OF ARTS AND SCIENCES. 35 



stage there is no visible sign of ascogonium or trichogyne, or indeed of 

 anything which might indicate in the least degree any sexual origin of 

 the apotheciura. Tlie apothecium originates rather above than below 

 the median line of the section, so that there exist beneath it consider- 

 able numbers of gonidia. As the apotheciura grows, it encroaches 

 more and more, not only on the overlying tissue, but also on that 

 below, the result being that the algie occupying the latter are pressed 

 together downward, and finally form a dense compact layer beneath 

 the apothecium. If we accept the view that the presence of the algae 

 may induce a more vigorous growth in the hyphas, this fact of the ac- 

 cumulation of the algas may account for the origin of the cortex which 

 is always found covering the lower surface immediately beneath the 

 mature apotheciura. There is one point in the development of the 

 apothecium to which attention must be drawn. The young paraphy- 

 ees, arising very early as perpendicular branches of the ui)per part of 

 the primordium, attain their full length before the cortex is ruptured. 

 The subsequent growth of the hymenium is entirely in area, and by this 

 means the cortex is soon ruptured ; but whereas in the other cases ex- 

 amined the Jater growth of the paraphyses and subhymenial tissue 

 raises the hymenium considerably above the surface, in Heppia the 

 upward growth has ceased by the time the cortex is ruptured, and the 

 apothecium remains sunk in the thallus and separated from the thalline 

 tissue only by a thin layer, one or two cells in thickness, formed of the 

 compressed thalline hyphge. Not until the hymenium is nearly or quite 

 exposed do the asci appear, rising from the dense subhymenial tissue 

 and pushing up between the paraphyses. Meanwhile the treatment 

 followed in other cases fails to show the presence of any differentiated 

 cells in the subhyraenial layer. I have been unable to see any trace 

 of a trichogyne, and maceration after treatment with potassic hydrate 

 shows a relation existing between the asci and paraphyses identical 

 with that seen in the other genera studied. (Plate IV. Figs. 23-25.) 



Pannaeia molybdea, (Pers.) Tuck. 



In this lichen we find a very peculiar thalline structure. The lower 

 part of the thallus is formed of a dense layer of hypha^ running parallel 

 with the substratum, or more or less obliquely to it. This layer is 

 continuous around the edge of the thallus, and forms upon the upper 

 surface a layer which is thinner and more strikingly parenchymatous 

 than the layer forming the lower surface. There is thus left a space 

 between these two parallel layers which is filled by a much looser 

 tissue of hypbae arising from the lower layer and growing upwards to 



