THE STAMEN 123 



small family, Cabombaceae, seems to complete the series in the Nym- 

 phaeaceae, with the sporangia abaxial in Cabomba. It is apparent that, 

 at least in many families, the primitive position for microsporangia is 

 adaxial. The abaxial position in primitive families with especially primi- 

 tive stamen form — Degeneriaceae, Himantandraceae, Lactoridaceae, An- 

 nonaceae, for example — has already been discussed. The Ranales, in their 

 most primitive stamens, present a major morphological problem in the 

 variety of microsporangial position; there are both adaxial and abaxial 

 types. Among ranalian families, some — Magnohaceae and Nymphaeaceae 

 — show phyletic migration of microsporangia from adaxial to abaxial 

 positions. ( Megasporangia in angiosperms are adaxial, without excep- 

 tion.) There seems no question but that the adaxial position is one 

 primitive position and that from this have been derived lateral and 

 abaxial positions. But it seems unlikely that the abaxial position in the 

 highly primitive stamens of Degeneria, Himantandra, and Lactoris is 

 secondary — that these stamens are highly specialized in sporangium 

 position. It is possible that both positions are primitive, a retention in 

 the stamen of morphological structure of an ancestral taxon. (In the 

 Pteridospermae, ovules were apparently borne on both surfaces of the 

 leaf. ) 



The old search (1820-1850) for evidence of basic sporangium posi- 

 tion by locating the stamen margins was well planned; it should be con- 

 tinued, with further critical studies of marginal meristems in stamens. 

 Ontogeny should add strong support to the theory of the migration 

 of sporangia. Leaves, petals, carpels, and broad stamens (Fig. 50) 

 show marginal meristems. In slender stamens, these meristems have been 

 lost wholly or in part; terete filaments apparently have no marginal 

 growth, but weak marginal growth is found in "flattened" filaments and 

 in the broader connectives. The somewhat flattened filaments of Lirio- 

 dendron have weak marginal meristems; the broader filaments of Mag- 

 nolm have well-marked marginal meristems. ( Liriodendron seems to have 

 been derived from ancient magnolian stock — its extrorse dehiscence 

 derived from introrse. ) Much more information about the development 

 of the sterile tissues of the stamen is needed, especially of semilaminar 

 forms. If the margin of the filament can be distinguished by linear meri- 

 stems and these meristems followed upward into the connective, the 

 position of the sporangia can hardly be questioned. 



Evidence of another type that all four sporangia belong on one side 

 of the sporophyll comes from the close histological association of each 

 lateral pair. The sporangia on each side often fuse, in late stages, and the 

 pollen grains mingle in a common chamber at dehiscence. Still further 

 support for the theory that sporangia migrate is present in stamens 

 where sporangium number is reduced to two. In these stamens, the 



