POLYT^ENIUM. 



C 526 ] 



POLYTRICHUM. 



and six. In some of the species a kind of 

 rind is formed subsequently, by a growth 

 from the base of the joints analogous to that 

 which occurs in BATRACHOSPERMUM and 

 Callithamnion. The fructification consists 

 of 1. ceramidia, attached to the sides of 

 branches, containing numerous pear-shaped 

 spores; 2. tetraspores, on distinct plants, 

 formed in the swollen central cell of distorted 

 branches (fig. 603) ; and 3. antheridia, elon- 



Fig. 603. 



Polysiphonia nigrescens. 



Distorted ramuli containing imbedded tetraspores. 

 Magnified 50 diameters. 



gated whitish sacs, collected in great num- 

 bers at the summits of the branches, accom- 

 panied by a dichotomous hair, and some- 

 times prolonged into a hair-like process at 

 the summit. Nageli described the sperma- 

 tozoids as consisting of a spiral filament. 

 Thuret disagrees with this, and states that 

 they are merely hyaline globules, about 

 1-5000" in diameter. The British species 

 are placed in two subgenera : Oligosiphonia, 

 where there are but four or rarely five peri- 

 pheral cells, and Polysiphonia, where there 

 are six or more. Twenty-six species are de- 

 scribed, many of which are common. 



BIBL. Harvey, Brit. Mar. Alg. p. 82. pi. 

 12 A; Thuret, Ann. des Sc. nat. 3 ser. xvi. 

 p. 16. pi. 6 ; Nageli, Zeitschr.f. wiss.Botan. 

 Heft 3 and 4 (1846). p. 207. pi. 6 & 7. 



POLYT^NIUM, Desv. A genus of 

 Tsenitideae (Polypodaeous Ferns). Exotic. 



POLYTHALAMIA. See FORAMINI- 



FERA. 



POLYTOMA, Ehr. A genus of Infuso- 

 ria, of the family Monadina (Hydromorina). 



P. uvella (PI. 24. fig. 69, undergoing di- 

 vision), the only species, is oblong or oval, 

 obtuse at the ends, colourless, furnished with 

 two flagelliform filaments; it has no cara- 

 pace. Aquatic; length 1-2200 to 1-960"; 

 size of body when the division is nearly 

 complete, 1-400". 



BIBL. Ehrenberg, In/us, p. 24. 



POLYTRICHACE^. A tribe of Mnioi- 

 deae (operculate Mosses of usually Acrocar- 

 pous habit). 



I. CATHARINEA. Calyptra narrowly 

 hood-shaped, subscabrous at the apex, ra- 

 ther hairy within. Peristome simple, com- 

 posed of thirty-two teeth, arising from a 

 narrow, cellular, basilar membrane, ligulate, 

 membranous, white, with many percurrent, 

 reddish, inarticulate filaments, somewhat 

 incurved, scarcely hygroscopic, firm. Colu- 

 mella dilated at the apex into a drum-like 

 epiphragm. Capsule equal. Inflorescence 

 monoecious or dioecious. 



II. POLYTRICHUM. Calyptra dimidiate, 

 but appearing campanulate on account of a 

 quantity of very close hairs descending from 

 it as a long villous coat ; otherwise resem- 

 bling the preceding genus. 



POLYTRICHUM, Dill. A genus of Po- 

 lytrichaceous Mosses, variously defined by 

 different authors. In the British Flora, it 



Fig. 604. 



Polytrichum commune. 



Plants in fruit. 

 One-half natural size. 



