HirsH : AIR CHAMBERS IN THE RICCIACEAE 75 
to the closing of the air chamber C that has broken out to the sur- 
face. All of the figures confirm the truth of the statements of 
Barnes and Land; and they cannot be interpreted in any other 
way than in consonance with their observations. 
FIGUR Ricciocarpus na- FIGURE 4. Riccta Frostii. Vertical longitudinal 
tans. A, cell division prepara- section, showing archegonium and origin of chambers 
tory to closing the chamber C. and filaments. 
An examination of Ricca Frost Aust., however, indicates an 
entirely different structure. FiGuREs 4, 5, and 6 present a condi- 
tion strikingly in contrast with that shown in the previous figures 
of Ricciocarpus natans, In FicuRE 6, A is the apical cell; M isa 
young archegonium; B, C, and D, are tiny depressions lying be- 
tween the rounded ends of the outermost layer of cells ; N and O 
show older spaces, and at P is shown the way in which the elon- 
gated, narrow air spaces are closed at the end. The air chambers 
of Riccia Frostii Aust. are, therefore, narrow chambers or canals, 
which are the spaces between the elongated filaments or rows of 
cells at right angles to the upper surface of the thallus. Ficure 
6 shows clearly the way in which these filaments and spaces origi- 
nate. Immediately back of the apical cell, the superficial cells 
arch outward in a papillate manner as a result of the cessation of 
growth at the lines of their junction. As they elongate they are 
divided by transverse walls so that filaments or rows of cells are 
formed, which are separate and distinct from one another. The 
intervening spaces in this species are formed, therefore, not by the 
cleavage or the separation of mature tissues, but in a manner 
