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
The first-formed xylem cells during growth/development
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
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
Tree on a Chip: Device generates passive hydraulic power (MIT, 2017)
How the Sap Runs... through a maple tree (Science Friday 5Apr2019)