THE BIRTHWORT TRIBE. X91 



thin, flat, and dark brown (fig. 5.), and contain a 

 small, dicotyledonous embryo, at the base of hard, 

 horny albumen {fi(j. 6. & 70' 



Asarum, the only other genus of this order you are 

 likely to meet with, has a regular three-lobed calyx, 

 and its stamens are distinct from each other ; the 

 adhesion of the stamens into a central column, does 

 not therefore form any part of the essentials of the 

 Birthwort Tribe, which is characterised by its inferior, 

 six-celled fruit, its six stamens, and by its tubular calyx 

 without corolla, divided into either one or three lobes ; 

 so that the type of its tructure is essentially ternary, 

 or thus, 



O >J O • tj • 



• • • • • • 



s s s or s s s 



s s s s s s 



c c c c c c 



c c c c c c 



which among Dicotyledons is very uncommon. 



It is, as you know, chiefly in Monocotyledons or 

 Endogens, that the number three prevails in the parts 

 of fructification, and it is not a little curious, that the 

 stem of Aristolochias should be almost intermediate in 

 structure between that of Exogens and Endogens. It 

 has the medullary processes of the former, and conse- 

 quently their pith ; but it wants the concentric layers 

 in the wood, which is formed of bundles of woodv 

 matter, collected indeed into wedges, but plunged 

 down into a pithy substance, as in Endogens. The 

 Birthwort Tribe may therefore be considered one of 



