PHYLOGENY OF THE ANGIOSPERMS 463 



donous families. The floral whorls of the dicotyledons are predominantly 

 pentamerous; those of the monocotyledons are trimerous. In the dicoty- 

 ledons, calyx and corolla are usually unlike in appearance, as well as 

 morphologically; in the monocotyledons, calyx and corolla are usually 

 alike in appearance and tend to unite to form a pseudowhorl. 



The absence of a typical cambium has been emphasized as a distinc- 

 tive character of the monocotyledons, but weakly developed cambium 

 is present in the free vascular bundles of several monocotyledonous 

 families, a cambium like that of similar free bundles in the stems of 

 herbaceous dicotyledons. 



Differences have been seen in root anatomy; the dicotyledons have 

 small numbers of xylem poles, two or four, basically; the monocotyledon 

 root is typically polyarch. Exceptions are numerous. 



The nectaries of the monocotyledons are chiefly of the septal type; 

 the characteristic type of the dicotyledons is the transformed stamen. 



But the dicotyledons and monocotyledons have many close morpho- 

 logical resemblances, in spite of their obvious differences in general 

 habit and gross anatomy. 



Cotyledon number has little value in separating the major groups of 

 angiosperms. Among the dicotyledons there are scattered genera and 

 species that have a single cotyledon, and some of tlie monocotyledons 

 have a second, smaller, usually vestigial cotyledon. 



In anatomical structure — in steles, nodes, leaf traces, meristems, vas- 

 cular tissues — there are no basic difiFerences. For example, the vessel and 

 the sieve tube have arisen independently in both monocotyledons and 

 dicotyledons — several to many times in each — and the histological proc- 

 ess and sequence are the same. (The vessels of the Gnetales also have 

 independent origin.) The scattered-bundle stele and the V-shaped, closed 

 vascular strand, characteristic of the monocotyledons, are present in 

 some of the Ranunculaceae. 



Differences in type of root system are those of degree only. The root 

 system of the monocotyledons is a modification of that of the dicotyle- 

 dons. Arrested growth of the primary root of the monocotyledon embryo 

 and seedling is present in all stages, especially in the woody types; 

 similar arrest is present in dicotyledons — Nymphaeaceae, Umbelliferae. 



Close similarities of the Ranunculaceae and the Nymphaeaceae to 

 the Alismataceae are seen as evidence of a monophyletic origin of the 

 angiosperms, but there are also prominent differences between these 

 families, especially in embryology. 



Though the phyletic relationships between monocotyledons and di- 

 cotyledons have been much discussed, no general agreement has been 

 reached as to which represents the more primitive stock. Each has 

 been considered the older, ancestral taxon, and they have been in- 



