5 2 4 
POL YGONUM 
the sta., and the firs, are visited by insects, but in very varying degree 
(see the interesting series of sp. described by Muller in Fert. of Firs . 
p. 509). Cleistogamic firs, are found under the ochrea in P . avicu - 
lare L. &c., and this sp. also is said to possess subterranean cleisto- 
gamic firs. In P . viviparum L. (an alpine Brit. sp. ) many of the firs, 
are replaced by bulbils in the lower part of the infl. (cf. Lilium, 
Allium). [For P. Fagopyrum L. see Fagopyrum.] 
Polypetalae (Benth.-Hooker). A division of Dicotyledons (p. 132). 
Polypodiaceae. Filicineae Leptosporangiatae (Homosporous). 55 gen. 
with 3000 sp. , cosmop. Mostly herbaceous perennials with a creeping 
rhizome or erect root-stock ; many are epiphytic. Leaves usually large 
and pinnate, with sori on the lower sides. Each sorus consists usually 
of a large number of stalked sporangia, each with a vertical incomplete 
annulus and dehiscing transversely. An indusium may or may not 
be present. Chief genera: Acrostichum, Davallia, Pteris, Polypo- 
dium, Platycerium, Adiantum, Asplenium, Aspidium, Nephrodium. 
Polypodium Linn. Polypodiaceae. 400 sp. cosmop. Several occur in 
Brit., viz. P. alpestre Hoppe, P. Dryopteris L. (oak-fern), P. Phego- 
pteris L. (beech-fern), P. vulgare L. (common polypody). They 
have circular naked sori. Many trop. sp. are epiphytic and show 
interesting features. The rhizomes creep over the supporting tree, 
and humus is collected in niches formed between it and the leaves. 
An ordinary P. leaf with its bare petiole below would be useless, 
but these sp. have leaves whose blade reaches down to the base. P. 
Heracleum Kze. is a good example. P. quercifolium L. and others 
are still more interesting, as they exhibit two kinds of leaves, the one 
of the ordinary branched type, projecting out from the support and 
serving for assimilation and spore-bearing, the others small leaves like 
those of the oak, close to the rhizome, forming humus-collecting niches 
(cf. Platycerium, and see Goebel Pflanzenbiol. Sck. 1. p. 216). 
Polypogon Desf. Gramineae (vm). 10 sp. warm temp, and trop.; z 
in Brit, rare (beard-grass). 
Polystachya Hook. Orchidaceae (9). 40 sp. trop., esp. Afr. 
Pomaceae (Warming). =Rosaceae (sub-order II). 
Pomaderris Labill. Rhamnaceae. 20 sp. Austr., N. Z. 
Pontederia Linn. Pontederiaceae. 2 sp. Am. Firs, trimorphic, hetero- 
styled (cf. Lythrum). P. crassipes Mart. —Fichhornia crassipes. 
Pontederiaceae. Monocotyledons (Farinosae). 6 gen. with 21 sp. 
trop. Water plants (p. 158), floating or rooted, of sympodial struc- 
ture, the successive axes ending in infls. (sympodial cymose pseudo- 
racemes). Often, e.g. in Eichhornia, the axillary shoot is adnate to 
the main shoot from which it springs (p. 42). Sometimes extra 
branches are formed, and the axis of the infl. is often pushed to one side 
so that it appears to spring from a leaf-sheath. FIs. zygomorphic. 
P (3 + 3), persistent; A 3 + 3, epiphyllous, G (3), 3-loc. with 00 ana- 
tropous ovules, or i-loc. with 1 ovule; style long, stigma entire or 
