. Principles of modern biology. Biology. 252 - Multicellular Plants sap, which prevents the column from break- ing as it is pulled upward in the vessels. The tensile strength of a column of water, which is almost equivalent to that of a sap column, has been measured in several ways. These measurements yield values that are equal to or greater than 60 kilograms per square centimeter, or 60 atmospheres. Such a force woidd be adequate to lilt sap to the tallest tree-top, even considering the high resistance encountered by the sap as it Hows through the very fine vessels of the plant. Also this fo
. Principles of modern biology. Biology. 252 - Multicellular Plants sap, which prevents the column from break- ing as it is pulled upward in the vessels. The tensile strength of a column of water, which is almost equivalent to that of a sap column, has been measured in several ways. These measurements yield values that are equal to or greater than 60 kilograms per square centimeter, or 60 atmospheres. Such a force woidd be adequate to lilt sap to the tallest tree-top, even considering the high resistance encountered by the sap as it Hows through the very fine vessels of the plant. Also this force would account for the rapidity of sap ascent, which reaches a maximum of some 200 feet per hour in some hardwood trees such as the oak. A large flow of sap is essential to the plant, but not because a great quantity of water is needed for photosynthesis. Actu- ally only about 1 percent of the water reach- ing the leaf is used this way. A very copious flow is necessary if the leaf cells are to receive adequate quantities of salts (for protein syn- thesis), since only very small amounts of in- organic salts are present in the sap. More- over, large quantities of water must be vapor- ized from the leaf to prevent an overheating of the tissues. Elevation of Sop in Tall Trees. The rise of sap in tall trees—which may grow to heights of more than 300 feet—may involve mechanisms that are not yet fully understood, although certainly both transpiration and root pres- sure must act in cooperation with one an- other. Root pressure is the positive pressure of the sap in the ducts at the point where these vessels emerge from the root to enter the stem. A rough measurement of root pressure may be obtained by cutting off the stem close to the root and joining the base of the stem to a pressure gauge, which records the force built up by the exuding sap (Fig. 13-19). Technical difficulty is encountered in making a junction that is both leakproof and noninjurious to the tissues in the reg
Size: 1262px × 1980px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
