Xylem and phloem are the pipes that let plants get tall
Vascular tissue is a multicellular transport system unique to vascular plants, built mainly from xylem and phloem that move water, minerals, and sugars. Associated meristems—the vascular cambium and cork cambium—add girth later. Together those tissues form the plant’s continuous plumbing network.
Cells in these tissues are typically long and slender, more like pipes than blobs. Phloem cells join end to end so sugars can flow through continuous tubes, and as tips elongate, newly differentiated strands stay aligned with older ones so the network never breaks. In stems and roots the strands travel as vascular bundles that pack xylem toward the interior and phloem toward the outside, sometimes with extra phloem islands in certain Asterales dicots.
Between xylem and phloem sits the vascular cambium, a meristem that peels off fresh conducting cells and thickens the axis. That secondary growth is how woody plants become stout; because expansion cracks the epidermis, a cork cambium arises among the phloem and builds protective cork that also cuts water loss. Wood and cork are therefore both products of secondary growth layered around the original bundles.
In leaves the bundles thread the spongy mesophyll with xylem facing the adaxial (usually upper) surface and phloem facing the abaxial underside—one reason aphids cluster beneath leaves where sugar-rich phloem is nearer. Each leaf vein is thus a miniature bundle, joined without a gap to the strands running down through stem and root.
The same phrase now names a medical frontier. Surgeons bypassing blocked coronary arteries still prefer a patient's own vein or artery, but varicose veins or earlier operations can leave nothing suitable to harvest, particularly in older people. One candidate substitute is bacterial nanocellulose, prized for strength and low toxicity yet still poor at letting endothelial cells attach. Another is chitosan, a chitin derivative whose porous hydrogels can be shaped into patient-specific scaffolds or tuned to release drugs in response to pH, light or heat; weak mechanics and rapid breakdown remain the hurdles.
Source: Vascular tissue
