structure of xylem and phloem Flashcards
what do xylem cells consist of
-vessels to carry water and mineral ions
-fibres to support plant
-living parenchyma cells
what do parenchyma cells act as
packing tissue to separate and support the vessels
how do xylem cells develop
-parenchyma cell walls in a column become strengthened with lignin
-impermeable and waterproof walls kill the cell
-protoplasm and end walls decay to make unrestricted flow of water
what’s the protoplasm
-cytoplasm and nucleus
what does lignin do
-strengthen cell wall preventing collapsing
-keep vessel open even at times when water may be in short supply
where is there no lignification in xylem and why
-where plasmodesmata present, leaving gaps in the cell wall
-gaps form bordered pits allowing water movement between vessels
what are plasmodesmata
-channels connecting the cytoplasm of adjacent plant cells
-allowing direct communication and transport of substances between them
what does lignin thickening cause
-spiral patterns in the cell wall
-can be annular (rings) or reticulate (network of broken rings) for flexibility
why are xylem tubes narrow
so that water column doesn’t break easily and capillary action can be effective
what do boarded pits allow
-allow water to move sideways from one vessel to another
what does the pattern of the deposited lignin allow
-allows xylem to stretch as plant grows and enables stem/branch to bend
why is the flow of water in the xylem unrestricted
-there are no cross-walls
-are no cell contents, nucleus or cytoplasm
-lignin thickening preventing walls from collapsing
what does lack of cell walls between xylem cells mean
means that the water flow isn’t slowed down
what forms rigid tubes that will not collapse and what does this mean for the xylem cell
-rings of lignin
-means xylem will not burst/collapse and provides support
what is the role of the phloem
-to transport assimilates
-such as sucrose and amino acids
-sucrose is dissolved to form sap
what does the phloem consist of
-sieve tube elements
-companion cells
what forms long sieve tubes
-individual sieve tube elements aligned end to end
what does phloem contain/not contain
-no nucleus
-very little cytoplasm
why does phloem not contain a nucleus or much cytoplasm
-to allow for mass flow of sap to occur
what do companion cells do
-carry out metabolic processes needed to load assimilates actively into sieve tubes
what are at the end of sieve tube elements
-perforated cross walls called sieve plates
what does perforation allow for in sieve plates
-allows movement of sap from one element to the next
what do companion cells contain
-a large nucleus and dense cytoplasm
-numerous mitochondria to produce ATP
where are plasmodesmata in phloem
connecting sieve tube elements and companion cells
