RATE OF ASCENT OF WATER THROUGH STEMS 213 



that mechanical compression of the stem, which would partially 

 close the cavities, lessens the transpiration stream. 



283. Demonstration of Path of Sap. Cut off a shoot of Impa- 

 tiens and place in water for an hour to equalize the negative pres- 

 sure, if it should exist. Now place the lower cut end of the shoot 

 in a beaker containing a saturated solution of eosin in water, and 

 stand in diffuse light. Note the appearance of the dye in the 

 stem a few hours later. Cut sections of the stems and determine 

 elements affected by the stain. It is to be kept in mind that the 

 dye is carried upward in certain elements, but that it also slowly 

 diffuses laterally and may be found after some time in cells not 

 concerned in the transportation of water upward. This test may 

 also be made with Zea, Helianthus, or any species of the mint 

 family. 



284. Comparison of the Capacity of Old and New Wood for Con- 

 ducting Water. Place a branch, which has been cut from an oak 

 or cherry tree, in water for a time to equalize the negative pres- 

 sure. Now immerse the base of the shoot in an eosin solution 

 and note the region through which the fluid is carried most read- 

 ily. Care must be taken not to injure the bark on the base of 

 the branch, so that it may absorb only by the cut surfaces. 



Select another branch and make a microscopical examination 

 of the stem with the view of finding differences between the wood 

 which conducts water readily and that which does not. Make 

 longitudinal sections and note thickness of wall, character of pits, 

 and chemical properties of wall with especial respect to lignin. 1 



285. Rate of Ascent of Current of Water through Stems. The 

 dye used in the above experiments does not readily pass through 

 plasmatic membranes, hence the roots of the plants on which tests 

 were made were cut away so that the solution might be taken up 

 directly by the dead elements. If salts are used which may be 

 taken up by the root hairs the entire plant may be used in the 

 experiment. Advantage may be taken of this fact in the demon- 

 stration of the rate at which solutions travel upward through stems. 



1 For lignin test see Zimmermann's Botanical Microtechnique. 



