36 PROCEEDINGS OF THE AMERICAN ACADEMY 



become merged in the upper layer. The position of the algae is in 

 conformity with this peculiar structure. Scattered very sparsely 

 through the dense lower layer, and corresponding in their course to 

 that of the hyphae in which they are embedded, are seen long filaments 

 of bluish-green cells. In the loose tissue forming the central part of 

 the thallus these filaments are shorter, and the cells composing them 

 are considerably larger, than those of the scattered filaments below, 

 (since the latter are decidedly compressed by the density of the sur- 

 rounding tissue,) and their general position is perpendicular to the 

 surface. This structure of the thallus must be fully understood in order 

 to enable us rightly to interpret the stages in the development of the 

 fruit. The spermogonia arise in the lower part of the loose central 

 tissue, and present in respect to their origin and development no pecu- 

 liarities worthy of notice. The origin of the apothecia is more 

 peculiar, and more difficult to follow. The first step takes place in 

 the form of an active branching, in a jjurely vegetative manner, of the 

 hyphas composing the upper part of the dense layer of parallel hyphae 

 already noted as bounding the thallus on its lower surface. This ten- 

 dency to increased activity in growth, starting at one point, is trans- 

 mitted to the loose tissue above, in which the algas are embedded. 

 These algae (seen by crushing the section to be filaments of Scytonema 

 even less altered than in Heppia) are, with few exceptions, pushed 

 aside, and the uppermost layer of parallel hyphas begins to take part 

 in this active growth. But whereas in the deeper layers the area of 

 growth is equally bounded on all sides by the thalline hyphre, and has 

 therefore no tendency to increase in one direction more than in an- 

 other, the hyphae forming the upper layers, when stimulated by the 

 growth below, can grow freely upwards. Such a growth takes place, 

 and the iso-diametric cells composing this layer begin to increase in 

 length in a direction perpendicular to the surface. Thus these cells 

 which here take the place of a true cortex are themselves transformed 

 into a layer of upward-growing hyphae, and, as we shall see, are to 

 be considered as forming a subhymenial layer. (Plate II. Fig. 12.) 

 We find accordingly the appearance of an inverted cone, the base com- 

 posed of these metamorphosed superficial cells rising to a height above 

 the normal surface of about 0.05 mm., the apex resting upon the dense 

 layer forming the lower portion of the thallus. This condition of 

 things is quite different from that seen before, where an existing cortex 

 was ruptured by a hymenial disk arising beneath it. The parallel 

 hyphre forming the subhymenial layer upon the surface now begin 

 to branch copiously, the branches arising parallel with the hyphae 



