244 DuPLerR: AIR CHAMBERS OF REBOULIA 
arising from successive segments. . . . Later, and often deep in 
the tissue, secondary splitting gives rise to intercellular spaces 
which may reach the surface or may break into a primary space. 
In Plagiochasma the situation is similar only the secondary splitting 
occurs more quickly and becomes quite extensive. The passages 
are not wide and shallow, but always deep and narrow open almost 
or quite uninterruptedly until the pore margin is well begun.” 
he problem thus opened up anew was attacked by different 
writers, dealing largely with the Ricciaceae and Targionia. Miss 
Hirsch (10) and Miss Black (2) both support the Leitgeb view for 
Riccia Frost. Campbell (3) regards their work, as well as that 
of Miss O’Keefe (15), as showing that Leitgeb’s account of the 
formation of air spaces in Riccia glauca and other allied species is 
entirely correct. Both Deutsch (6) and Miss O’Keefe (15), in 
their studies of Targionia hypophylla, agree that the chambers 
arise by splitting, but that the splitting begins superficially and 
proceeds inward, instead of arising internally and proceeding 
outward. Miss Starr’s study of Plagiochasma (16) leads her to 
conclude that there is no doubt as to the internal schizogenous 
origin of the air chambers of both thallus and receptacle. The 
development of the barrel-shaped air-pore on the receptacle is 
described in detail. Evans (7) reviews the recent work on the 
problem pointing out that in all cases, even including the work of 
Miss Hirsch and Miss Black, the origin of the air chambers is 
probably due to splitting of the cell walls, although it may begin 
superficially in some cases. From his study of Grimaldia fragrans 
he concludes that “the chambers all owe their origin to a splitting 
of cell walls in closely united tissue. In the case of the dorsal 
chambers the split sometimes begins below the surface and extends 
outward, sometimes at the surface and extends inward.” Further, 
‘that there still seems to be no conclusive evidence that Leitgeb’s 
explanation ever applies.” Haupt (9) in his recent study of 
Reboulia hemisphaerica states that ‘the air chambers of Reboulia 
arise immediately behind the apical cell of the thallus by inter- 
cellular splittings which start at the surface of the thallus and 
progress inward, reaching the line of differentiation between the 
dorsal and ventral regions. Secondary splittings occur deep 
within the dorsal region and do not reach the surface.” No 
