The structure of xylem

• what are the main cell types in xylem?

  • vessels
  • tracheids
• what are vessels?

  water-conducting structures in angiosperms comprising cells fused end-to-end making hollow tubes, with thick, lignified cell walls
• what are tracheids?

  spindle-shaped, water-conducting cells in the xylem of ferns, conifers and angiosperms
• where do tracheids occur?

  in ferns, conifers and angiosperms (flowering plants), but not in mosses
• why are mosses poorer at transporting water and at growing as tall as other plants?

  as they have no water-conducting tissue
• where do vessels occur?

  only in angiosperms
• what happens as lignin builds up in xylem vessels cell walls?

  • as lignin builds up in their cell walls, the contents die, leaving an empty space, the lumen
  • as the tissue develops, the end walls of the cells break down, leaving a long hollow tubes, like a drainpipe, through which water climbs straight up the plant
  • the lignin is laid down in a characteristic spiral pattern and, unlike cellulose of phloem cell walls, stains red so xylem is easy to identify in microscope sections
• what are the functions of the xylem?

  • transport of water and dissolved minerals
  • providing mechanical strength and support
• what do tracheid cell walls contain and what do they do?

  they contain lignin, which is hard, strong and waterproof
• what do tracheid cell walls have?

  they have gaps, called pits, through which water travels
• what is the shape of tracheids and what does it mean?

  they are spindle-shaped so water takes a twisting, rather than a straight, path up the plant
• why have angiosperms become the dominant plant type on earth?

  as water moving straight up the plant in vessels is so much more efficient than the twisting path through tracheids
• why are vessels more efficient than tracheids?

  as water moving straight up the plant is so much more efficient than the twisting path through tracheids
• what is this an image of?

  transverse section of xylem vessels showing large empty vessels

  
• what is this an image of?

  3D view of xylem

  
• what is this an image of?

  a longitudinal section of tracheids to show water taking an indirect path upwards