244 Evans: AIR CHAMBERS OF GRIMALDIA FRAGRANS 
Pietsch’s work is remarkable for its thoroughness and accuracy. 
It deals with species of Riccia and Ricciella, and his account is 
therefore based on the group of plants from which Leitgeb drew 
his conclusions. Although he criticises the work done by Barnes 
and Land, his observations lead to similar conclusions, so far as 
the development of the air chambers is concerned. He finds that 
even in Riccia the chambers originate from a splitting of cell walls, 
the split beginning below the surface = then extending upward 
until the surface is reached. 
Deutsch’s paper, devoted to Targionia hypophylla L., includes 
an interesting observation on the development of the air chambers. 
He states that they arise by a splitting apart of cells close to the 
apical cell but maintains that the split begins on the outside and 
extends inward, instead of beginning below the surface and extend- 
ing outward. The f. 3, which he cites as evidence, would be more 
convincing if the youngest chamber shown did not extend into 
the hypodermal tissues; as the figure stands it might equally well 
bear the opposite interpretation from the one drawn. Deutsch 
does not consider that his account differs in any essential respect 
from the explanation of Barnes and Land, in spite of the super- 
ficial origin which he assigns to the chambers. Miss O’Keeffe, 
who also worked on Targionia, fully supports Deutsch in his 
statements about the origin of the chambers. Fortunately, the 
youngest chamber which she shows (f. z, A, a) seems to be con- 
clusive; it appears in longitudinal section as a split between two 
superficial cells and does not extend beyond them. 
Miss Black’s paper deals with Riccia Frostii, one of the species 
investigated by Miss Hirsh, and the same conclusions are drawn 
as to the origin of the air chambers. Her f. 6, however, is open to 
the same criticism as Miss Hirsh’s figures. It represents the 
apical region of a thallus cut longitudinally and including five 
young air chambers, but even the youngest of these projects below 
the original surface, showing that a splitting of a cell wall must 
have taken place. Miss Black emphasizes the fact that she ob- 
served no cases in which an intercellular space appeared below the 
surface and then broke through to the outside, so that her con- 
clusion regarding the superficial origin of the chambers seems 
justified. At the same time her figure presents no evidence that 
