90 
Cavers .- — On the Structure and 
cells then grow out horizontally and form a roofing layer (epidermis), 
the cells of which divide only by vertical walls. The epidermis thus forms 
a single layer of cells, a pore being left above each chamber (Plate VI, 
Figs. 17, 18). Each pore is at first surrounded by from five to eight 
cells, each of which undergoes repeated division by walls tangential to 
the pore, giving rise to five or six concentric rings of cells (Plate VI, 
Fig. 10). The dorsal region of the thallus thus comes to be occupied by 
a series of wide air-chambers, forming a single layer and separated from 
each other by vertical partitions, which are for the most part one cell 
in thickness and are united with each other so as to form a network. 
It is of course to these vertical walls between the chambers that the 
areolation of the upper surface of the thallus is due. 
The concentric rows of cells surrounding each pore do not all lie 
in the same plane, the inner rows being higher than the outer, so that 
the pore becomes situated on the summit of a dome-like elevation. 
Immediately around the pore there is a thin but fairly wide membrane 
of cellulose, which shows in surface view a coating of granules ; these 
are readily dissolved in alcohol, and probably consist of resin or wax. 
A similar coating of resinous or waxy grains is found on the cells 
surrounding the barrel-shaped pores of Marchantia , and it was suggested 
by Kny (1890, p. 369), that these serve to prevent the entrance of water 
into the chambers. 
Whilst the chambers have been growing in width the cells forming 
the floor of each chamber grow out and divide by transverse walls, so 
as to form short rows of cells containing abundant large chloroplasts ; 
the latter are also found in the cells forming the floor and sides of the 
chambers, and in much smaller numbers in the epidermis itself. At 
the sides of the chamber the filaments are generally attached both above 
and below, but the central filaments, lying below the wide pore, have 
a peculiar form, the terminal cell of each filament being elongated and 
produced into a narrow tapering point. In each of these pointed cells 
the chloroplasts are confined to the swollen basal portion of the cell, 
the upper portion containing only clear sap with a thin lining layer of 
protoplasm (Plate VI, Fig. ix). The writer has repeated the experiments 
of Kamerling (1897, p. 50), which show that these pointed terminal cells 
have the function of giving off relatively large quantities of water-vapour. 
A plant having its rhizoid-bundles intact is kept for several hours in a weak 
(i per cent.) solution of red prussiate of potash (potassium ferricyanide), 
and afterwards treated with alcohol so as to precipitate the salt. On 
next placing the plant in a solution of ferrous sulphate it is found that 
the blue precipitate is almost entirely confined to the pointed terminal 
cells of the chlorophyll-bearing filaments, the cells below showing hardly 
any precipitate. This clearly shows that the evaporation of water is 
