EEB340 - Lecture 7.docx

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18 Mar 2012
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Lecture 7: Evolution of Vascular Tissue: Phloem
Evolution of Vascular Tissue
Phloem
Lateral Meristem Evolution
Vascular Cambium
Cork Cambium
Vascular Plants
- phloem tissue: contains cells that conduct food food conducting tissue
o sieve tube elements in flowering plants (more derives)
- liverworts and mosses have some water conducting cells so could be considered
“vascular”
- phloem tissue:
o sieve cells gymnosperms and onward (basal)
no perforation plate
have lateral sieve areas
companion cell is not developmentally related to the sieve cell
- shoot and root apical meristems are responsible for vertical growth
- xylem cells have fiber and are lignified so give plants a lot of support
- cork cambium gives rise to the bark
o as divides, gives rise to cork cells and fibers
cutin (hydrophobic)
- sieve area is like a sieve plate but the pores are much smaller
o allow food to move laterally from one sieve tube element to another
- sieve tube elements are living at maturity
o most derived form of food conducting cell
o companion cell has all components needed to function (organelles, etc.)
o sieve tube elements becomes modified:
nucleus degenerates (no nucleus)
no ribosome’s no protein synthesis (also because no nucleus)
still a plasmamembrane
no vacuole (would interfere with rapid movement of food)
o there is an interdependence of the sieve tube element on the companion cell
- protostele: central core of water and food conducting tissues
- Silurian Lower Devonian
o These organisms can only get so tall
o Over time the land moved off the poles and many land masses moved
towards the equator more tropical, glaciers disappeared, sea levels rose,
still high CO2 levels
o Dichotomous branching stems
Some along or under ground
o No leaves
o Small (2 meters in height max)
- Vascular cambium produced xylem towards inside, phloem towards outside
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