Exchange and Movements of Gases and Liquids. 77 



water, and after the cut end has remained immersed for an 

 hour, set it in a beaker containing a solution of eosin or red 

 ink. An hour later begin at the base and cut away portions of 

 the stem in sections about 2 cm. long, to ascertain the distance 

 which has been penetrated. A current of water will traverse 

 the stem more rapidly than that shown by this test, however, 

 since the coloring matter is filtered out as it ascends, and the 

 water carrying the uppermost traces of eosin will have traversed 

 the stem to some distance above the colored tissues when the 

 examination is made. A much more accurate test is made by 

 the use of lithium salts. The base of the stem is to be placed 

 in a beaker containing a 2 per cent, solution of lithium nitrate. 

 After an hour cut the stem into sections about 2 cm. long, and 

 number them to preserve their relative position. Dry these 

 sections, and then, beginning at the basal portion, burn each 

 one in succession, in a colorless Bunsen flame, observing the 

 carmine-red tint given by the lithium salt. This may be best 

 observed through a sheet of blue glass. Note the first section 

 from which the lithium flame reaction is absent, and find dis- 

 tance from the base of the stem. This will give the approxi- 

 mate rate of ascent of the sap. 



73. Diffusion streams in the plant. Water and 

 mineral salts ascend from the roots to the leaves chiefly through 

 the non-living woody cells, from which diffusion constantly 

 takes place laterally in the stem, and at the termination of 

 the woody elements in the leaves. The stream is thus seen to 

 start in living cells, to traverse dead elements, and end in living 

 cells which use some of the water and salts as food and exhale 

 the superfluous water as vapor. On the other hand, sugars and 

 other organic substances are formed in the leaves and slowly 

 diffuse downward through the body of the plant, through the 

 living cells. The principal pathways for such conduction con- 

 sists of the sieve, companion cells, and other tubular elements 

 in the phloem. The killing of trees by girdling is effected by 



