Vascular bundle

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Vascular bundles (fascicles) are conduction pathways for the long-distance transport of water, dissolved substances and organic substances ( assimilates , mainly sugar) in the shoot , in the leaf and in the roots of higher plants ( vascular plants ). Vascular bundles consist of the xylem (wooden part) with cell elements for water transport ( e.g. trachea and tracheids ) and the phloem (bast part, sieve part) for the transport of the assimilates with sieve cells or sieve tubes and escort cells .

Vascular bundle types

Types of vascular bundles (blue: xylem, green: phloem, white: cambium)
A  concentric, periphloematic
B  concentric, perixylematic
C  radial with inner xylem, here with four xylem poles, closed on the left, open on the right
D  closed  collaterally
E open collaterally
F  open bicollaterally

There are several types of vascular bundles

  • Simple vascular bundles only consist of a sieve part or a wooden part.
  • Composite vascular bundles have a sieve part and a wooden part.
  • In the case of concentric vascular bundles, the sieve part lies around the wooden part (periphloematic vascular bundle), or the wooden part lies around the sieve part (perixylematic vascular bundle).
  • The most common type is the so-called collateral vascular bundle, in which the sieve part is opposite the wood part (the xylempol is facing the center of the stem axis ).
  • With open collateral vascular bundles, there is also a cambium between the xylem and the phloem. This arrangement occurs in dicotyledonous plants.
  • If the collateral vascular bundles are closed, the cambium is absent and the vascular bundles are surrounded by a closed, sclerchymatic vascular bundle sheath. This arrangement occurs in monocotyledonous or (other) herbaceous plants.
  • A special form of the collateral type is the bicollateral vascular bundle, as it occurs, for example, in the cucurbits and nightshades . Here another phloem part is added to the vascular bundle on the side facing the center of the stem axis.
  • In roots the vascular bundles are combined into a radial vascular bundle system, where the wooden part is arranged like the spokes of a wheel - the bast part lies between the spokes ( actinostele ).


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