215 Howe and Underwood: The Genus Riella 



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 two 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, Riella was at first placed 

 with the Ricciaceae, together with SpJiacrocarpiis, 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 Thallocarpiis, they constitute the simplest mem- 

 bers. The genus Riella 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 Riella 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, Riella Gallic a, was reduced by M. Cor- 

 Ibiere 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 R. Paulsenii from Turkestan was 



