14 LICHEN FLORA OF THE UNITED STATES 



The Proper Exciple. As the name indicates, the proper exciple is in reality 

 a portion of the ascocarp, being merely an upward extension of the hypothecium 

 and, like it, either plectenchymatous in structure or composed of closely inter- 

 woven hyphae. Both hyphal and plectenchymatous areas are found in a thalloid 

 exciple, so that it is not always easy to distinguish between the two kinds of 

 exciples by microscopic structure alone. The fundamental distinction is that if an 

 exciple can be traced to the hypothecium it is to be regarded as a proper exciple, 

 and if it can be traced to the thallus it is thalloid. But an evanescent thallus 

 may disappear and a thalloid exciple still be present. In such instances the algal 

 cells are usually present and the structure, further, does not appear to be continu- 

 ous with the hypothecium. The hypothecium is always present, and the proper 

 exciple may always be traced back to it in vertical sections through the center 

 of the ascocarp. Some authors seem to consider any exciple that does not contain 

 algal cells a proper exciple, but such a disposition disregards morphological rela- 

 tionships and leads to confusion in the study of species. The proper exciple may 

 be pale in section or varying to a black color. It may be permanent or evanescent, 

 and thin to very thick and conspicuous. In stating that certain exciples become 

 covered by the growing hymenium, it is by no means certain that some of these 

 exciples do not disappear instead of becoming covered. Again, they seem some- 

 times to become covered and then to disappear. The exciple is a protective cover- 

 ing about the hymenium and is often produced into a perithecial wall, which com- 

 pletely surrounds the hymenium, except for the ostiole at the summit. The proper 

 exciple also aids in the dispersion of spores. 



The Hypothecium. This area lies immediately below the hymenium and 

 varies considerably both in thickness and structure. Sometimes it exceeds the 

 hymenium in thickness, but in most instances it is considerably thinner in vertical 

 section. In structure it may be composed entirely of interwoven hyphae or en- 

 tirely of plectenchyma similar to that of a plectenchymatous cortex, or it may be 

 partly plectenchymatous and partly hyphal. In some of the higher lichens, even 

 when the structure is hyphal throughout, the hypothecium is more or less plainly 

 differentiated into two layers, with the hyphae extending in a generally vertical 

 direction in the upper layer and more nearly horizontally in the lower layer. The 

 cells and hyphae of the hypothecium are smaller than the similar structures of 

 the thallus, and the walls are more inclined to become gelatinized so that the 

 structure is obscured; and the same statement applies to the proper exciple and 

 the perithecial wall, which, when present, are continuations of the hypothecium. 

 The color of the hypothecium, like that of the exciple, varies from the palest 

 shades to black, and likewise the sections may appear perfectly hyaline. Both 

 exciple and hypothecium often become darker with age, so that there may be a 

 considerable amount of variation in color in the same species. The hypothecium 

 has not been carefully studied in the pyrenomycetous lichens. Sometimes the 

 perithecial wall is open below, and sometimes it is entire. Data on this point 

 would have been valuable, and yet it seems that it has been possible to distinguish 

 the species in most instances without taking this feature into account. 



The Amphithecium. In those lichens in which the proper exciple is pro- 

 duced into a perithecial wall, there is sometimes a dark layer outside and a lighter 

 and often hyaline layer within between the dark outer covering and the hymenium. 

 This inner layer is known as the amphithecium. In some pyrenocarpic lichens, in 

 which the fruits are immersed in the thallus, as in Dermatocarpon, the dark outer 

 protective layer is not needed, and the whole of the tissue surrounding the hyme- 

 nium is hyaline or colorless, but it is also known as an amphithecium. 



