OF ARTS AND SCIENCES. 23 



of emberldii)g in paraffine seems ill adapted to the lichen-tissue. In 

 those lichens which are invested with a tough cortex, it is wellnigh 

 impossible to secure the requisite degree of infiltration with the re- 

 agents usually employed, and the attempt to use the microtome re- 

 veals the fact that the paralfiue has penetrated very slightly, if at all, 

 into the inner tissue of the thallus, while the gelatinous lichens are, 

 from their very nature, liable to much distortion during the process of 

 embedding. 



To ascertain the mutual relationship existing between the asci and 

 paraphyses, I have again had recourse to thin sections of young apo- 

 thecia cut by hand, selecting for that purpose apothecia in which the 

 young asci were just visible among the more fully developed para- 

 physes. By treatment of such sections with various clearing and stain- 

 ing reagents, and subsequently crushing them under the cover glass, 

 the origin of both asci and paraphyses may be readily seen. 



Sticta anthraspis, Ach. 



The medulla of this lichen consists of a loosely interwoven layer of 

 comparatively thick-walled, sparingly septate, hyaline hyphaj. Owing 

 to the slight degree of complexity in the tissue, the generally regular 

 course and the large size of these hyphoj, they may be followed sepa- 

 rately in their course, often for a long distance, especially after treat- 

 ment with alcohol to expel all air from the section, and subsequently 

 with dilute potassic hydrate. The youngest stages of the apothecia 

 can thus be made out with considerable accuracy. The surface of this 

 lichen presents a peculiar pitted appearance, due to an irregular net- 

 work of raised veins or ribs, which in section present the normal thalline 

 structure. It is upon these ribs almost exclusively that the sper- 

 mogonia and apothecia are borne. The former appear to the naked 

 eye as slight elevations on the ribs, pierced in the centre by a minute 

 dark brown pore. They arise in the upper part of the medullary 

 layer as small spherical knots of hyphoe. As one of these knots in- 

 creases in size, it becomes more and more dense, and soon reaches the 

 outlying groups of the algal layer. Hitherto the structure of the knot 

 has been more or less clear ; now, however, it becomes very dense, and 

 from spherical becomes ovoid, the pointed end being directed upwards. 

 As it grows, it pushes aside the groups of algae composing the gonidial 

 zone, crowding them together on all sides, and frequently pushing 

 them well back into the medullary layer. As soon as the apex reaches 

 the thick parenchymatous cortex at one point, the cells of the latter at 

 this point apparently increase in size, this appearance being due to the 



