254 



stained by hcematoxyline (fig. 174 A). Trichocytes may occur. The greater part of 

 the crust is polystromatic, being composed of vertical cell-rows, up to 8 cells high 

 or more. The older parts of the crusts are very irregular, being composed of several 

 smaller crusts or lobes growing partly over each other. These partial fronds or 



lobes are usually polystro- 

 matic to the very margin. 

 The compound crusts may 

 1)6 composed of lobes of 

 the same frond or of dif- 

 ferent fronds growing to- 

 gether; it is therefore im- 

 possible to state the dia- 

 meter of the single frond. 

 In the thicker fronds, the 

 upper ordinary frond-cell 

 may bear more than one 

 cortical cell, frequently one 

 over the other (fig. 174 A, D). 

 In the last-named figure the 

 three undermost cells at 

 right bear each a cortical 

 cell, the explanation of 

 which must be that they 

 represent the monostroma- 

 tic border of the frond, 

 which has been overgrown 

 by tissue produced by the 

 neighbouring thicker part 

 of the same frond. A si- 

 milar process often takes 

 place in various parts of 

 the frond and gives rise to 

 the complicated structure 

 of the old crusts, new lobes 

 developing from certain 

 parts of the frond, and growing over the neighbouring parts. — The length of the 

 cells of the vertical cell-rows is highly variable, usually one to three times as long 

 as the breadth, and irregularly varying in the same filament. The undermost cell- 

 layer is not distinct from the others, its cells being of variable height. The cells 

 contain often numerous starch-grains. Transversal pores between the cells of difFerent 

 cell-filaments never occur, but transversal fusions are frequently met with, between 

 the cells of the basal layer and between cells of the upper parts of the vertical 

 cell-rows as well (figs. 174, 175). 



Fig. 174. 



Melohesia Iricliostoma. A, vertical section of the monostromatic part of a 

 frond growing over tlie conceptacle of another frond of the same .species. 

 B, rrionostromatic frond seen from above. C, vertical section of older, com- 

 pound crust. D, vertical section of a thick lobe of frond. B, vertical section 

 of sporangial conceptacle. C, 65: 1, the rest 350: 1. 



