HoltZ : OBSERVATIONS ON PELVETIA. 



31 



The cortical part of the holdfast passes without any marked 

 change into that of the stipe. The epidermal cells however are 

 not elongated as much radially as those of the other parts of the 

 surface of the plant. It is covered with a cuticle, thicker than 

 on the stipe or lamina. 



Cross-sections of the central part of the holdfast show (Fig. 

 8) that the vertical rows of cells seen on vertical section are not 

 disposed in any regular order. The intercellular substance is 

 not nearly as abundant as in the stipe. Toward the margin of 

 the holdfast the cells show power of dividing. Here we find, 

 interspersed with cross-sections of the vertical rows, sections 

 through cell rows slanting up toward the axis of the plant. Still 

 nearer the outside we come upon the meristematic zone. Here 

 are principally slanting rows of cells dividing dichotomously in 

 the radial direction. These divisions repeat the dichotomy, 

 running directly to the surface. 



All the living cells in the holdfast have chromatophores. 

 The central cells contain but few grains, the cortical are crowded 

 with them. 



Stipe. But little need be added here to what has been said 

 under tissues in general. The young stipe has a nearly cylin- 

 drical structure, with a slight notch on the end where a growing 

 point is situated. No differentiation is noticeable between stipe 

 and lamina. Older stipes become flattened, partly on account 

 of the flattening of the cells parallel with the longer axis of the 

 cross-section, but more on account of the greater growth toward 

 the thin margins. A cross-section of an older stipe shows two 

 principal planes of fission by the arrangement of the cells in rows 

 parallel with the major axis of the section and the other obliquely 

 across this axis. This is especially noticeable in the pith. The 

 cortical cells show a distinctly concentric arrangement (Fig. /). 



The only differentiation seen in cross-section is that the pith 

 and inner rortex cells near the ends of the ellipse are somewhat 

 larger than those of the central part. This differentiation how- 

 ever does not even suggest a midrib. Longitudinal sections of 

 the stipe, cut parallel to the flat surface, show a similar appear- 

 ance, except that the typical pith cells are reached sooner in 

 passing from the surface along the minor axis. The appear- 

 ance of the cells in both cross and longitudinal sections has been 

 discussed under tissues in general. 



Lamina. The general tissues of the stipe and lamina are so 



