438 MORPHOLOGY OF THE ANGIOSPERMS 



ments and basifixed anthers. Some highly specialized stamens — those of 

 Potamogeton, Posidonia, Lilaea, Phyllospadix — are described as having 

 an expanded laminalike connective or filament. The carpels are basi- 

 cally follicles — reduced to achenes in many genera — with a sessile or 

 nearly sessile stigma. 



The morphological story in this order is one of reduction throughout 

 the plant, in adaptation to moist or aquatic habitats. In the flowers, the 

 numerous spiral sporophylls are reduced to trimerous whorls, like those 

 of the perianth, and the whorls are progressively reduced to one and 

 to a solitary, "terminal" carpel or stamen. (The gynoecium of the most 

 reduced forms closely resembles that of the dicotyledon Cerato- 

 phyllum. ) 



In monocotyledons, two types of perianth have been distinguished: 

 one, with calyx and corolla free and unlike in form, structure, and 

 color; the other, with the two whorls closely alike in every way and 

 tending to unite. Both these types are present in the Helobiales. (The 

 first type is rare elsewhere in the monocotyledons — in some primitive 

 members of the Liliaceae, the Trillieae.) In the aquatic taxa, the peri- 

 anth is greatly reduced and often lost. It may persist in vestigial form, 

 even in submersed flowers — Wisneria, a highly specialized alismataceous 

 genus, with submersed flowers. Reduction of the stamen whorls brings 

 about the formation of one, rarely two, outer whorls, in which the 

 stamens stand in pairs (Fig. 48). (Morphologically, these pairs have 

 been formed by telescoping of the whorls, not by dedoiiblement.) 

 Lateral enlargement of the receptacle in Thalassia and Cymodocca, en- 

 closing the carpels, forms a false inferior ovary. Ventral adnation of 

 the carpels to the enlarged receptacle forms pseudosyncarpy in the 

 Alismataceae, Aponogetonaceae, Scheuchzeriaceae. 



The placentation is fundamentally laminar — Butomaceae, Hydro- 

 charitaceae — with many ovules. The Ahsmataceae show, in Dammonium, 

 a series of stages in reduction from laminar to basal placentation: D. 

 polyspermiwi has several ovules; D. stcllatum, two ovules; and D. cali- 

 jornicum, a solitary ovule. The vascular anatomy of these carpels is ap- 

 parendy unknown, and the relation of this placentation type to that of 

 Cahomha and Brasenia — which shows similar reduction — is uncertain. 

 Laminar placentation in the monocotyledons perhaps differs somewhat 

 from that in the dicotyledons; absence of ovules along the midrib line 

 has not been reported in laminar placentation in the monocotyledons. 



The ovules are anatropous, as in primitive dicotyledons, with possible 

 exceptions among solitary ovules; some basal ovules are described as 

 "erect" and some suspended ovules as "doubtfully anatropous." Emphasis 

 has been placed on the presence of several archesporial cefls in the 

 ovules of some genera as indicative of primitiveness and as suggesting 



