428 MORPHOLOGY OF THE ANGIOSPERMS 



Exospermum and Zygogynum, the ovules are borne on the dorsal face 

 of the locule wall (the basis for the name Exospermum) in two longi- 

 tudinal rows, and their traces are derived from the dorsal vein. This 

 ovule position was called "extremely rare" in 1900, and probably no 

 other examples have been described. The two genera appear unique 

 among angiosperms in their placentation. [In some achenes, ovule 

 traces appear to be derived from dorsal bundles, but, in these fruits, 

 the dorsal and lateral veins are greatly fused and condensed, and the 

 ovule trace is derived from the united veins (Fig. 42).] 



The Ovule. The primitiveness of the anatropous ovule is strongly sup- 

 ported by the Ranales, where it is apparently present in all families. 

 Ovule type is perhaps the most constant character in the order. 



Nectaries. The Ranales show stages in the development of nectaries 

 from poorly defined areas of secretion to prominent secretory organs 

 that represent stamens or petals. Where pollination is by chewing in- 

 sects, beetles— Eupomatia and Calycanthus — food bodies (Fig. 69) 

 take the place of secretory structures. 



Pollen and Pollination. Pollen in the Ranales is dominantly mono- 

 colpate or of types (dicolpate and acolpate) derived phylogenetically 

 from monocolpate, the form characteristic of lower seed plants. The 

 higher, tricolpate type is present in families that are most specialized 

 in habit — Ranunculaceae — or in flower structure — Berberidaceae. Pollen 

 in permanent tetrads — generally considered an advanced character — is 

 present in the Winteraceae, Lactoridaceae, and some of the Annonaceae. 



Within the Ranales, there are all methods of pollination, from the 

 most primitive, that by beetles only — by a single species of beetle in 

 Eupomatia — to pollination by various advanced methods. The Ranales 

 support the theory that pollination by wind is associated with the de- 

 velopment of unisexuality. Pollination types and the related nectary 

 morphology have been largely neglected in phylogenetic studies. The 

 Ranales show the primitiveness of beetle pollination by their structural 

 adaptations to this type, and by transitions in structure to pollination by 

 other insects and by wind. The food bodies of Eupomatia and Calycan- 

 thus are replaced by the simplest of nectaries — mere areas of diffusion 

 or secretion; these areas are replaced by emergences, and these emer- 

 gences by transformed organs or parts of organs. 



Receptacle. Modifications in the form of the receptacle that are steps 

 to the complex gynoecial forms in higher orders are already present in 

 the Ranales. The large flowers of the Magnohaceae and Annonaceae, 

 with many sporophylls, have the primitive elongate receptacle. Accom- 

 panying the reduction in number of appendages and change in ar- 

 rangement from spiral to cyclic is progressive shortening of the re- 

 ceptacle by "telescoping"— shortening and elimination of internodes. 



