264 



layers of cells, not including the cortical cells, and further transversal divisions fre- 

 quently do not occur except in the immediate vicinity of the conceptacles; it may 

 even happen that the frond is monoslromatic in almost its whole extent. The long 

 cells in the upper layers are always connected with transversal pits (fig. 184). 

 The thickness of the frond is rather variable. Monostromatic fronds were 25—42// 

 thick, fronds consisting of two layers of cells 67 — 105 // and fronds containing three 

 layers were 91 — 123/;! thick. The fronds are frequently growing over each other. 

 It also frequently happens that new growing edges are produced from certain parts 

 of the frond, growing over the neighbouring parts the growth of which has ceased. 

 The long cells contain a small nucleus in the upper part of the cell, and a number 



of small chromatophores spread in 

 the cell. The cortical cells are pro- 

 duced early, immediately after the for- 

 mation of the long cells by the divi- 

 sion of the marginal cell. But at some 

 distance from the margin a new cor- 

 tical cell may be cut off under the 

 primary one by a horizontal or in- 

 clined wall (fig. 184 B), and this pro- 

 cess may be repeated several times. 

 Hyaline hairs may be produced from 

 long cells seemingly not different from 

 the others, and provided, like these, 

 with a cortical cell (fig. 184 B). The 

 length of the long cells of the frond 

 varies greatly; when the crust is po- 

 lystromatic, the cells of the under- 

 most layer are often rather short. When these cells or those of the monostromatic 

 frond are long, their undermost part is usually more inclined than their upper part 

 (fig. 184 B, comp. Nichols, 1. c. fig. 12, 15). 



The sporangial conceptacles are very prominent, conical with rounded or ap- 

 planated top, 300—500 /^ in diameter. Under the conceptacle 1—3 layers of sterile 

 cells are present. Papillae projecting inward and upward, lining the pore, as de- 

 scribed and figured by Nichols, may be found in the under part of the pore, but 

 they are usually slightly developed. Seen from above, the superficial cells sur- 

 rounding the pore appear scarcely different from the others, the nearest being only 

 a little smaller (fig. 185 B). The sporangia are only placed in the peripheral part 

 of the conceptacle, the central part being occupied by sterile cells forming a conical 

 columella. Nichols found also sporangia in the central part, though less numerous 

 there than at the periphery. As shown by this author, each sporangium is born 

 by a stalk cell. A "plug" was found in some rare cases in the ostiole, forming a 

 continuation of the central sterile cells (fig. 185 A), but it seems to be usually wanting, 



Fi^ 184. 



Lithophijlluin maci ocarpiini {. iiitei media. Vertical sections of 

 fronds. A, margin of frond. B, part of monostromatic frond 

 showing a hair-cell and two cortical cells cut ofT from one cell. 

 C, part of thicker crust; transversal pits between the cells 

 of the two upper layers. 350 : 1 



