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 region 

 of the joint; and on this account early in- 

 vestigators failed to record pressures that 

 were adequate to lift the sap to any sig- 

 nificant height. However, recent experiments 



fn\ 



Fig. 13-19. A rough method of measuring root pres- 

 sure. The stem of a potted plant is cut off and re- 

 placed by a closed tube containing water and mercury. 

 As water is absorbed, sap exudes from the cut stem, 

 forcing the mercury upward. 



prove that even a small plant, such as the 

 tomato, can generate a root pressure amount- 

 ing to at least 2 to 3 atmospheres. 



Root pressure results from the hyper- 

 tonicity of the root sap, relative to the soil 

 water that surrounds the root. A 10 percent 

 glucose solution separated from soil water 

 by a membrane that is permeable to water 

 but not to the sugar will continue to absorb 

 water until a pressure of more than 12 at- 

 mospheres has been developed. This would 



