POTERANTHERA 535 



highly modified type of all. [All the above are Brit. sp. ; they are 

 best studied at first in a herbarium.] Interesting phenomena con- 

 nected with the above are to be seen in the germ, of the seeds, and 

 the anatomy. 



Hibernation in different ways; some remain green all winter; 

 P. natans, &c. die down and leave only the rhiz. ; P. pectinatus L. 

 forms peculiar tubers on special branches (Schenk, Wassergewachse, 

 p. 86) ; P. crispus and others form winter buds with broad 1. (not 

 closely packed as in Utricularia, &c., but wavy, like holly 1.); P.obtn- 

 sifolius forms winter buds of the ordinary kind. The fls. are of simple 

 structure, arranged in spikes which project above the water. Each 

 has 4 sta. in two whorls, and 4 cpls. From the connective of each 

 sessile anther there grows out a cup-shaped expansion simulating a P 

 leaf. The fl. is protog. and wind-fert. The outer layer of the peri- 

 carp contains air, so that the achene floats on the water and may thus 

 be carried to a distance, sinking when the air escapes. 

 Potamogetonaceae (EP. ; Naiadeae\>.'p. BH.}. Monocots. (Helobieae). 

 9 gen., i 20 sp., cosmop., all water plants, several marine. There is in 

 most a creeping stem or rhiz., mono- or sym-podial, attached to the 

 soil by adv. roots, and sending off erect branches upwards into the 

 water. These usu. have ribbon leaves, submerged (exceptions in 

 Potamogeton), arranged in \ phyllotaxy. The base is sheathing, and 

 within the sheath are the small scales (sijnamulae intravaginales] 

 which occur in most of the Helobieae. The infl. is a spike or cyme, 

 or the fls. solitary. The fls. possess no true P (exc. Zannichellia ? ), 

 but Potamogeton shows leaf-like outgrowths from the sta. which 

 perform the P functions ; similar structures occur in Zostera and 

 Ruppia. In several gen. it is not easy to decide what is to be regarded 

 as the fl. (see Zostera). Fl. or unisex., reg., i 4-merous. Cpls. 

 free or only i. Ovules i in each cpl., pend. orthotr. Fruit one- 

 seeded. No endosp. Embryo with well-developed hypocotyl. For 

 details see genera. 

 Chief genera: 



A. Fls. in spikes: 



1. saltwater: Zostera, Phyllospadix, Posidonia, Ruppia. 



2. fresh or brackish : Potamogeton. 



B. Fls. in cymes or sol. : Cymodocea, Zannichellia. 

 Potamopbila R. Br. Gramineae (6). i Austr. 

 Potaninia Maxim. Rosaceae (in. 2). i Mongolia. 



Potato, Solatium tuberosum L. ; sweet-, Ipomoea Batatas Lam. 



Potentilla L. (incl. Comarum L., Tormentilla L.). Rosaceae (ill. 2). 

 225, chiefly N. temp, and arctic, a few S. temp, and Andes; 9 in 

 Brit., incl. P. Anserina L. (silverweed), P. reptans L. (cinquefoil), 

 P. Coniarum Nestl., P. Tormentilla Neck, (tormentil), &c. Herbs, 

 usu. with creeping stems which root at the nodes and thus multiply 

 the plant veg. There is a well-marked epicalyx of small green 1. 

 outside and alt. with the seps. These are the slips, of the seps. 

 united in pairs; often one or more may be seen with two lobes or 

 even completely divided. Fls. of class AB, homogamous, fly-visited; 

 honey secreted by a ring-shaped nectary within the sta. 



Poteranthera Bong. Melastomaceae (i). 4 Brazil, Guiana. 



