196 



MOVEMENTS AND EXCHANGES OF FLUIDS 



of the tube should be at least 80 cm. in length. Set the prepara- 

 tion in a tall jar filled with water, and pour mercury into the long 

 arm of the tube. After the mercury reaches a certain height the 

 air between it and the wood is forced through the wood very 

 slowly and may be seen to bubble up in the water. This is due 

 to the presence of very small intercellular spaces. The amount 



Fig. 97. Epidermis and stomata of the lower surface of leaf of Helleborus foitidm; 

 A, in cross section ; B, surface view, e, epidermal cells, c, cuticle. /, strengthen- 

 ing ridges of the outer walls, f, folds of the lateral walls. S, stoma, s, guard 

 cells, sp, opening, u, stomatal chamber, cl, mesophyl. d, chloroplasts. After 

 Prantl. 



Fig. 98. A, cross section of stoma of Cypripedium venustwn, with large entrance 

 chamber shown at v. JB, cross section of stoma of Dasylition filiferum with entrance 

 chamber divided into two parts, by folds in the walls of the bounding epidermal cells. 

 After Haberlandt. 



of air passing in this manner is small. The tracheids are imper- 

 vious to gases under pressure when wet. 



270. Stomata. The elements of the epidermis join closely to- 

 gether, so that no facility is afforded for gaseous interchange with 

 the atmosphere, under the influence of filtration pressure. At 

 numerous points on the surface of the leaf regular pores are 



