230 MORPHOLOGY OF THE ANGIOSPERMS 



carpel building has been the basis for many of the errors of interpreta- 

 tion that have formed a basis for the peltate theory, with its obscure 

 "meristem," the "cross zone." 



Anatomy of the Carpel 



In vascular structure, carpels are closely like leaves. A three-trace, 

 three-gap system was generally accepted as basic for the carpel through 

 most of the first half of the twentieth century, but studies of the woody 

 Ranales during the later part of this period have shown that a two-trace, 

 one-gap is probably primitive for all appendages of the stem (Fig. 6). 

 The three-trace system, doubtless an early modification of the two-trace 

 system, is common in carpels. Phylogenetically, it has been formed by 

 the union of the two traces of the primitive organ to form a mor- 

 phologically double median trace, with two additional lateral traces, 

 one on each side, with separate gaps. The several-ti^ace and the single- 

 trace types have been developed from the three-trace, three-gap, as in 

 leaves. 



A two-bundle, median carpel trace, consisting of two independent 

 bundles, is rare in carpellary modes (although common in cotyledonary 

 nodes), but double and two-forked midveins are frequent. Some two- 

 lobed stigmas are supplied by the two midveins or their branches. 

 Carpels with more than three traces are frequent. Carpels that are 

 structurally reduced have one, or three in various stages of reduction to 

 one. Reduction in vascular supply — traces and laminar bundles — paral- 

 lels reduction in ovules. Both median and lateral supplies are reduced 

 by distal shortening and by lateral union. That loss of lateral (placental) 

 bundles is by progressive distal shortening is apparent from comparisons 

 in achenes of related genera; that the reduction may extend to elimina- 

 tion of the traces and their gaps is seen in some species where individ- 

 ual carpels have from one gap a single trace representing traces fused 

 before entering the carpel. (Lateral fusion is a stage in the loss of the 

 lateral traces.) Reduced follicles and achenes show stages in the sim- 

 plification by reduction of the vascular supply — traces and laminar 

 strands (Fig. 42). Shortening of the ventral (placental) bundles ulti- 

 mately brings the ovule to a position where it appears to be borne on 

 the dorsal (midrib) bundle, but anatomical comparison with other 

 achenes shows that the ovule is morphologically borne on a fusion 

 bundle that represents the united dorsal and ventral bundles. An ovule 

 in this position has been called "basal" (a satisfactory term for simple 

 description, but incorrect morphologically, because the ovule belongs 

 on a submarginal placenta). An ovule in this position has also been 

 called "cauline," because it appears to be strictlv basal (if anatomy is 

 disregarded) and merely surrounded by the carpel. 



