110 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



The system is completely filled with alcohol at room temperature 

 and sealed. The spring with its index is placed under a bell-jar, 

 which is then evacuated. On cooling the bulb, the spring will 

 close somewhat, due to the internal stress set up by the con- 

 tracting liquid. This stress may be demonstrated by heating 

 the capillary locally, thus breaking the liquid column, when the 

 spring will be observed to fly back suddenly to the unstressed 

 position. The stress exerted by the liquid at the time may be 

 found by determining the external pressure necessary to produce 

 the same distortion of the spring. 



The channels in the stem of a tree through which water is 

 conducted are divided into minute compartments by means of 

 numerous longitudinal and transverse partitions. This con- 

 figuration seems ill adapted to the conduction of fluids; for while 

 the walls are permeable to water, they offer a great resistance to 

 its flow. But from the standpoint of the cohesion theory of the 

 ascent of sap this structure becomes, as Dixon 14 has shown, a 

 beautiful adaptation of the plant to confer stability on the ten- 

 silely stressed transpiration stream. The ascending column of 

 water, interlaced by these innumerable permeable partitions, is 

 given great stability even though subjected to great tensile 

 stress. 



Observers agree that air bubbles are of common occurrence 

 in the water ducts of the stems of plants. It is evident from 

 Dixon's experiments that dissolved air is not prejudicial to co- 

 hesion. But if a minute bubble appears in an experimental 

 tube in which the water is subjected to great stress the bubble 

 expands rapidly and the column is at once ruptured. How then 

 can the presence of air in the conducting channels be harmonized 

 with the cohesion theory of the rise of sap? Here the plant 

 again shows in the minute subdivisions of its conducting chan- 

 nels a beautiful provision against the interruption of its water 

 supply through the development of bubbles in the stem. If an 

 air bubble appears in one of the minute compartments, it may 

 expand, as Dixon has pointed out, until it fills the compartment, 



14 Dixon, H. H. Transpiration and the ascent of sap. Progressus Rei Botan- 

 icae, 3 : 1-66. 1909. 



