180 BOTANICAL GAZETTE [September 



growing points show clearly that its longer axis is the vertical one 

 (fig. 2). 



Very rarely at the forward end of the thallus can only one apical 

 cell be found (figs. 5, 6). Usually two indentations are separated 

 bv a narrow marginal uroiection and each sinus contains two apical 



mam 



cells (figs. 9-1 1). One set carries on the 



other produces a branch. The apical cell of the branch originates 



in a segment of the axial apical cell by a curved vertical wall bent 



the last cutting one. 



that it strikes the wall of the segment 



more 



to the ventral side (fig. 2). The wall-formation in either 

 therefore, shows an obliquity which later is distinctly 1 

 horizontal. The primary segment is divided by a vert 



ma 



one. Walls parallel to the surface come in now, followed by more 

 vertical ones. The order of vertical, transverse, or longitudinal 

 does not seem fixed. Clearly these three directions give thickness, 

 length, and width to the thallus. 



the 



dichotomously 



other 



recognizable as an indentation on the lateral margin, where it may 

 remain growing very slightly if at all; or if the main axis be injured, 

 it becomes the apical region of the main thallus, growing rapidly. 

 Characteristic of the apical cell are the mucilage hairs borne on 

 the ventral surface of the thallus (fig. 12); 6-10 of them curve 

 inward and upward around the growing point. Kny and Leitgeb 

 describe the mucilage cells as bearing no direct relation to the 

 thallus in arrangement. They show an alternation, however, 

 corresponding to that of the apical cell segmentation. The super- 

 ficial cell from which the hair originates first projects from the 

 surface, and then divides into two cells. The basal inner cell 

 retains its plastids permanently; the chloroplasts of the outer one, 

 after some growth, are transformed into a mucilaginous stuff, which 

 stains very deeply. These are the hairs which are sloughed off as 

 the thallus grows. The basal cell divides like any superficial cell. 

 Apparently it is the posterior ventral surface cell, cut from the 



