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This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of vascular plants. Several groups of plants later developed pitted tracheid cells independently through convergent evolution .
The xylem is oriented toward the adaxial surface of the leaf (usually the upper side), and phloem is oriented toward the abaxial surface of the leaf. This is why aphids are typically found on the undersides of the leaves rather than on the top, since the phloem transports sugars manufactured by the plant and they are closer to the lower surface.
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 ...
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There is also a tissue between xylem and phloem, which is the cambium. The xylem typically lies towards the axis ( adaxial ) with phloem positioned away from the axis ( abaxial ). In a stem or root this means that the xylem is closer to the centre of the stem or root while the phloem is closer to the exterior.
Xylem sap is mostly made of water. This is because one of the main roles of xylem is to transport water and inorganic nutrients throughout the plant. [13] Water is not the only thing that makes up xylem sap though. Xylem sap contains long-distance signaling hormones, proteins, enzymes, and transcription factors.
This is incorrect. Most clades of xylem-bearing plants lack vessel elements. The defining feature of xylem in recent cladistic and morphological literature seems to be the presence of the compound lignin, not a cell type. In fact, some of the "xylem" in the Silurian rhyniophytes has been found not to be true xylem, despite the structure of the ...
On October 31, 2011, Xylem Inc. completed its spinoff from ITT Corporation, [18] and had 2011 revenues of $3.8 billion. [19] Goulds Water Technology is a brand of Xylem Inc. providing pumps for residential, commercial and agricultural applications and Goulds Pumps remains a part of ITT Corporation providing pump products for the industrial market.