94 
F. Cavers. 
divides the Marchantiaceae into three sub-orders—Corsinioideae, 
Targionioideae, and Marchantioideae. These, it is now suggested, 
should be raised to the rank of orders. There is no sharp distinction 
between the Ricciaceae and Schiffner’s Marchantiaceae, except in 
the structure of the sporophyte, and even in this respect Tessellina 
shows an approach to a connecting link between the two groups. 
At any rate, it seems hardly justifiable to set the Ricciaceae against 
the whole of the remaining Marchantiales. The genus Monoclea, 
which must now be removed from the Jungermanniales, should in 
the writer’s opinion be placed in an order (Monocleaceae) by itself, 
standing between the Corsiniaceae and the Targioniaceae. With 
the Marchantiaceae in the restricted sense (corresponding to 
Schiffner’s Marchantioideae), we thus get five orders of Marchantiales. 
Ricciaceae. 
In most species of Riccia the upper zone of the thallus consists 
of narrow vertical air-canals, each bounded by four of the filaments 
produced by outgrowth and repeated transverse division of a super¬ 
ficial cell. The terminal cell of each filament is usually larger than 
the rest, forming with the corresponding cells of the neighbouring 
filaments a sort of epidermis of hyaline cells containing few or no 
chloroplasts (Fig. 4). In many species, each of these epidermal 
cells becomes divided into an upper anil a lower cell, and the former 
eventually dries up and collapses, leaving only the lower part of its 
