108 Lillian O'Keeffe. 
and by Deutsch for Targionia. On plasmolysing hand-sections of 
fresh material, however, it was found that as the irrigating salt 
solution reached the section, the cells on shrinking, separated and 
a flask-like cavity was seen (Fig. 1, C); hence the crack between the 
cell-walls was evidentlycontinuous, though owing to their turgescence 
the cells remained in contact at the middle. At a slightly later 
stage, a little further behind the apical cell, the young chamber is 
flask-like and open to the surface (Fig. 1, A, b, c ), but it soon becomes 
closed again in the following manner. The epidermal cells grow 
upwards and towards each other, and each undergoes division by an 
oblique wall (Fig. 1, A, d), cutting off a cell which contributes to the 
formation of the dome-like roof of the chamber and these roofing 
cells come into contact so as to close the chamber (Fig. 1, A, e). 
That this closure is due to the turgor of these primary roof-cells is 
easily shown by plasmolysing fresh sections, when the roof-cells 
separate and the chamber becomes open'again to the surface. This 
closed phase may last for a considerable time (Fig. 1, D, E). For 
instance, perfectly closed chambers were seen in which the diameter 
of the roof consisted of as many as twelve cells (as seen in median 
longitudinal section of thallus)—horizontal (surface) sections were, 
of course, also examined and showed the same. 
The comparison of longitudinal and horizontal sections showed 
that the re-opening of the chamber takes place almost simultaneously 
with the earlier of the divisions which give rise to the concentric 
rings of cells which surround the pore, though in some cases the 
first of these divisions occurred while the pore was still closed. 
After a time, however, the pore becomes open, evidently owing to 
the lateral expansion of the chamber as the thallus grows in length 
and width, and the failure of the roof-cells to keep pace with this 
expansion. The innermost ring is, as stated by Deutsch, composed 
of dead cells, the contents of which become lost at a fairly early 
stage while the walls, especially on the free portion of the cell, 
become thickened and cutinised, this free portion growing inwards 
towards the centre of the pore and the cavity of the cell here 
becoming obliterated. The pore becomes widened with the surface 
growth of the thallus, and in the older parts of the thallus the 
pores gape widely and are elliptical in outline, as described by Miss 
Maybrook (10) in the case of Fegatella. The assimilating filaments, 
compact tissue and ventral scales have been sufficiently described 
by Cavers and no additional features of interest were observed in 
connexion with these. 
