HELOBIALES 439 



derivation of the Helobiales from the Ranunculaceae, but multiple arche- 

 sporial cells are present in some high families. The absence of endo- 

 sperm in the seeds is an important character. 



The leaves of the Helobiales vary greatly in type; those with cordate 

 or sagittate blades are prominent. Probably the only free, paired 

 stipules in the monocotyledons are in the Potamogetonaceae. In other 

 orders, the stipules are merged with the leaf sheath; in the Scheuchzeria- 

 ceae, the stipules are still prominent as distal appendages of the sheath. 



The vascular structure of the Helobiales is little known; its reduced 

 condition makes interpretation difficult. The vessel elements of the 

 Alismataceae are of the simply perforate type, a type, perhaps, not to 

 be expected in this primitive family, but this type of vessel is charac- 

 teristic of herbaceous, and especially of aquatic, plants. 



Within the Helobiales, the Alismataceae and Butomaceae are generally 

 considered the most primitive families. Though primitive in apocarpy 

 and general absence of fusion in Horal organs, they are advanced in the 

 possession, in some taxa, of achenes, unisexuality, whorled perianth 

 (with clear distinction of calyx and corolla), whorled androecium, 

 gynoecium ranging from spiral to whorled, solitary ovules, and simply 

 perforate vessels. As compared with the most primitive dicotyledons, 

 these families have fewer primitive characters. 



The genera of Alismataceae are closely interrelated; Alisma is most 

 primitive on cytological evidence — In chromosomes, in contrast with 

 much higher numbers in other genera. The pollen grains of this family 

 — multiporate with elaborate apertures — are markedly unlike the mono- 

 sulcate type characteristic of most of the higher monocotyledons. In 

 the Butomaceae, all the genera, except Butomiis, have pollen resembling 

 that of the Alismataceae, but But 011ms has monosulcate pollen grains. 

 On the basis of pollen morphology, it has been suggested that Biitomiis 

 alone be retained in the Butomaceae and the other genera placed in 

 the Alismataceae. Relationships among these aquatic or marsh-living taxa 

 are difficult to determine because of their great reduction. 



The carpels of Alisma perhaps show the origin of septal nectaries. 

 Between the bases of adjacent carpels, nectar is secreted from simple, 

 unspecialized areas of the carpel walls. With connation of the carpels 

 in higher taxa, the secretory areas are enclosed, and a specialized aper- 

 ture developed. 



The Hydrocharitaceae are important in showing, with the Butomaceae, 

 primitive laminar placentation. The morphology of the gynoecium in 

 this family needs anatomical study; the relation of the carpels to the 

 receptacle is obscure and has been variously interpreted. 



The other families in the Helobiales show specialization of various 

 types, including a reduction series in flower structiue — Aponogetonaceae, 



