THE STAMEN 133 



two traces enter the filament and soon fuse to form a single concentric 

 bundle. In the Betulaceae and Fagaceae, there is evidence of the 

 double nature of the stamen bundle; in several genera, there are, at 

 the base of the filament, either two separate bundles or one bundle with 

 two xylem strands or two xylem poles; in Conjlus, the single bundle 

 arises from two traces. 



The phylogenetic history of the trace supply of the stamen has been 

 clarified by the better interpretation of nodal structure in the shoot. 

 Changes in number and relation of traces have been the same as those 

 of the leaf. From the basic two-trace supply — characteristic of all ap- 

 pendages — have been derived the three- and the several-trace types 

 by addition of lateral traces to a median double trace (representing 

 the basic two, fused); from the basic two (by fusion) and from more 

 than two (by loss of laterals) has been derived the single trace. 



Most laminar and semilaminar stamens have three traces and three 

 major veins. Three traces characterize some of the primitive families: 

 Degeneriaceae, Himantandraceae, Annonaceae, Lardizabalaceae, many 

 of the Magnoliaceae, and Nymphaeaceae. Eupomatia has five or seven. 

 Three traces, often with additional laterals, are present in most mono- 

 cotyledons — Musaceae, Zingiberaceae, Marantaceae. The common one- 

 trace condition has obviously arisen by reduction from a higher num- 

 ber in association with the narrowing of the filament. 



Evidence of reduction in trace number from three or more to one by 

 loss of the lateral traces — as also occurs commonly in leaves and 

 carpels — is well shown in the Magnoliaceae. In Magnolia, the lateral 

 bundles may be weak and discontinuous; in Michelia, some species have 

 three traces, others one, with vestiges of the lost bundles. In these 

 genera, distortion and rupture of lateral bundles are associated with 

 reduction in width of the stamen, accompanied by the development of 

 an anther and the migration of the sporangia. As early as 1824, there 

 was a surprising understanding of stamen anatomy; it was stated that, 

 in slender stamens, the lateral veins vanish and the median persists. 

 The present-day view that the single-bundle supply was derived by 

 reduction from a three-trace supply is very old, not recent, as generally 

 believed. 



Branching of the vascular supply is uncommon or rare in stamens 

 with one trace, but is frequent in stamens with more than one trace. 

 Minor lateral branches may connect with major veins or anastomose to 

 some extent with one another, but only rarely does the branching and 

 fusion suggest the reticulate structure of most carpels. 



Appendages of the stamen usually have no vascular tissue, but, in 

 the Melastomaceae, the anthers have prominent adaxial and abaxial 

 projections or lobes that are completely vascularized. These appendages 



