EvAxs: Air chambers of Grimaldia fr.\graxs 247 



portion of the thallus to the right of the apical cell and derived 

 from lateral segments. In drawing Fig. 9 the microscope was 

 focused on the surface of the cells in a circumscribed area; in 

 drawing Fig. id it was focused a little below the surface of the 

 same area. Fig. 9 shows a series of cells in close union and two 

 air chambers which have reached the surface; Fig. 10 shows the 

 same two chambers and six additional ones. The latter clearly 

 represent schizogenous spaces below the surface and demonstrate 

 an origin like that of the youngest chamber in Fig. 6. 



Fig. 8. Transverse section through a growing point, X 500. x, apical cell. 



Fig. 9. Superficial cells to the right of the apical cell shown in Fig. 8, X 500. 



Fig. 10. The same region as that shown in Fig. 9, but at a slightly lower focus, 

 more very young dorsal chambers being visible, X 500. 



Fig. II. Slightly older dorsal chambers than those shown in Fig. 10, the section 

 parallel with the surface, X 500. 



Fig. 12. Section just below the one shown in Fig. ii, the numerous intercellular 

 spaces being the beginnings of more deeply situated chambers, X 500. 



The rudimentary chambers shown in Figs. 6-10 represent the 

 beginnings of the complex dorsal chambers shown in Figs. 1-4. 

 The later stages in the development of these chambers and the 

 origin and development of the more deeply situated chambers are 

 exceedingly difficult to follow. For a while the dorsal chambers 

 are distinct enough in sections cut immediately below the epider- 

 mis. Such a section is shown in FiG. 11, where two complete 

 chambers and parts of six others are represented. The increase 



