26 



MORPHOLOGY OF THE ANGIOSPERMS 



or examples of fused organs — are now seen to be persisting examples of 

 early nodal structure in angiosperms. The odd-numbered types, so char- 

 acteristic of angiosperms in general, have been derived by the union of 

 the tv^'O traces of the primitive node, as a part of the specialization of 

 stem and leaf. Fusion of the two traces of a unilacunar node gives a 

 one-trace unilacunar node; the addition of lateral traces in pairs, with 



their individual gaps gives tri- and 

 multilacunar nodes with odd-num- 

 bered traces (as under the older 

 theory of nodal specialization) (Fig. 

 6C, D ) . Advanced types of unilacunar 

 nodes with a single trace have devel- 

 oped by the loss of lateral traces and 

 by the lateral union of three or more 

 traces and their gaps (Fig. 6E, F, 

 G). In these nodes, the "compound" 

 trace may be three-parted or massive 

 — Asclepias. The lateral union of 

 three or more traces — the median 

 trace of double nature — is more com- 

 mon in floral shoots, where the ap- 

 pendages are crowded, than in 

 vegetative shoots. The carpels of 

 some dicotyledons may show stages in 

 the fusion of three traces from three 

 gaps to form one trace from one gap — 

 Ruhus spp. Ranunculus Ficaria, Anem- 

 one canadensis. (In these examples, 

 the reduction and simplification of 

 nodal structure accompanies the evo- 

 lutionary modification of a follicle in 

 achene development. The carpels appear to be one-trace organs until the 

 origin of their vascular supply is determined. ) 



Two-trace unilacunar nodes are known in the stems of the Austro- 

 baileyaceae (Fig. 5), Chloranthaceae, Lactoridaceae, Trimeniaceae, and 

 some Verbenaceae, Labiatae, and Solanaceae. In most of these taxa, the 

 traces are independent well down in the eustele. In cotyledons and 

 sporophylls also, there are frequent examples of paired or fused median 

 traces. Paired traces are probably more numerous in cotyledonary than 

 in vegetative nodes (Figs. 7 and 8). In reproductive nodes, they are 

 doubtless more frequent than is known; tiieir presence cannot be so 

 readily determined as in leafy stems, but two-trace floral organs are 

 occasional. Double median bundles are frequently found in the carpels 



Fig. 7. Forms of cotyledonary vena- 

 tion showing double traces and ex- 

 tent of independence of the two vas- 

 cular systems within the cotyledon. 

 {After Bailey.) 



