Water Transport
The Vascular Bundle
Substances are transported around a plant within the xylem and phloem. These
tissues are collected into a structure called a vascular bundle. The
distribution of vascular bundles within a plant stem is illustrated below.
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Root Hair to Xylem
Water is taken up by the root hairs , crosses the cortex and enters the xylem
in the centre of the root. This movement is driven by a water potential gradient-
the water potential inside the xylem being the lowest. Water can travel threough
the cortex through one of two possible paths- the apoplast and symplast
pathways.
Which one of these diagrams shows the apolplast, and which shows the symplast
pathway? Can you explain what is going on at each numbered stage? Mouse-over
for the answer
In the apoplast pathway model, water moves from cellulose cell wall to cellulose cell wall, and does not enter the cell.
In the symplast pathway, water is viewed as moving through cell cyctoplasm or vacuoles and through to interconnected cells via the plasmodesmata
The prevalence of each pathway varies from plant to plant. On reaching the stele, the water cannot follow the apoplast pathway any longer as the the cells in the outer layer (the endodermis) have a thick, waterproof, waxy band of suberin in their cell wall. This is called the Casparian strip. Water has to pass through the cytoplasm of the endodermal cells. This may play a role in governing the uptake of mineral ions from the soil and with the developemnt of root pressure. The two pathways, and a cell from the endodermis are shown below.
Once past the endodermis, the water moves across the pericycle to the xylem
Click here to remind yourself of where you find phloem and xylem in root and stem sections.
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