TRANSPIRATION. 205 



the emulsion through the cylinders, using it instead of the 

 water in the preceding experiment. 



In the case of Pine or other Coniferous wood, colourless 

 water passes from the lower end of the wood. In the case 

 of Dicotyledonous wood, containing vessels, the particles 

 pass through unless the cylinder is longer than the 

 longest vessels in it. In each case examine the wood to 

 see whether it is coloured throughout or whether the colour 

 is confined to the young outer wood ; also cut longitudinal 

 sections and note with the microscope the distribution of 

 the particles. 



(e) Another method, which gives interesting data as 

 to the length of the vessels in Dicotyledonous wood, is 

 to suck through a cut stem first a mixture of 1 part 

 " dialysed iron " (oxy chloride of iron, which is a colloid 

 and incapable of diffusing through membranes) and 

 3 parts water, and then a solution of ammonia which 

 produces with the iron salt a red-brown precipitate. 



Cut a piece about 10 cm. long from the middle of a 

 twig of Alder or Lime about eight years old (other trees 

 may be tried), holding the stem under water. Join the 

 upper end of the twig by strong rubber tubing to a 

 piece of glass tube, and join this in turn to a gently- 

 acting aspirator. Let the lower end of the stem dip into 

 diluted iron, and note that the fluid that passes through 

 the stem is colourless instead of brown if none of the 

 iron has traversed the stem. After an hour, transfer the 

 lower end of the stem to a solution of ammonia, and 

 apply suction again until the water that passes through 

 smells strongly of ammonia. Examine sections with the 

 microscope and note the distribution of the precipitate 

 formed. 



268. Sucking Force of Transpiring Leaves. Con- 

 nect a leafy shoot e.g. a Willow twig by rubber tubing 

 to a straight glass tube about 20 cm. long. Fill the tube 

 with water and dip it into mercury in a dish. As the water 

 is used up by the leaves, mercury enters the tube and may 

 rise several centimetres in a few hours. 



