40 



divide by walls parallel to the surface of the 

 thallus. For this reason the opening is sur- 

 rounded with from four to eight tiers of cells. 

 The opening is narrowed at its upper and under 

 surfaces, and therefore shows a barrel-shaped 

 form. 



2. Intercellular Spaces. 



Under each stoma is found a large, intercellu- 

 lar space, into which project from below 

 threads of cells (two or three cells long) occa- 

 sionally branched. These cells are especially 

 rich in chlorophyll, and they arise fiom the flat- 

 tened cell layer next below which is poor in 

 chlorophyll. The walls bounding the air-chamber 

 laterally are constructed of loosely-combined cells, 

 which also contain chlorophyll. 



3. Cellular Tissue. 



The tissue next under this consists of broader 

 cells, which have reticulately thickened walls. 

 They are almost free from chlorophyll and have 

 no interstices. 



4. Ventral Cortical Layer. 



Approaching the -ventral surface the last two 

 layers are flat and narrow, and contain chloro- 

 phyll and form the so-called ventral cortical 

 layer. 



5, Epidermis. 



The epidermis of the under surface is much 



