Palisade cells, 55, _25_. — tissue in leaves, 58. — — — fossil leaf, 59, _34_. Palms, 85. Parenchyma, 55, _24_. Petrifaction of cells, 4. Petrifactions, 17. — of forest débris, 18. — treatment of specimens of, 184. _Phyllotheca_, 173. Plant, parts of, the same in living and fossil, 59. — world, main families in, 44. _Platanus_, 83. Polypodiaceæ, 124. Polystelic anatomy, 63, 72. _Populus_, 83, 85. Poroxyleæ, 88. — description of group of, 96. _Poroxylon_, anatomy of, 97, 116. Primitive plants, 46. Primofilices, 132. Protococcoideæ, 47, _17_. _Protodammara_, 89. Protoplasm, 47. Protostele, 62, 70. Protoxylem, 57, _31_. _Psaronius_, 129, 130. — stem anatomy of, 131, _91_. Pteridophytes, development of secondary wood in fossil forms of, 72. Pteridosperms, 44, 104, 114, 131. — description of group, 114 et seq. — general distribution of in time, 177. — summary of characters of, 123. _Pteris aurita_, 62. Quarries, 7, _1_. _Quercus_, 83, 85. Race senility, 180. Ranales, 103. Renault, 2, 156, 187. Reproductive organs, likeness between those of living and fossil plants, 67, _45_, _46_. — — peculiar characters of some from the Palæozoic, 74. — — simplicity of essential cells of, 52. Rocks, persistence of mineral constituents, 36. — fossils varying in according to the geological age, 37 et seq. Roof of coal seam, 24, _13_, 25, _14_. Roots, likeness of structure in living and fossil, 60, _35_.