INTRODUCTION TO CRYPTOGAMIC BOTANY. 881 



of the medullary mass, whether simply presenting a cellular 

 mass, or more or less decidedly filamentous, or furnished with 

 hair-like simple or branched processes. In proportion as the 

 supporting cellular tissue of the hymenium makes its way 

 through the outer coat, more or less perfectly, the shields are 

 bordered by the thallus as in Lecanora, or by their own basal 

 substance, as in Lecidea. The hymenium consists of dis- 

 tinct asci, as in Pyrenomycetous fungi, accompanied by articu- 

 lated and often branched paraphyses. The application of iodine, 

 even without sulphuric acid, produces a deep blue in the asci and 

 paraphyses ; the terminal portion of the latter, however, which 

 is altered in chemical composition by constant exposure to 

 the weather, is not affected. The asci and their paraphyses 

 are so closely connected by a firm gelatinous substance that it 

 is often very difficult, without the assistance of some acid, to 

 separate them from one another, so as to obtain a clear view 

 under the microscope. Mere compression is not always sufii- 

 cient, and very strong compression is seldom advisable in 

 microscopical observations. The contents of the spores become 

 brown on the application of iodine, while their epispore is un- 

 affected, and does not strike blue on the application of sul- 

 phuric acid, which does not affect it, except highly concen- 

 trated. Though the sporidia are often far more complicated 

 in structure than the sporidia of Fungi, their epispore is usually 

 smooth, except in such cases as Solorina saccata, Thelotrema 

 exanthematicum, and Biatora marginiflexa (Fig. 79, h), 

 which latter has enormous sporidia, attaining a length of -^ J o 

 of an inch. In Pertusaria Wulfenii, though smooth when 

 expelled from the ascus, they soon become minutely granulated ; 

 but these granules are merely the commencement of a peculiar 

 germination. 



422. Lichens are essentially the creatures of light. They 

 cannot, of course, exist without periodic moisture ; but they are 

 capable of enduring long drought, accompanied by heat, and 

 are not impatient of considerable degrees of cold. The same 

 species, therefore, which will pass through all the phases of 

 vegetation, up to the perfection of its fruit when properly 

 exposed, will in a less favourable situation perfect only its frond, 



