OP ARTS AND SCIENCES. 25 



diately under the cortex, the disk has no time to enhxrge sufficiently to 

 rupture the cortex, as seen in the first case, but the paraphyses first 

 formed pierce it at one point, and, acting like a protruding cone, 

 force ajxirt its tissue. In either case, the expansion and upward 

 growth of the hymenium soon cause it to appear on the surface of 

 the thallus in its normal disk-like form, the elevated border or exciple 

 being formed partially by the marginal paraphyses, but almost entirely 

 by the cortex, which by the elongation of its cells in the direction of 

 the course of the hypha?, i. e. perpendicular to the surface, becomes 

 much thickened. 



The algas composing the gouidial layer, when freed from the thallus 

 by maceration, are seen to exhibit the bluish-green color of the Cya- 

 nophyceaj, and form almost unaltered colonies of Chroococcus, Na?g. 

 The density of the spherical mass of hyphce forming the young apo- 

 thecium excludes the alga? from it, and with the growth of the former, 

 and the subsequent rupture of the cortex, the algfe are forced aside, so 

 that the mature apothecium consists only of a dense subhymenial layer 

 arising from and resting upou the normal medullary hyphas, and giving 

 rise above to the hymenium, the whole being surrounded by the up- 

 turned and thickened cortex. It occasionally happens that here and 

 there an algal colony is enclosed and carried up in the lower tissue of 

 the apotheciuna ; but the conditions are unfavorable to its growth and 

 multiplication, it soon dies, the contents of the cells disintegrate, and 

 the shrunken hyaline membranes alone remain. Not until the young 

 apothecium has burst through the cortex, and has attained a diameter 

 of almost or quite 1 mm., do the asci appear. They can then be dis- 

 tinguished by treatment with iodine, and are seen to be scattered very 

 sparingly through the whole hymenium. The iodine brings out sev- 

 eral important features. First, the asci alone are colored, and the blue 

 color does not extend appreciably into the subhymenial tissue. The 

 ascogenous cells then, if they exist, do not partake of the chemical 

 nature of the ascus membranes as exhibited by a blue coloration with 

 iodine. Secondly, the whole subhymenial layer is colored a deeper 

 brown than the surrounding tissue, and this coloration is homogene- 

 ous throughout the layer. The ascogenous cells then, if they exist, 

 do not differ materially in size or in their reaction with iodine from 

 the cells which give rise to the paraphyses. These apparent facts are 

 borne out by further investigation. If a thin median section through 

 a young apothecium in which asci are just beginning to appear, be 

 treated first with alcohol to expel all trace of air, and then with dilute 

 potassic hydrate, it becomes very transparent, and a dilute tincture of 



