198 MOVEMENTS AND EXCHANGES OF FLUIDS 



Examine also the stomata of Marchantia or ConacepJialus which 

 are permanently open. Place leaves and thalli of the above spe- 

 cies in the sun to wilt, and cut a thin surface section of the stom- 

 atal surface and examine diy. Compare the appearance of the 

 stomata, with that seen in similar sections of fresh leaves. Place 

 both kinds of sections in water and note results. If a number of 

 species are examined it may be found that immersion in water 

 will cause some stomata to close and others to open, owing to 

 the different behavior of the epidermal cells. Take strips of epi- 

 dermis from the lower surfaces of Tradescantia discolor standing 



FIG. 99. Diagram of cross section of stoma showing the action of the guard cells. 

 The heavy walls indicate the outlines of the guard cells when turgid and with the 

 stoma open ; the thin walls indicate the contour of the guard cells when relaxed 

 with the stoma closed. After Schwendener. 



in sunlight and examine in water. Run a five per-cent. solution 

 of cane sugar under the cover-glass and note result. 



Any study of stomata in which their action is observed by 

 means of a microscope will be vitiated with many errors, because 

 in taking the epidermis from a leaf and mounting it for examina- 

 tion, stimuli are set up, which may cause the stoma to open or 

 close before its original condition can be observed. 



Practically all of the water given off by a leaf in transpiration 

 passes through the stomata in the form of vapor, and the best 

 method of ascertaining whether the stomata are opened or closed, 

 is to use some means of detection of watery vapor. This may be 

 done in two ways, viz., by the cobalt method, in which paper 



