RANALES 423 



examples or suggestions of the imbranched "durian" habit, as might be 

 expected in this large and diverse primitive order if the durian theory 

 were valid. 



In leaf structure, some attention has been given to differences in 

 form and arrangement of the subsidiary cells of the stomata, with the 

 view that these differences — "haplocheile" versus "syndetocheile" — may 

 be of phylogenetic significance, as they have been shown to be in the 

 gymnosperms. But subsidiary-cell patterns seem not to be clear in 

 angiosperms; some families appear to have no pattern, and, in other 

 families — even within the genus Magnolia — more than one pattern can 

 be distinguished. There may have been evolutionary advance in angio- 

 sperms from a simple subsidiary-cell pattern to a more complex one, 

 and to a loss of distinction from surrounding epidermal cells, but there 

 is, as yet, insufficient information to determine this. 



The unilacunar node is characteristic of many of the ranalian families, 

 and the origin of the trilacunar and multilacunar types is well shown in 

 Austrohaileija and other genera. The origin of the odd-number trace 

 system, long supposed characteristic and unique in angiosperms, is well 

 shown in this order. The basic number of traces for appendages is 

 clearly two, from one gap (Fig. 5). (Cotyledons, carpels, and stamens 

 also show the basic number two in various orders. ) Within the Ranales, 

 nodal structure shows stages in advance from unilacunar to trilacunar 

 and multilacunar and is rarely reduced to a secondary unilacunar with 

 one trace. 



In the shoot apex, the two-layered tunica is characteristic of several 

 families and seems to be the primitive form in angiosperms. 



Xylem and Phloem. Four ranalian families have vesselless xylem and 

 accompanying highly primitive tissue structure and cell arrangement: 

 the Winteraceae (Fig. 25), Trochodendraceae, Tetracentraceae, and 

 Amborellaceae (Fig. 24). (Outside the order, only Sarcandra (Fig. 23) 

 in the Chloranthaceae is known to be vesselless.) Only a step in 

 histological advance above these taxa are the Eupomatiaceae and 

 Cercidiphyllaceae; their tracheary cells differ little from those of 

 the vesselless taxa except in the presence of perforations in the scalari- 

 form-pitted end walls. Other families have only scalariform or both 

 scalariform and simply perforate vessel elements. The various families 

 show progressive shortening of the scalariform-perforate end wall of 

 the vessel element; reduction in number and increase in size of 

 openings lead to the short, simply perforate element of the Berberida- 

 ceae and Ranunculaceae, with the exception of Hydrastis, which prob- 

 ably does not belong in this family. (Paeonia is now generally recog- 

 nized as a dillenialian taxon. ) Similar progressive modification is present 

 in the tracheid-fiber and vascular-ray series. 



