DuPpLER: AIR CHAMBERS OF REBOULIA 247 
Origin and development.—The young chambers evidently arise 
schizogenously, the splitting beginning as close to the apical cell 
as between the third and fourth segments (Fics. 6, 8). The 
splitting probably begins internally (Fic. 7), although a superficial 
splitting is also initiated very soon, and the process apparently 
proceeds simultaneously from both these points, the two splits 
meeting about midway (FIGs. 6, 8). The turgor conditions which 
effect splitting can reasonably bring about external as well as 
internal separation. As the thallus thickens back of the apex 
the splitting may continue to deeper levels, although the increased 
s 
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Fic. 6. Longitudinal section through a growing point, showing the apical cell 
and the beginning of an air chamber, with both internal and superficial splitting, 
X 433. Fic. 7. Similar section, showing air chamber with internal origin, x 433- 
G imi ti ith slightly older air chamber, x 433 IG.9. -Longitudinal 
Titiadt SCCL LWitil siigs yY O1GCT ait 43. 
section of young air chambers, showing beginning of secondary partitioning and 
intercalary division in the primary partition, x 433. Fic. 10. Air chamber a little 
older than that of Fic. 9, x 433. Fic. 11. Epidermis of a nearly mature chamber, 
X 433. 
depth of the chambers is due in large part to intercalary growth 
of the partitions themselves (Fic. 14). The earliest chambers 
are horizontal but with the growth of the thallus become obliquely 
inclined. Early in the development of the chambers the secondary 
plates have their origin as lateral outgrowths from the primary 
- partitions (Fics. 9, 10). These are so situated in the narrow 
