THE STAMEN 



119 



of Degeneria and Himuntandra, which are median or low in position.) 

 In form, the most primitive stamen in the famihes is that of Victoria 

 and some species of NytJipliaea (Fig. 54/). This stamen is broad and 

 flat, without division into anther and filament. The four sporangia are 

 elongate, located in the distal part of the sporophyll and deeply em- 

 bedded. Like the sporangia of laminar stamens in the woody genera of 

 the Ranales, they are wall-less but covered externally by a fibrous 

 layer, commonly called exothecium (Fig. 59). This layer is associated 

 with dehiscence; it is obviously not the sporangium wall, because it 

 extends along the connective beyond the fertile areas (Fig. 55). 



Fig. 53. Diagrams showing progressive changes in vascular structure in the anther 

 accompanjing evolutionary migration of sporangia from adaxial to abaxial posi- 

 tions. The change, essentially a dovvn-roUing involving the entire sporophyll, is 

 shown by the inversion of the outer veins in C and D. Xylem in solid black. 



The various taxa in these families show stages in a progressive nar- 

 rowing and thickening of the lamina throughout its length (Fig. 54). 

 The sterile basal part is greatly reduced in width and thickened, with 

 the formation, in early stages, of a narrow, flat filament; in the last 

 stages, of a terete filament — Nelianbo, Barclmja, the Cabombaceae. The 

 fertile section is greatly modified, with the formation of an anther. Tissue 

 relations in this part of the lamina are much changed, and the spo- 

 rangia and even the vascular bundles, in part, occupy new positions. 

 The anther is a complex structure. A thickened median strip, the con- 

 nective, is built up as the marginal areas are reduced in width and 

 retracted. Reduction and rearrangement of the tissues of the laminar 

 wings leave the sporangia surrounded by a wall of laminar tissue, which 

 forms the anther-sac wall. The sporangia appear "free" but are still 

 embedded in laminar tissue. 



The sterile, terminal part of the sporophyll is less modified than the 

 fertile part. It is reduced in length and width, becoming a distal ap- 



