THE SKIN, OR BIND, OF PLANTS. 81 



It may be proper to state, that Link and Nees von 

 Esenbeck, deny the existence of any perforation in 

 these pores, which are covered, they say, with tissue. 

 R. Brown, again, describes them as glands whose disc 

 is perhaps sometimes perforated. 



Pores. a, portion of the surface of a walking cane; b, a por- 

 tion of the epidermis or outer skin, with three pores placed among 

 the meshes j both figures greatly magnified. 



Air Cells. 



The pores lead to small cavities which have been 

 termed air cells, very variable in size, figure, and 

 arrangement. M. Dutrochet, by putting leaves under 

 water in the exhausted receiver of an air pump, ob- 

 served the water forced into the cells. He thus proved 

 that the white colours of flowers as well as the spots on 

 leaves are caused by the superabundance of these air 

 cells. Similar cells may be seen in the stem of the 

 rush placed one above another, and separated by 

 membranous partitions. 



It confirms the preceding opinion that the structure 

 of the pores is so very similar to that of the spiracles or 



