62 



POPULAR HISTORY OE LICHENS. 



perithecium, as in Verrucaria. In regard to its mode of 

 fixture to the thallus, the apothecium may be sessile, when 

 adnate or immediately adherent to, and seated on, the sur- 

 face of the thallus; partially innate when its base is im- 

 mersed in the superficial thalline tissues; pedicellate or 

 stipitate when fixed on the apex of a stalk-like projection, 

 as in Calicmm. In respect to its position on the thallus — in 

 foliaceous species it may be central, as in many Parmelias, 

 or marginal, as in PeUigera ; while in fruticulose species it 

 is either lateral, as in many Ramalinas, or terminal, as in 

 Spkserop/wron and Stereocaulon. Abnormal or abortive states 

 of the thalamium or exciple, but particularly of the latter, 

 frequently give rise to great irregularities in the characters 

 of the apothecium in the same species. The scutellate apo- 

 thecium sometimes becomes patellate from the border of the 

 exciple being so reflexed as to allow the thalamium to cover 

 and conceal it, as in Lecanora ventosa ; and on the other hand 

 the patellate may become scutellate by an inflexed condition 

 of the margins of the thalamium, and the assumption, real or 

 apparent, of a thalline exciple. Some apothecia in the young 

 state are covered or veiled by an extension of the cortical 

 thalline layer or by a delicate dehiscent membrane, as in Pel- 

 tig era. The rudimentary apothecium appears as a globular 



