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Cross section of celery stalk, showing vascular bundles, which include both phloem and xylem Detail of the vasculature of a bramble leaf Translocation in vascular plants. Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem ...
in secondary xylem, laid down by a meristem called the vascular cambium in woody plants; as part of a stelar arrangement not divided into bundles, as in many ferns. In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.
An example of this is that secondary wall in wood contains polysaccharides called xylan, whereas the primary wall contains the polysaccharide xyloglucan. The cells fraction in secondary walls is also higher. [7] Pectins may also be absent from the secondary wall, and unlike primary walls, no structural proteins or enzymes have been identified. [4]
After the primary growth, lateral meristems develop as secondary plant growth. This growth adds to the plant in diameter from the established stem but not all plants exhibit secondary growth. There are two types of secondary meristems: the vascular cambium and the cork cambium. Vascular cambium, which produces secondary xylem and secondary ...
The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the flowering plants. It was once thought that vessel elements were an evolutionary innovation of flowering plants, but their absence from some basal angiosperms and their presence in some members of the Gnetales suggest that this hypothesis must be re-examined; vessel elements in ...
The cambium present between primary xylem and primary phloem is called the intrafascicular cambium (within vascular bundles). During secondary growth, cells of medullary rays, in a line (as seen in section; in three dimensions, it is a sheet) between neighbouring vascular bundles, become meristematic and form new interfascicular cambium ...
Primary and secondary phloem; Vascular cambium; Secondary and primary xylem. Cork cell walls contain suberin, a waxy substance which protects the stem against water loss, the invasion of insects into the stem, and prevents infections by bacteria and fungal spores. [21]
Later, more complex pits appeared, such as bordered pits on many tracheids, which allowed plants to transport water between cells while reducing the risk of cavitation and embolisms in the xylem. As tracheids evolved along with secondary xylem tissues, specialized inter-tracheid pits appeared. [ 3 ]