Download / swf / 169 Kb
Most plants secure the water and minerals they need from their roots.
The path taken is: soil -> roots -> stems -> leaves
The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells).
Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration.
Soil water enters the root through its epidermis. It appears that water then travels in both
* the cytoplasm of root cells — called the symplast — that is, it crosses the plasma membrane and then passes from cell to cell through plasmodesmata.
* in the nonliving parts of the root — called the apoplast — that is, in the spaces between the cells and in the cells walls themselves. This water has not crossed a plasma membrane.
However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of suberized matrix called the casparian strip. Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele.
Once inside the stele, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and/or tracheids [link to views of the structure of vessels and tracheids]. These are nonliving conduits so are part of the apoplast.
Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids.
At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues.
At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.