water
- These pits can also divert water around a cell that has an air bubble or embolism
Thickening on the xylem cell can have different patterns depending on location and maturity (see below for examples)

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 the 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

Overview of the arrangement of vascular tissues in cross-section
Softwood and hardwood trees

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

'Resurrection' millet—a plant that revives after severe drought (Phys.org 4Jun2025)

└Stewart et al. (2025) Xylem embolism refilling revealed in stems of a weedy grass

Tree on a Chip: Device generates passive hydraulic power (MIT, 2017)
How the Sap Runs... through a maple tree (Science Friday 5Apr2019)

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