THE PLANT BODY 23 



The ectophloic stele is the common type, but amphiphloic steles are 

 frequent — Apocynaceae, Asclepiadaceae, Campanulaceae Compositae, 

 Convolvulaceae, Cucurbitaceae, Gentianaceae, Myrtaceae. Internal 

 phloem is especially well developed in the Solanaceae and Cucurbita- 

 ceae, where, in herbaceous genera, it may play a prominent part in 

 conduction. The phylogenetic relations of these stelar types in the 

 angiosperms are uncertain. The sometimes apparently vestigial or de- 

 generate condition of the internal phloem and its late ontogenetic de- 

 velopment suggest that the amphiphloic stele may be more primitive 

 than the ectophloic, but the absence of internal phloem in primitive 

 families and its presence in some advanced families does not support 

 this concept. If the angiosperms are polyphyletic, both stelar types 

 may have been present in the stock from which they arose. 



Anomalous vascular structvire is frequent in angiosperm roots and 

 stems. The peculiarities are various and are chiefly in the secondary 

 tissues and in their method of origin and development. The original 

 cambium of the young stem or root may cease to function and may be 

 replaced by other secondary cambia which arise outside of the phloem. 

 A cambium of normal type and persistence may form xylem and phloem 

 of unusual distribution and arrangement. Excessive multiplication of 

 parenchyma in restricted regions may break the original cylinder into 

 strips. A cambium that develops tissue only centripetally may form 

 complete vascular bundles embedded in "interfascicular tissue." Anoma- 

 lous steles are doubtless in large part adaptations to unusual function, 

 habit, or habitat; they occur in many woody vines and parasites, and in 

 food-storage organs. They may characterize families — Chenopodiaceae, 

 Amaranthaceae. 



Anatomy of the Root. The root is typically protostelic; in specialized 

 types, a pith may be present. Angiosperm root steles range from mon- 

 arch to polyarch, but are most commonly diarch and tetrarch. The basic 

 types are clearly the diarch and tetrarch, and each of these has been 

 claimed to be the most primitive. The diarch type is characteristic of 

 large groups of herbaceous genera — members of the Ranales, Rhoead- 

 ales, and Urticales, for example. Woody taxa commonly have tetrarch 

 roots. The less common stelar types seem more likely to have been 

 derived, phylogenetically, from the tetrarch than from the diarch. 

 Diarch and monarch are probably reduction types; monarch steles are 

 found in greatly reduced roots. The roots of the monocotyledons are 

 typically polyarch and usually have a pith. It seems probable that the 

 tetrarch stele is the primitive type; it best fits, structurally, into hypo- 

 cotyls and into the pattern of the two-trace node; it is characteristic of 

 most of the Archichlamydeae and is rare in the Metachlamydeae. The 

 polyarch stele with a pith seems to be highly specialized. 



