BIOL 2242 Lecture Notes - Lecture 13: Sieve Tube Element, Cucurbita, Phloem

• School: University of Manitoba
• Department: Biological Sciences
• Course: BIOL 2242
• Professor: Isobel Waters

Document Summary

Water stress less severe: negative pressure (tension) weaker, less risk of collapse, vessels with larger bore. Move water faster: also because less resistance due to absence of end walls. Water stress more likely: stronger tension in air, more risk of collapse, narrower bore vessels, tracheids. Sacrifice speed for lower risk of blockage. Note: leaf veins often have tiny tracheids with helical or annular secondary walls: exception: herbaceous vines e. g. , cucurbita. Phloem tissue is cucurbita (squash) stem in c. s and l. s (p. 551): complex tissue. Sieve tube elements, companion cells are unique to phloem. Also contains parenchyma, sclerenchyma fibres: share traits of sieve tubes and companion cells. Elongated: same length (arise from same mother cell) Dependent of companion cells: end wal with sieve plate (perforations) Sealed off with callose when not in use: transport sucrose in aqueous solution. Intact cytoplasm with nucleus: tapered end wall, with no sieve plate, aid transport in sieve tube.