Xylem and Phloem
Vascular plants have two main transport systems which are xylem and phloem. The Xylem is a narrow tube that is involved in transporting water and minerals from the soil to the other parts of the plant while phloem is mainly responsible in transporting sugar and other byproducts of the plants.
The term xylem comes from the Greek word “xylon” which means wood, although xylem tissue is found in wood it is basically found in every part of the plant. Although the basic function of xylem is to transport water into the other parts of the plant, it can carry other nutrients as well.
The long tracheary elements that transport water are the most distinctive xylem cells. Vessel elements and tracheids are distinguished by the mass of spots that transpire on the vessel elements. Both of the inner and outer walls are missing in these spots and the vessel elements are joined together in long tubes known as vessels.
Xylem is present in vascular bundles, found in non-woody plants; in secondary xylem that is laid down by the vascular cambium, a meristems, in woody plants; and in ferns, as part of a stellar arrangement which is not divided into bundles.
Aside from being a transport system of water and other soluble nutrients from the roots to the other parts of the plant, xylem is also used in replacing the lost water during the process of photosynthesis and transpiration.
The xylem sap is mainly composed of inorganic ions and water, but it can also contain certain amount of organic chemicals.
Flow in the Xylem
The tracheary elements cannot provide all the energy needed for such transport system to occur, as they are mainly dead by maturity. There are two phenomena that enable the flow of the xylem sap:
- Transpirational pull. The evaporation process of water from the mesophyll cells to the atmosphere causes the flow of the xylem sap. Transpiration process gives rise to the formation of millions of minute menisci to the mesophyll cell wall thus, resulting in surface tension that pulls the water up from the soil and roots.
- Root pressure. Water can be movede from the soil to the roots through the process of osmosis, this occurs when the water potential of root cells is more negative than what the soil is, normally because of solute high concentration level. This causes certain pressure that will force the sap to the xylem into the other parts of the plant. Root pressure is greater in the morning before the opening of the stomata, which allows the beginning of transpiration.