Hower and UNnperwoop: THE Genus RIELLA 215 
Riella is not apical in the ordinary sense of the word, but inter- 
calary. The whole tissue of the young shoot appears to be meri- 
stematic at first and of one layer of cells. Later, the growth ac- 
tivities are concentrated at either one or two points on the margin of 
the shoot intermediate between its distal and proximal extremities. 
If at vo points, these are situated on opposite margins of the shoot. 
The new cells formed on the upper or distal side of the growing 
point now contribute to the growth of the unistratose wing, which 
is dorsal in position. The new cells formed on the lower or prox- 
imal side of the growing point go to constitute the multistratose 
stem, which is ventral in position. If two growing points on oppo- 
site margins of a young shoot persist, a double or twin plant is 
formed, the two branches of the axis bearing a single continuous 
dorsal wing. If, however, only one growing point persists, the 
plant or branch is apparently one-sided, with stem on one side and 
wing on the other, though in reality the wing is dorsal and the 
stem ventral. 
On account of the absence of elaters, Aze//a was at first placed 
with the Ricciaceae, together with Sphaerocarpus, in which, like- 
wise, elaters are not developed. <A better understanding of the 
morphology of these two genera has led to placing them in the 
order Jungermanniales, of which, together with the exclusively 
American genus Thallocarpus, they constitute the simplest mem- 
bers. The genus Rie//a forms the subfamily Rielloideae and in 
the usual arrangement stands between the Sphaerocarpoideae and 
Metzgerioideae in the family Metzgeriaceae (the Jungermanniaceae 
anakrogynae of Leitgeb and Schiffner). 
The geographical distribution of this strongly characterized 
genus Riel/a is of interest. Up to within a few months ago, the 
genus was supposed, as far as the literature on the subject is con- 
cerned, to be confined to the Mediterranean drainage basin of 
Africa and Europe, with seven species, as commonly recognized. 
One of these, however, Ried/a Gallica, was reduced by M. Cor- 
biére in the last number of the Revue Bryologique for 1902 to 
forma Gallica of the Algerian R. Battandieri, To these seven 
known species, or six, as now conceived, another from a region 
far removed was added by Morten P. Porsild in a recent number 
of the Botanisk Tidsskrift, where A. Paw/seni from Turkestan was 
