TRANSPIRATION. 



219 



until it fills the gauge from end to end ; air is then admitted above 

 and the mercury allowed to find its natural level ; then put the 

 rubber tubing on again, with a clip at its free end, and pinch it 

 below the clip, at once releasing it so that the air column is forced 

 down round the bend to the bulb and allowed to spring back, when 

 a part of the mercury will be forced out and replaced by water, 

 which should fill rather less than half of the bulb ; then remove the 

 clipped tubing, seal the end of the glass tube with a drop of hot 

 shellac applied with a knife, and stand the gauge in the water for 

 two hours to let the plug of shellac harden. 



Join the short limb to the cut stem stump by means of thick 

 rubber tubing and wire or string, and note the exact height of the 

 mercury column at the beginning of the experiment. The pressure 

 developed is calculated by Boyle's Law. 



279. Escape of Liquid Water from Leaves. This process 

 of "guttation" is shown commonly by plants on a cool evening 

 following a hot day. When the stomata close at night, water is 

 still absorbed by the roots in this way 

 a plant that had become somewhat 

 wilted on a hot day recovers its tur- 

 gidity at night, and drops of water may 

 be forced out of the leaves, usually 

 through special non- motile stomata 

 (water-pores) on the edges of the leaves. 

 In Grasses the water escapes between 

 the ridges on the upper side of the leaf, 

 and in seedlings at any rate from the 

 tip of the leaf. 



( 1 ) Cover various growing plants with 

 a bell-jar overnight, and look for water- 

 drops excreted by the water-stomates. 

 The plants should be growing in pots, 

 and the following will usually give good 

 results : Fuchsia, Tropaeolum, London 

 Pride (a Saxifrage, with chalk-glands). 

 In a cut twig of Cherry, set in water 

 and kept under a bell-jar, drops of water 

 are seen oozing from the glands on the 

 leaf-stalk. 



(2) Cover seedlings of Wheat or Maize 

 with a bell-jar, and note the excretion 



of water from the tips of the young leaves. 



(3) Fix a cut piece of a Fuchsia into the short limb of a J-tube, as 

 shown in Fig. 51. Pour some water into the tube and then pour 

 in mercury. Drops of water are caused to escape from the " water- 

 glands" on the teeth of the leaf-margin. A water-gland is a mass 

 of tissue on the end of a vein, communicating with the watei'- 

 stomates on the leaf-teeth. 



GLANDS- 



WATER ' 



_Hg 



Fig. 51. Mode of demonstrat- 

 ing the Excretion of Fluid 

 Water from Leaves. 



