B. SPECIAL STRENGTHENING STRUCTURES 103 _a._ Sclerotic strands _b._ Chondroid axis C. SURVEY OF MECHANICAL TISSUES 105 D. RETICULATE FRONDS 106 E. ROOTING BASE IN FRUTICOSE LICHENS 108 IV. STRATOSE-RADIATE THALLUS 1. STRATOSE OR PRIMARY THALLUS A. GENERAL CHARACTERISTICS 111 B. TISSUES OF PRIMARY THALLUS 112 _a._ Cortical tissue _b._ Gonidial tissue _c._ Medullary tissue _d._ Soredia 2. RADIATE OR SECONDARY THALLUS A. ORIGIN OF THE PODETIUM 114 B. STRUCTURE OF THE PODETIUM 114 _a._ General structure _b._ Gonidial tissue _c._ Cortical tissue _d._ Soredia C. DEVELOPMENT OF THE SCYPHUS 117 _a._ From abortive apothecia _b._ From polytomous branching _c._ From arrested growth _d._ Gonidia of the scyphus _e._ Species without scyphi D. BRANCHING OF THE PODETIUM 119 E. PERFORATIONS AND RETICULATION OF THE PODETIUM 120 F. ROOTING STRUCTURES OF CLADONIAE 121 G. HAPTERA 122 H. MORPHOLOGY OF THE PODETIUM 122 I. PILOPHORUS AND STEREOCAULON 125 V. STRUCTURES PECULIAR TO LICHENS 1. AERATION STRUCTURES A. CYPHELLAE AND PSEUDOCYPHELLAE 126 _a._ Historical _b._ Development of cyphellae _c._ Pseudocyphellae _d._ Occurrence and distribution B. BREATHING-PORES 129 _a._ Definite breathing-pores _b._ Other openings in the thallus C. GENERAL AERATION OF THE THALLUS 132 2. CEPHALODIA A. HISTORICAL AND DESCRIPTIVE 133 B. CLASSIFICATION 135 I. CEPHALODIA VERA II. PSEUDOCEPHALODIA C. ALGAE THAT FORM CEPHALODIA 136 D. DEVELOPMENT OF CEPHALODIA 137 _a._ Ectotrophic _b._ Endotrophic _c._ Pseudocephalodia E. AUTOSYMBIOTIC CEPHALODIA 140 3. SOREDIA