218 



TRANSPIRATION. 



plants for root-pressure experiments are Fuchsia, Begonia, 

 Marguerite, Phaseolus. 



(b) A. simple method of demonstrating exudation is the 

 following. Cut across the stem of a Phaseolus seedling, 

 or a small pot plant, tie a bit of rubber tubing about 10 cm. 

 long over the stump, and fill this with water coloured with 

 red ink. Then insert in the rubber tube a capillary glass 

 tube, and support this horizontally e.g. by means of 

 a cleft stick placed in the soil. Pinch 

 the rubber tubing so as to force some 

 water out of the open end of the fine 

 glass tube, and absorb it with blotting- 

 paper ; then release the rubber tube 

 so that air is drawn into the glass 

 tube. Watch the advance of the 

 coloured water along the glass tube, 

 from the open end of which it falls 

 in drops. 



(c) That the sap exudes under 

 pressure may be simply demonstrated 

 by joining to the cut stump, by means 

 of rubber tubing, a long straight glass 

 tube of the same diameter as the 

 stem, keeping this tube vertical by 

 lashing it to a stick placed in the soil 

 or fixing it to a retort- stand. Fill 

 up the tube with water to a point 

 about 5 cm. above the rubber tubing, 

 marking the place with a file-scratch 

 or a strip of gummed paper, and 

 pour in a little olive oil to prevent 

 evaporation of the water. 



(d) In order to measure, roughly at any rate, the pressure set 

 up during exudation that is, root-pressure it is necessary to use 

 a manometer or pressure-gauge instead of an ordinary glass tube. 

 An excellent form of manometer, supplied by the Bausch and Lomb 

 Optical Company, is shown in Fig. 50. 



p* Clean and dry the gauge with alcohol ; hold it vertically with the 

 shorter (limb in boiled (air-free) water ; through a piece of thick 

 rubber^ tubing slipped over the end of the long limb pour mercury 



Fig. 50. Manoinet 

 demonstrate 

 Pressure. 



