440 MORPHOLOGY OF THE ANGIOSPERMS 



Potamogetonaceae, Najadaceae. Family lines are difficult to draw, 

 morphologically, in this order, because of its greatly reduced floral 

 structure and the questionable interpretations of some of the floral 

 organs, especially in those families that have been described as pos- 

 sessing stamens with remarkable, more or less sepallike, expansions of 

 the connective — Potamogetonaceae sensu l-ato, Lilaeaceae, Scheuchzeria- 

 ceae. Anatomy shows, however, that these peculiar stamens have been 

 misinterpreted, morphologically; the pseudoperianth parts of the potamo- 

 getons and their close relatives are compound organs, stamens with 

 adnate sepals, the adnation varying in degree. The sepal and the stamen 

 have separate vascular supplies and are, morphologically, independent 

 organs. 



The interpretation of these peculiar organs has been extended to 

 cover the flowers of the Scheuchzeriaceae ( Juncaginaceae ) , Triglochin 

 and its closely related genera, where sepal and stamen are closely as- 

 sociated but structurally independent. The "flower" of these genera is 

 an inflorescence, and it seems most doubtful that Triglochin and its re- 

 lated genera belong in the Potamogetonaceae. The association of Tri- 

 glochin with Scheuchzeria seems unnatural. The "perianth" parts of Tri- 

 glochin are bracts; those of Scheuchzeria are tepals. The leaves of 

 Scheuchzeria are bifacial, those of Triglochin are terete. The rhizome of 

 Scheuchzeria has a fibrous sheath; that of Triglochin has a sheath of 

 stone cells. 



Resemblances to the Ranunculaceae and to the Nymphaeaceae have 

 been seen in the Helobiales, and these similarities used as evidence of 

 derivation of the monocotyledons from the dicotyledons. But the simi- 

 larities are largely those of general primitiveness and reduction related 

 to habitat. The sharply distinct calyx and corolla suggest the similar 

 differentiation in the Ranunculaceae, but this character alone can 

 hardly be significant. The presence, in the Alismataceae, of laticiferous 

 cells, and the absence of these cells in the Ranunculaceae is evidence 

 against relationship of these two families. 



Among the major primitive monocotyledonous taxa, the well-differ- 

 entiated calyx and corolla set the Helobiales apart from the Palmae 

 and the primitive Liliales, 



LILIALES 



The Primitive Liliaceae 



The Liliaceae are generally considered, in taxonomic treatments, to 

 be one of the primitive monocotyledonous families, an interpretation 

 supported by morphology, anatomy, and palynology. Most primitive in 

 the family are the Melanthioideae, a subfamily with genera rather di- 



