Structure & Development of Targionia hypophylla. 107 
the t middle one when three equally long were present— was 
invariably wider than the others and was evidently the single apical 
cell, while the cells on either side were its youngest segments (Pig. 
1, A, B). 
According to Leitgeb the air-chambers in the thallus of the 
Marchantiales appear as depressions in the surface caused by definite 
points, typically situated where four superficial cells meet, lagging 
in growth and so becoming overgrown by the adjacent cells, the 
young chamber thus arising in the same manner as the pit in which 
the sexual organs (both kinds in Riccia, the antheridia in higher 
Marchantiales) become embedded, while the roofing-in of the 
chamber was attributed to the growth and division of the upper cells 
of the lamellae thus formed. As shown by Barnes and Land (1), 
however, the chambers arise by the splitting apart of the superficial 
cells, in essentially the same way as the schizogenous intercellular 
spaces in higher plants. Deutsch confirms this account for 
Targionia, but states that while in the other Marchantiales described 
this splitting originates in an angle between the epidermal and 
hypodermal layers of cells, and proceeds outward toward the surface, 
in Targionia the process is reversed, the cracking apart starting at 
the surface between two epidermal cells and proceeding inward, 
subsequent divisions enlarging the space thus formed as well as the 
breadth of the roof. 
The careful examination of sections taken in different planes 
shows that the chambers of Targionia arise in the way described 
briefly by Deutsch, but some interesting additional points were 
brought out. The first indications of the chambers were found at 
a very short distance behind the apical cell (Fig. 1, A), in which 
respect Targionia more closely resembles Riccia than the other 
Marchantiales figured by Barnes and Land, the first signs of splitting 
between the superficial cells being in fact detected in some cases 
only three]cells]behind the apex. The cleft begins at the surface 
but after it has proceeded inwards to about half the depth of the two 
cells (as seen in median longitudinal section of thallus) its progress 
seems to be arrested and a cavity is then formed by splitting of the 
walls between the epidermal and hypodermal cells (Pig. 1, A, a). 
Prom the regularity with which this appearance was presented in the 
early stages it was at first thought that the chambers in Targionia 
arose by the almost simultaneous occurrence of the two methods 
of cleavage (from without inwards and from within outwards) 
described respectively by Barnes and Land for various Marchantiales 
