22 MORPHOLOGY OF THE ANGIOSPERMS 



commonly added a secondary body, by the activity of secondary meri- 

 stems of the cambium type. The secondary tissues so formed may ob- 

 scure, distort, or destroy parts of the primary body. The various organs 

 may consist entirely of primary tissues (formed by primary meristems) 

 or partly of primary and partly of secondary tissues. The primary body 

 is complete in itself in so far as the presence of all basic organs is con- 

 cerned — root and shoot (stem and leaves, sterile and fertile). In all 

 these parts, vascular tissues — forming the vascular skeleton — are promi- 

 nent structural features. In the axis (root and shoot), a central core, 

 the stele or central cylinder, is more or less clearly set apart from the 

 surrounding cortical and epidermal tissues. Limiting the stele is the 

 endodermis, a uniseriate layer of specialized cells. Morphologically, the 

 endodermis is probably best considered the outermost layer of the stele. 

 In the angiosperms, as compared with lower vascular plants, the endo- 

 dermis is greatly reduced in distribution and in histological structure. 



The anatomical structure of the primary body of the angiosperms, 

 especially that of the shoot, is complex. The modifications of high 

 specialization, with reduction, accompanying the herbaceous, aquatic, 

 epiphytic, and parasitic habits, have brought about vascular structure 

 that may be difficult to interpret. 



A basis for the interpretation of the vascular structure of the stele 

 was provided by the stelar theory, late in the nineteenth century. This 

 theory has been somewhat modified and new terms added, but satis- 

 factory definitions are difficult to make and there has been much loose- 

 ness in use of the terms applied to the stele and its vascular prolonga- 

 tions. The terms protostele and siphonostele were applied to two basic 

 types: protostele, to a central cylinder with a solid (pithless) vascular 

 core; and siphonostele, to a cylinder with a tubular vascular core and 

 central pith. The siphonostele, when broken by openings (gaps) where 

 traces pass out to lateral appendages, is phyllosiphonic; where gaps are 

 formed by branch traces alone, it is cladosiphonic. The term solenostele 

 has been applied to siphonosteles in which the leaf gaps in the inter- 

 node above are closed before the gaps associated with traces next above 

 appear, and dictyostele where the vascular tube is dissected into a 

 meshwork by overlapping leaf gaps. But, in general use, the term 

 dictyostele has been incorrectly applied to angiosperm steles; by origi- 

 nal definition, the vascular bundles of a dictyostele are concentric, and 

 this is a type unknown in angiosperms. The newer term eustele is now 

 in general use for steles with a vascular skeleton of collateral or bi- 

 collateral bundles which anastomose more or less freely. This is the 

 common type in angiosperm shoots. Steles in which phloem is present 

 only external to the xylem are ectophloic; those in which there is also 

 phloem internal to the xylem are amphiphlvic. 



