Different types of vascular bundles evolved in the different plant groups. Monocots have usually collateral vascular bundles, a type that is also called closed. The vascular bundle is enclosed by a bundle sheath of parenchyma cells.
Xylem and phloem of the shoots of mono- and dicots are usually arranged opposite to each other in the vascular bundles (collateral vascular bundles). The xylem is normally at the inside, the phloem at the outside. In dicots, both components are separated by the fascicular cambium. It is also spoken of an open vascular bundle in contrast to the 'closed' bundles of monocots that lack the cambium.
Xylem and phloem are surrounded by a bundle sheath of parenchyma that is often starch-containing. The prime object for the demonstration of structure and arrangement of monocot bundles is the shoot of corn (Zea mays). Although the vascular bundles seem to be scattered in cross-section, do they occur at the periphery in larger numbers. Central bundles are usually bigger than peripheral ones.
Many monocots have hollow stems. It seems as if their vascular bundles were arranged in a ring, because space is rare. In the stems of some aquatic monocots (Elodea, Potamogeton), they are united in a central (axial) strand. This arrangement is of secondary nature, since it developed as an adjustment to aquatic life. A comparable structure can be found in some submerse dicots. It decreases the strength of the stem, while at the same time increasing its flexibility. Exactly this property is needed to meet the demands of running water. Selection favouring efficient water transport cannot be expected in submerse living plants anyway. In one of the next sections, we will deal with the structure and arrangement of vascular bundles in the shoot.
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