INDEX 



Pistia Stratiotes, air tissue of leaves, 82, 

 J 54. 256; comparison with Lemna- 

 ceae, 74, 82, 316; hairs, 82, 83; vigour 

 of vegetative growth, 83, 213, 214- 

 water pores, 82 (Fig. 53), 83, 167, 267 



"Pith" helmets, 191 



Plantago, 233, 313 



"Plantago aquatica," 20 



Plantago major, 241 



Plantain, Lesser Water. See Echino- 

 dorus ranunculoides 



Plantain, Water. See A lisma Plantago 



Podostemaceae (including Tristicha- 

 ceae), 112-122, 327-333; affinities, 

 310, 319; anatomy, 117, n8(Fig. 80); 

 ancient aquatics, 321; and natural 

 selection, 327-333; and wading birds, 

 300; anemophily, 120, 121 ; anthocya- 

 nin, ii2, 113, 276, 277; cleistogamy, 

 121 (Fig. 82) ; dependence on aeration, 

 257; distribution, 295, 306; dorsi- 

 ventrality, 121, 122, 327-329; en- 

 tirely aquatic, 318; flowers, 120, 121 

 (Fig. 82); germination in situ, 248; 

 "gill-tufts," 118, 119 (Fig. 81), 255; 

 habit, 114, 115 (Figs. 76, 77), 116 

 (Fig. 79), H7;haptera, 113, 114, 121; 

 in biological classification, 7; inhabit 

 rapids, 112, 113, 119, 257; "Kiemen- 

 biischel," 118, 119 (Fig. 81), 255; 

 lack of adaptation, 328-333; lack of 

 intercellular spaces, 118 (Fig. 80), 

 257; morphology, 73, 121, 122; 

 mucilaginous seeds, 300; polymor- 

 phism of thallus, 114-117; rarity 

 outside tropics, 112, 113; reduction 

 of primary root, 244 ; root . thallus, 

 114, 115 (Figs. 76, 77, 78), 116 (Fig. 

 79), 117, 208; secondary shoots, 114; 

 seedling, 114, 115 (Fig. 78), 117; 

 seeds, 121, 300; shoot thallus, 117; 

 silica, 117; simulation of lower plants, 

 114-117; vegetative reproduction, 

 216; water reservoir in nucellus, 121 



Podostemon, Alga-like form, 114; cleisto- 

 gamy, 121 (Fig. 82), 234; distribution, 

 306 



Podostemon Barberi, 121 (Fig. 82), 234 



Podostemon subulatus, 114 



Pollination, anemophilous, 57, 120, 121, 

 230, 232, 233 



Pollination, aquatic. See Pollination, 

 hydrophilous 



Pollination, cleistogamic. See Cleisto- 

 gamy 



Pollination, entomophilous, 9, 57, 80, 230 



Pollination, hydrophilous, i, 6, 8, 55- 

 57, 70, 71, 84, 85, 124, 127, 129, 130, 

 134, 235-238, 345. 34$ 



Polygonaceae, 311 



Polygonum, affinities, 311, 318; colonis- 

 ing new waters, 289; land and water 

 forms, 150, 151, 152 (Figs. 99, 100), 



197. 198', heterophylly, 150, 151, 152 

 (Figs. 99. 100); length of shoot 

 system, 215; mucilage, 271; vegeta- 

 tive reproduction, 225 



Polygonum amphibium. 150-152 (Figs. 

 99, loo), 197, 198, 215. 271, 289, 311, 

 3i8 



Polygonum vivtparum, 225 



Polypetalae, 309-312, 319 



Polystely, 37, 180-182, 346 



Pondweed. See Potamogeton 



Pond weed. Horned. See Zannichellia 



Pontederia, geotropic curvature of fruit 

 stalk, 239. 240 (Fig. 155); phyllodic 

 leaf anatomy, 341 (Fig. 169). 342 

 (Fig. 170), 343, 344 ; specialised genus, 

 317; stomates, 166 



Pontederia cordata, 166, 341 (Fig. 169), 

 342 (Fig. 170), 343, 344 



Pontederia rotundifolia, 239, 240 



Pontederiaceae, affinities. 316, 317; 

 cleistogamy, 234 (Fig. 153); entirely 

 aquatic, 318; geotropic curvature of 

 fruit stalk, 239. 240 (Fig. 155), 282; 

 heterophylly, 154, 160, 161; phyllodic 

 leaf structure, 337, 341 (Fig. 169). 

 342 ( Fig. 170)-344 ; vegetative multi- 

 plication, 213 



Poplar, 187, 316 



Portulacaceae, 310 



Posidonia, apical openings absent, 269 ; 

 chlorophyll in epidermis, 164; fibres 

 in leaf sheath, 133; habit, 124; leaf 

 anatomy, 132 (Fig. 89); marine 

 Angiosperm, 123; pollen thread-like, 

 124, 125 



Posidonia Caulini, 125, 132 (Fig. 89) 



Potamogeton, 58-72; air tissue in fruit 

 wall, 71, 72 (Fig. 46); altitude, 290, 

 291; anatomy of inflorescence axis, 

 65; anatomy of root, 65 (Fig. 41), 66. 

 208; anatomy of stem, 62 (Fig. 39), 

 63, 64 (Fig. 40). 65, 175; apical 

 openings of leaves, 167 (Fig. 108), 

 268; chlorophyll in epidermis, 164; 

 choking mill sluices, 210; cortical 

 bundles, 65; cuticle, waxy, 254; de- 

 layed germination, 71, 72; dia- 

 phragms, 65, 184 (Fig. 118); dispersal 

 by ducks, 301, 302; distribution, 295, 

 297, 298; dwarfing due to heat. 275; 

 exudation of water drops, 269; fibres 

 in leaves. 61 (Fig. 38). 169; flowers. 

 69-71: fruits. 71, 72 (Fig. 46). 297; 

 germination, 280; heterophylly. 151, 

 153. 154 (Fg- i3). 157- 38 (Fig. 

 104). 159 (Fig. 105), 3.vj( Figs. 167 and 

 1 68); in biological classification, 6; 

 land forms. 61. 195. 196 (Fig. 125); 

 leaf forms, 61,339 (Figs. 167 and 168); 

 oil drops, 62 ; perenniation. 215 ; phyl- 

 lodic interpretation of leaf. 339 (Figs. 

 167, 168), 340 



