177 



a thin covering of much-branched rather small-celled hyphae, a tissue 

 comparable to the endoderm or root-sheath of higher plants. 



The prevailing color of the thallus is light green to grayish green, 

 sometimes changing to a reddish brown. In a few species, es- 

 pecially U. barbata,\ho. main branches are covered by minute second- 

 ary thalli which never bear apothecia ; these are for the purpose of 

 increasing the assimilating surface and are functionally comparable 

 to the leaves of higher plants. Soredia sometimes occur. 



As a rule the apothecia are few ; sometimes, however, individuals 

 occur on which they are numerous ( U. barbatd} ; they are large 

 and usually terminal. The disk is much flattened and thin, the 

 margin bearing numerous small thalloid branches and sometimes 

 somewhat torn ; the epithecium is of about the same color as the 

 thallus ; thecium and hypothecium are colorless. The spores are 

 simple, colorless, without any special characters. 



The Usncas are somewhat northern in their distribution ; U. bar- 

 bata seems to be quite cosmopolitan ; they occur most frequently upon 

 trees. 



PLATE 50. 



USNEA BARBATA (L.) Fr. 



1. Small portion of thallus natural size. 



2. Section of apothecium. 



3. Radial longitudinal section of thallus. 



4. Paraphyses and spore-sac. 



5. Spores. 



Family 7. VERRUCARIACEAE. 



This family is quite definitely limited by its apothecial characters* 

 The apothecium is more or less spherical and disk closed. In the 

 lower forms ( Trypethelium, Pyrenula] the apothecia are partly or 

 almost entirely hvpophloeodal. In the higher forms (Derma tocar- 

 pon, Endocarpoii) the apothecia are immersed in the thallus. In 

 the intermediate forms (Conotrema, Thclotrcnia, I'cn-itcariu} they 

 are sessile upon the substratum and partially enclosed by the rudi- 

 mentary thallus. In the non-foliose forms the thallus is mostly hv- 

 pophloeodal. The paraphyses have a tendency to become gelatin- 

 ized, especially in the higher forms ( Vcrnicaria^ Dcrniatocarpou, 

 Endocarpon^). In the lower forms the paraphyses are long, slender 

 and very soft or partially gelatinized. Older lichenologists have 



