566 Transactions of the Society. 



principal walls being flat and parallel to each other, and the outer 

 wall cylindrical. 



In the more highly developed forms, the apical cell is smaller 

 than in the simpler species, the usual shape, as seen from above, 

 being that of a transverse section of a bi-convex lens ; sometimes 

 the two faces are equally curved, as in Bhodi/menia laciniata ; 

 generally the anterior wall is more convex than the posterior. 

 The segment is watch glass-shaped with the concave wall next the 

 base of the apical cell ; this segment by subdivision gives origin to 

 the mass of cells forming the thallus. The mode of segmentation 

 characteristic of vascular cryptogams, in which several daughter- 

 cells of equal value are simultaneously cut off from the apical cell, 

 does not occur in this group, although when growth is very active 

 the segment is so soon cut up, that its components present the 

 appearance of having been directly cut off from the apical cell ; 

 but later in the season, when cell-development is somewhat retarded, 

 the segment can be seen intact. In all cases when the thallus is 

 composed of more than single rows of cells, the segment first 

 divides into an axial cell, surrounded by a varying number of 

 pericentral cells ; these last, owing to the watchglass shape of the 

 segment, stand at a higher level than the apical cell, which thus 

 becomes buried in the surrounding tissue, consequently the organic 

 apex or growing point is much below the geometric apex of the 

 thallus. The species of Chondriofsis and Laurencia illustrate this 

 mode of growth, which also occurs in some monosiphonous genera 

 as Batrachospermum aadHalurus, where the last whorl of branches, 

 which are lateral extensions of the segment, arch over the apical 

 cell. CaUithamnion roseum and G. polyspermum present the 

 peculiarity of having two distinct methods of segmentation of the 

 apical cell, which in the main axis is cut into two daughter-cells by 

 a septum inclined at an angle of 45° to the axis of growth ; the 

 septa are all in the same plane, but slope alternately to right and 

 left, so that the cells just below the growing point are more or less 

 triangular in shape, and the septa form a zigzag line ; as the cells 

 increase in size, the triangular form is lost, and at some distance 

 behind the apex tbey are cylindrical and the septa transverse. In 

 all the branches the segments are cut off by septa, which are from 

 the first at right angles to the axis of growth. 



Branches originate either by lateral budding or by division of 

 the apical cell. The first method is most general, the branches 

 showing as minute protuberances from the segment, as in Ptilota 

 elegans and Cystoclonium pmjMrascens, or more frequently from 

 a cell further back, as in Ahnjeldtia plicata and Plocamium cocci- 

 nenm. All species with a flattened thallus appear to branch by this 

 method. According to Sachs* the lateral branches show as promi- 

 * ' Text-Book of Botany,' second English ed, 1882, p. 140, fig. 108. 



