454 



METZGEKIACEAE. 



Archegonia superficial, borne on the upper surface of ordinary thallus- 

 branches or of short, more or less specialized branches, often protected by 

 scales and sometimes by tubular pseudoperianths. About 25 genera and 

 350 species, most abundant in tropical regions. 



Thallus not developing a median strand of elongated cells. 1. Riccartlia. 



Thallus developing a distinct median strand of elongated cells. 



Thallus apparently dichotomous, antheridin and archegonia 



borne on short ventral branches. 



Thallus branching usually by adventive ventral branches, some- 

 times apparently dichotomous ; antheridia and archegonia borne 



2. Mctzgeria. 



on ordinary thallus-branches. 



3. PaUavicinia. 



I. RICCARDIA S. F. Gray? 



Thallus dark green, linear, with distinct lateral branches, the latter some- 

 times perpendicular to the substratum, composed of parenchyma with little 

 differentiation. Antheridia borne in two rows on short branches with involute 

 and often crenulate or dentate margins. Archegonia borne in irregular clus- 

 ters on short branches, surrounded by irregular and minute, scale-like or fila- 

 mentous structures. Calyptra fleshy, carrying up on its surface some of the pro- 

 tective structures, together with the unfertilized archegonia. Pseudoperianth 

 none. Capsule oval, the wall splitting into four equal valves two cells thick, 

 some or all of the cells with ring-like thickenings. Elaters usually with a single 

 band of thickening, remaining attached to the free tips of the valves. Gemmae 

 oval, mostly two-celled, formed directly from the protoplasmic contents of 

 superficial thallus-cells. [Probably in honor of F. Eiccardi, an Italian marquis.] 

 About 150 species, mostly tropical. Type species: R. multifida (L.) S. F. Gray. 



Thallus irregularly pinnate or palmate, the margins opaque and entire. 1. R. latifrons. 

 Thallus regularly bipinnate or tripinnate when well developed, 



the margins translucent and often crenulate. 2. R. multiflda. 



1. Riccardia latifrons Lindb. 

 BROAD KICCARDIA. (Fig. 499.) 

 Thallus mostly 2"-5" long, irregu- 

 larly pinnate or palmate, the 

 branches mostly \"-V wide, often 

 broadening out from a narrow 

 base, opaque, thinning out to an 

 entire margin one cell thick and 

 one cell wide. Inflorescence autoe- 

 cious; ring-like thickenings present 

 in inner layer of capsule wall, ab- 

 sent (or nearly so) from outer 

 layer. 



On roots oi plants and on the 

 ground in swamps. Devonshire 

 Marsh. Widely distributed in Europe, 

 Asia, and North America. 



