Exchange and Movements of Gases and Liquids. 75 



table ^^■hen laid upon it, and the bristle should point to the 

 upper division on the scale. If this apparatus is placed on a 

 leaf, the exhalation of watery vapor will be detected by the 

 straightening tendency of the strip and the movement of the 

 bristle pointer over the scale. Test the upper and lower sur- 

 faces of leaves of various parts of the plant, which should in 

 every instance be placed horizontally. 



Care must be taken not to disturb the action of this delicate 

 instrument by the vapor of the breath, or that coming from the 

 hand. Do all stomatal surfaces transpire constantly? Make 

 tests in different conditions of light and temperature. 



69. Action of stomata. — Take thin surface sections of 

 a number of leaves of different species, and examine in water, 

 with magnifications of about 300. Stomata of some species are 

 closed, and others are opened, by the action of the guard cells 

 with regard to the water. Replace the water with a 5 per 

 cent, sugar solution, and note results in one or two species. 

 Iris, Tradescantia, or tomato will be found suitable for this 

 work. Allow a growing specimen of tomato to become wilted. 

 Now take a surface of the leaf, and also one from a fresh leaf, 

 and examine in a dry condition. Run in water, and examine 

 again. 



70. Path followed by sap in ascending from the 

 roots to the leaves. — It has been shown that water is taken 

 up by the roots, and that water is constantly being exhaled from 

 the leaves in the form of vapor. It yet remains to demonstrate 

 the path followed by the stream which replaces the loss and 

 carries mineral salts upward through the stem. This may be 

 done if some colored fluid is given the plant in such manner 

 that it may be conducted through the ordinary channels, stain- 

 ing the, walls in its course. To do this, cut off a stem of Zea, 

 Impatiens, Helianthus, tomato, or mignonette, under water^ and 

 allow it to remain immersed for an hour. Support the excised 

 shoot with the base of the stem in a beaker containing a strong 



