Ascent of sap
Encyclopedia
The ascent of sap in the xylem
tissue of plants is the upward movement of water from the root
to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain the phenomenon, the cohesion-tension mechanism has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism.
The more recently proposed compensating pressure (CP) theory favors a version of vital theory
proposed by Jagdish Chandra Bose
. However, experimental evidence has not supported it
An alternative theory points out that liquids are at the walls submitted to intermolecular forces inferring density gradients making heterogeneous liquid layers. The crude sap motion takes the disjoining pressure gradient into account and the sap flow dramatically increases such that the watering of nanolayers may be analogous to a microscopic flow. Application to microtubes of xylem avoids the problem of cavitation and enables to understand why the ascent of sap is possible for giant trees. [5]
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
tissue of plants is the upward movement of water from the root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain the phenomenon, the cohesion-tension mechanism has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism.
The more recently proposed compensating pressure (CP) theory favors a version of vital theory
Vital theory
"Vital force theory" is a proposed mechanism for the ascent of sap through the xylem tissue of plants.According to the vital force theory, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue....
proposed by Jagdish Chandra Bose
Jagdish Chandra Bose
Acharya Sir Jagadish Chandra Bose, CSI, CIE, FRS was a Bengali polymath: a physicist, biologist, botanist, archaeologist, as well as an early writer of science fiction...
. However, experimental evidence has not supported it
An alternative theory points out that liquids are at the walls submitted to intermolecular forces inferring density gradients making heterogeneous liquid layers. The crude sap motion takes the disjoining pressure gradient into account and the sap flow dramatically increases such that the watering of nanolayers may be analogous to a microscopic flow. Application to microtubes of xylem avoids the problem of cavitation and enables to understand why the ascent of sap is possible for giant trees. [5]