426 



PLANT SOCIOLOGY 



Opegrapha, 291 



saxicola assn., 188 

 Opuntia, 130 

 Order, definition of, 364 

 Orders of communities, 364 

 Omithopus, 236 

 Orobanche, 290 

 Orographic factors, 261 

 Orthotrichum, 268, 269 



lyellii, 16 

 Oryzopsis, 292 

 Oscillatoria, 75 



Osmotic changes, regulative power 

 against, 193 

 concentration, 192 

 for halophytes, 130 

 measurement of, in soil, 131 

 of cell sap, 129 

 of desert succulents, 130 

 of Salicornia, 192 

 of soil solutions, 164 

 of soil water, 131 



range of, in plant associations, 130 

 seasonal range of, 130 

 pressure {see Osmotic concentration) 

 theory, of Gola, 164 

 value of epidermal cells, 201 

 of saline soil solution, 205 

 of soil solution, 202 



and zonation of vegetation, 206 

 Oxalis, 37, 98, 226 



acetosella, 101, 229, 253 

 Oxidation of soils, 243 

 Oxygen content of soil, 224 

 Oxyria, 308, 310 

 Oxyrietum, 308 

 Oxylropis campestris, 71 

 montana, 188, 242 

 jyilosa, 211 



Paleoendemics, 189 



narrowly localized, 62 

 Paleontologic records, 351 

 Paleosynecology, 82 

 Paleosyngenetics, 82 

 Panicum capillare, 328 



ischaemum, 364 

 Papaver alpinum, 59 



Parallel communities, 319 



species, 343 

 Paraplectrum foetidum, 243 

 Parasitism, 5, 6 

 Parietaria ramiflora, 373 

 Paris, 239, 290 



quadrifolia, 229 

 Parkland of Alberta, 350 

 Parmelia lanala assn., 155 

 Parmelietum acetabuliic, 241 

 Paronychia pidmnala, 150 

 Pastures, permanent, 344 

 Pasturing and plant growth, 271 



effect of, 277 

 Peat borers, 338 



fossils, 337 



kinds of, 251 



pollen content of, 337 

 Pedicularis grayi, 90 



verliciUala, 71 

 Peganurn harmala, 128, 276 

 Pelargonium, 296 

 Pelitic soils, 160 

 Pellia, 291 

 Peltigera rufescens, 72 



spuria stage, 14 

 Peplis erecta, 220 

 Perhalicole species, 165 

 Perhaloid-anastatic associations, 197 

 Periodic phenomena, related to tem- 

 perature, 92 

 Periodicity, 26, 47 



diagrams of, 47 



influence of habitat conditions upon, 

 297 



of climates, 113 



plankton, 75 



temperature and, 91 

 Periods, archeological, 339 



climatic, post-glacial, 339 

 Permanent communities, 322 



quadrats, use of, 328 

 {See also Quadrats) 

 Permeability, changed by ions, 180 



of soil, 232, 283 

 Petasiteteum nivei, 183 

 Petrosimonia crassifolia assn., 202 

 Peucedanum palustre, 59, 226 

 pH curves, 169, 170, 171, 172 



explanation of, 166 



