POTAMOGETON 
5 2 5 
slightly lobed. Capsule or nut. Embryo central in the seed, scarcely, 
or not, shorter than the rich mealy endosperm. Chief genera: Eich- 
hornia, Pontederia. Placed in Coronarieae by Benth. -Hooker, in 
Liliiflorae by Warming. 
Populus Linn. Salicaceae. 18 sp. N. temp. P. alba L. (white poplar) 
and P. trejnula L. (aspen) in Brit. Like Salix in most features, but 
the llrs. are wind-pollinated and have no honey; correlated with this 
is the fact that they have usually more sta. than those of Salix. The 
wood of P. alba is useful, and also that of P. nigra L. (black poplar) 
with its variety pyramidalis Spach (P. fastigiata Desf.), the Lombardy 
poplar, often grown in parks &c., P. canadensis Michx. (cotton-wood 
tree, N. Am.) and others. P. balsa 7 tiifera L. is the balsam poplar, 
and yields a resin known as Tacamahac (see Calophyllum). 
Porana Burm. f. Convolvulaceae (i. 4). 10 sp. Indo-mal., Austr. 
Porlieria Ruiz et Pav. Zygophyllaceae. 3 sp. Mexico and Andes. The 
leaflets of P. hygrometrica Ruiz et Pav. spread out horizontally at night, 
but during the day are folded up in pairs against one another, avoid- 
ing excessive transpiration. 
Porrum (Tourn.) Linn.-— Allium Tourn. 
Portulaca Linn. Portulacaceae. 20 sp. trop. and subtrop. The flr. 
has a semi-inferior ovary and 4 — go sta. It is homogamous with self- 
fertilisation on withering, and remains closed in bad weather. The 
sta. of P. oleracea L. are sensitive to contact and move toward the 
side touched. 
Portulacaceae. Dicotyledons (Archichl. Centrospermae). 17 gen. with 
144 sp., cosmop., but esp. Am. Most are annual herbs, often with 
fleshy leaves, and with stipules (sometimes represented by axillary 
bundles of hairs). Firs, usually in cymes (often dichasia with tendency 
to cincinni), regular, $ . K 2, the lower sepal (usually anterior) over- 
lapping the upper (the two are often regarded as bracteoles) ; C 5; 
A 5 + 5, or 5 opp. the petals, or some other number ; G (2 — 8) usually 
(3), superior exc. in Portulaca, i-loc. with several stigmas and 2 — 00 
campylotropous ovules on a central basal placenta. The firs, secrete 
honey and are mostly insect-pollinated. Fruit a capsule with albu- 
minous seeds; that of Claytonia and Montia is explosive; embryo 
more or less curved round the perisperm. Chief genera: Calandrinia, 
Spraguea, Claytonia, Montia, Portulaca, Lewisia. Placed in Caryo- 
phyllinae by Benth. -Hooker, in Curvembryae by Warming. 
Portulaceae (Benth. -Hooker) = Portulacaceae. 
Portulacaria Jacq. Portulacaceae. 2 sp. S. Afr. 
Posidonia Kon. Potamogetonaceae. 2 sp. Austr., Medit. 
Posoqueria Aubl. Rubiaceae (1. 8). 5 sp. S. Am. 
Potamogeton (Tourn.) Linn. Potamogetonaceae. 50 sp. cosmop. 
11 or more (cf. Rubus) in Brit, (pond-weed). Water plants with 
creeping sympodial rhizomes and erect leafy branches; leaves all 
submerged or some floating. A whole series of types occurs, begin- 
