24 MORPHOLOGY OF THE ANGIOSPERMS 



Anatomy of the Shoot. The vascular structure of the shoot axis in 

 the angiosperms is complex and has a pattern that is repeated from 

 node to node. The nodal sections are more complicated than the inter- 

 nodes, because of the presence of divergent vascular strands — leaf traces 

 — which extend from the internodal system to the leaf. Branch traces, 

 always two, arise just above the leaf traces and similarly connect the 

 vascular stele of the branch with the stele of the mother axis. The num- 

 ber of leaf traces ranges from one to many and is characteristic of taxa; 

 it is phylogenetically significant. 



The term leaf trace was first applied to the traces of a leaf, collec- 

 tively, but is now applied to the individual strand, because number of 

 strands per leaf is important. A leaf trace cannot be delimited rigidly, 

 because it is continuous from stele to leaf. Distally, it is considered to 

 end where it passes into the cortex. In the cortex, it may fork or unite 

 with other traces. (The base of the petiole may show a number differ- 

 ent from that of the stele. ) Proximally, the trace unites with the stelar 

 cylinder or with bundles of the internodal steles at various levels, ac- 

 cording to patterns related to trace number and phyllotaxy. The stele 

 may be siphonostelic, with an essentially unbroken vascular cylinder, or 

 eustelic, with "free" bundles of various sizes. The eustelic type has 

 been incorrectly considered characteristic of herbs, but many herbaceous 

 genera — Nicotiana, Salvia, Aster, Hypericum — have siphonostelic stems. 

 Woody angiosperms have both types of steles. The eustele, with re- 

 duced vascular tissue, is perhaps the advanced stelar form. 



Areas where cortex and pith are continuous are termed leaf and 

 branch gaps, respectively. Where the primary vascular stele has the 

 form of a more or less unbroken cylinder, breaks are present above and 

 often lateral to the leaf and branch traces where they depart from the 

 cylinder. Each trace may have its own gap, or two or more traces may 

 have a common gap. Nodes are termed unilacunar, trilacunar, and 

 multilacunar, where the number of gaps is one, three, and many, with- 

 out regard to number of traces per gap. In a unilacunar node, more 

 than one trace may be associated with the gap. 



The primary vascular cylinder is often weakly developed, and the 

 gaps may not be obvious, becoming prominent only after secondary 

 thickening has begun. In typical eusteles, where the cylinder consists 

 of more or less isolated sti'ands, gaps are usually not apparent; they 

 have been considered as merged with the spaces between the bundles. 

 In the floral shoot, nodal anatomy is like that in the vegetative shoot. 

 This similarity is strong evidence that floral appendages are of leaf 

 rank, that they are not organs sui generis, not mere lobes of the axis, 

 not mere spore-bearing areas (Chap. 3). 



The trilacunar node, with three traces (Fig. 6C), has long been 

 looked upon as the primitive type in angiosperms, a type associated 



