98 HOFMEISTER, ON 



ser., t. xviii, p. Ill ; Duriena, 1. c, t. i, p. 228 ; t. ii, p. 50) 

 is the most remarkable, on account of its very peculiar 

 habit. The Algerian Riella (Duriena) helicophylla is the 

 most striking of all ; its upright leaf, which is three inches 

 high, and shaped like a winding staircase, being one of 

 the most wonderful of vegetable forms. My investiga- 

 tions of this remarkable genus were made on a species 

 found by Reuter at Geneva, which represents in miniature 

 the vegetative phenomena of the North African species. I 

 am indebted for the materials for my work to the kindness 

 of the discoverer, who sent me numerous living specimens. 



Young individuals, whether produced from spores or 

 adventitious shoots (PL XIV, figs. 1 — 4), are formed of 

 short rows of cells, which pass at the fore end into a small, 

 cellular surface. The arrangement of the cells is that 

 which is common to the Ricciese and the Marchantiese, 

 viz., in pairs and flabelliform, originating from two cells of 

 the first degree, which are divided alternately by transverse 

 and longitudinal septa. In the young state of the plant 

 there is an excess of formation of transverse septa, nearly 

 at right angles to its median line, and consequently of 

 longitudinal growth. At an early period the multiplica- 

 tion and expansion of the cells of one side of the fore edge 

 considerably exceed that of the other side, so that the 

 punctual vegetationis of the young Riella is turned on one 

 side (PL XIV, fig. 2). 



Contemporaneously with the appearance of the first 

 leaves, the plant develops a mid-rib, by the production in 

 certain cells of septa parallel to its surfaces ; this mid-rib 

 is a strip of massive cellular tissue, consisting sometimes, 

 of as many as six layers of cells, which runs along the less 

 highly developed side of the shoot. The rib forms one 

 margin of the flat stem, which may be compared to a stem- 

 joint of Marchantia, of which the membranous left-hand 

 wing has been removed. The helicoid winding of the 

 stem is produced by the lateral twist which takes place in 

 the axis as it grows obliquely upwards, and which is caused 

 by the more rapid development of the left-hand side wing. 

 The twist is always to the right. 



Leaves are formed only on the mid-rib. The fraction %\ 



