Vascular, Vascular Bundle, Vein

vascular [ VAS-kyuh-ler ] adjective: in botany, pertaining to or having vessels through which fluids pass in a plant

vascular [ VAS-kyuh-ler ] bundle [ BUHN-dl ] noun : a strand of specialized vascular tissue of higher plants consisting mostly of xylem and phloem

vein [ veyn ] noun: any of the vascular bundles forming the framework of a leaf

In the human cardiovascular system, the heart pumps blood, which circulates throughout the body in a closed loop through tubular vessels: arteries deliver blood rich with oxygen obtained from the lungs to the rest of the body; veins return oxygen-depleted blood to the heart; and capillaries connect very small arteries and veins to allow oxygen and nutrients and carbon dioxide and waste products to pass into and out of cells.

In a plant’s vascular system, fluids do not move in a loop or through tubes but rather from one end of the plant to the other through specialized cells—phloem and xylem. Phloem cells deliver the products of photosynthesis. Xylem cells deliver the water and minerals absorbed by the roots.

Yucca filamentosa ‘Color Guard’ leaves upper and lower surfaces

Yucca leaf cross section showing the vascular bundles "arranged in parallel series across the breadth of the leaf with smaller bundles towards the periphery and larger bundles in the center of the leaf." Photo courtesy of Berkshire Community College Bioscience Image Library

Left to right: Upper and lower leaf surfaces of Yucca filamentosa ‘Color Guard‘ (Adam’s Needle); Yucca leaf cross section showing a parallel series of vascular bundles with large bundles in the center of the leaf and smaller bundles along the edges.

The phloem and xylem are always located next to each other. In leaves and stems, they form a structure called a vascular bundle surrounded by one or more layers of compact parenchyma cells called a bundle sheath. In roots, vascular tissue is arranged in a central cylinder. In most vascular plants, vascular cambium lies between the phloem and xylem in the stems and roots. The stem bundles are considered open. In a leaf, the veins are the vascular bundles. Usually, no cambium lies between their xylem and phloem so these bundles are referred to as closed.

The arrangement of the phloem and xylem relative to one another in the vascular bundles differs according to the species and where they are found on the plant. The most commonly found arrangements in leaves and stems are:

CONJOINT

Collateral
xylem is adaxial—toward the axis/upper surface; phloem is abaxial—toward outer/lower surface (e.g., Zea leaf)

Zea leaf cross section showing collateral and closed vascular bundles

Left to right: Zea mays ’Pink Zebra’ (ornamental corn) leaves; Zea leaf cross section showing closed, collateral vascular bundles with xylem (adaxial) and phloem (abaxial) wrapped by bundle sheaths of thin walled parenchyma cells.

Bicollateral
phloem lies on the outer and inner sides of the xylem
(e.g., Curcurbita stem)

Cross section of a Curcurbita stem in which “the vascular bundles are bicollateral

Left to right: Curcurbita moschata (butternut squash) stems; Curcurbita stem cross section showing a bicollateral vascular bundle where “the central xylem is bound by an inner and outer cambium, and topped by a larger outer and a smaller inner phloem.”

CONCENTRIC

Amphicribral (hadrocentric)
the phloem encircles the xylem
(e.g., fern rhizome)

Pteridium aquilinum (bracken fern) rhizome. Photo © Bibliothèque de l'Université Laval CC BY-SA 4.0 DEED

Pteridium aquilinum (bracken fern) rhizome showing large xylem cells (reddish-purple color) encircled by phloem tissue surrounded by parenchyma and sclerenchyma cells (brown cell wall). Photo © Anatoly Mikhaltsov CC BY 4.0 DEED

Left to right: Pteridium aquilinum (bracken fern) rhizome and cross section showing large xylem cells (reddish-purple color) encircled by phloem tissue surrounded by parenchyma and sclerenchyma cells (brown cell wall).

Amphivasal (leptoconcentric)
the xylem surrounds the phloem
(e.g., Ficus leaf midrib)

Ficus carica (common fig) palmate leaf showing branching venation.

Ficus leaf cross-section in which the vascular bundles of the petiole and midrib are "leptocentric with a ring of xylem surrounding central phloem” wrapped in parenchymatous bundle sheaths. Photo courtesy of Berkshire Community College Bioscience Image Library

Left to right: Ficus carica (common fig) leaf; Ficus leaf cross-section in which the vascular bundles of the petiole and midrib are “leptocentric with a ring of xylem surrounding central phloem” wrapped in parenchymatous bundle sheaths.

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How the veins are arranged in a leaf—venation—also differs according to the species. For example, monocots (like yucca and corn) tend to have parallel veins, while eudicots (like squash and fig) tend to have branched veins.

Since plants do not have a pump to move fluids through the phloem and xylem, they rely primarily on the processes of translocation and transpiration. In photosynthesis, which generally takes place in the leaves, the energy from sunlight is converted to chemical energy to produce sugars and oxygen from carbon dioxide (absorbed from the atmosphere through a leaf’s pores or stomata) and water (supplied to the leaves by the roots). The stomata release the oxygen into the atmosphere and sugars move into the phloem. The high concentration of sugar draws water into the phloem from the xylem creating pressure that moves the sugars through the phloem to where they are needed in the plant to grow tissue or to store for later use. In this case, leaves are the sources of the sugars and the delivery points—shoots, fruits, and roots—are referred to as the sinks. The phloem translocates the sugars to the nearest sinks: leaves at the top of the plant send sugars up to the new shoots; leaves at the bottom send sugars down to the roots; and leaves in the middle send sugars both upward and downward.

Flow of fluids through phloem and xylem. Photo © Nefronus CC BY-SA 4.0 DEED [dark orange arrows added to original] — The common flow of fluids through phloem and xylem.
Photo © Nefronus CC BY-SA 4.0 DEED
[dark orange arrows added to original]

WHAT HAPPENS WHEN PLANTS DO NOT HAVE LEAVES?

Since a deciduous plant lacks leaves in early spring, the source of the sugars for new growth becomes the roots or bulbs, where the sugars from the previous season have been stored. The shoots and budding leaves become the sinks until the leaves can produce sugars again.

In cacti, stems are the main source of photosynthesis. Although they absorb sunlight and convert carbon dioxide and water to oxygen and sugars during the day, the stomata do not open until night, at which time they exchange gases, releasing oxygen made during the day and storing carbon dioxide for use the next day. This not only minimizes the loss of water during transpiration, but it alters the taste of the cactus. Since the carbon dioxide is stored in the form of malic acid, cacti harvested for food at night or early morning will have a sour taste. For a sweeter yield, harvest cacti mid-morning to mid-afternoon when the acid content is lowest.

Xylem, on the other hand, usually transports water and minerals upward. [An exception is the California redwood where, in heavy fog, the xylem flow reverses bringing the fog droplets deposited on and absorbed by the leaves downward toward the soil.] Root pressure, which builds as water is absorbed from the soil through osmosis, and capillary action, where liquids rise in a narrow tube in defiance of gravity, propels some water upward through the xylem cells. However, they can only drive water so far—maybe a few yards—and certainly nowhere near the top of a tall tree. Transpiration is responsible for the primary movement of water through the xylem. When the stomata open to exchange gases, they also release water in the form of vapor. The Cohesion-Tension Theory posits that this water loss causes the water from the roots to be sucked up through the xylem cells to the shoots. Hydrogen bonds the water molecules together so as each water molecule evaporates, it pulls up the next one in an uninterrupted chain. Researchers estimate that up to 95% of the water absorbed by the roots is lost through transpiration.

References

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Cacti / Desert Succulents. 2021. National Park Service. (accessed March 24, 2024).

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Prickly Pear Cactus Production. 1989. Small Farm Center, University of California, Davis, CA.

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