THE MOLECULAR FORCES IN PLANTS, 



183 



wash them, and examine both the isolated portion of the branch, 

 and that still in connection with the parent plant. It is found 

 that the solution has penetrated many centimetres, as is indicated 

 by the staining of the wood, whether the plant be herbaceous or 

 woody. The experiments succeed particularly well, as I have 



FIG. 66. Apparatus for proving the negative pressure of air in the vessels. 



often found, with Ampelopgis and Phaseolus shoots, and yield 

 particularly striking results when we work with potted bean 

 plants which have been allowed to wither to some extent, or shoots 

 of the wild vine cut through on a very hot, dry summer day. 



These experiments prove that the air in the vessels is at a 

 negative pressure, for the considerable rise of the coloured solution 



