Chapter 12 



PHYLOGENY OF THE ANGIOSPERMS 



Relation of Monocotyledons to Dicotyledons 



Division of the angiosperms into two major groups, monocotyledons 

 and dicotyledons, has long been maintained, although there is, mor- 

 phologically, no clear-cut separation. Primarily, the bases for the dis- 

 tinction have been differences in cotyledon number, leaf venation, and 

 vascular structure of the stem. The dicotyledons have two cotyledons, a 

 terminal plumule, netted venation, and steles with cylinders of vascular 

 tissue; the monocotyledons have one cotyledon, a so-called lateral 

 plumule, parallel venation, and steles with scattered vascular bundles. 

 But there are many exceptions. Cotyledon number is not constant; taxa 

 with one cotyledon are frequent among dicotyledons, and some mono- 

 cotyledons have a second vestigial cotyledon (Chap. 9). In the dicotyle- 

 dons, there are examples of parallel venation — the Epacridaceae, espe- 

 cially DracophyUiim; in the monocotyledons, some of the Araceae and 

 Liliaceae have netted venation. A difference of detail in venation, the 

 presence of free vein endings in the dicotyledons, and closed venation 

 — absence of free vein tips — in the monocotyledons has been empha- 

 sized. But there is closed venation in many dicotyledons. 



Characters of less morphological significance are helpful in the sepa- 

 ration of dicotyledons and monocotyledons. In general habit, the mono- 

 cotyledons stand apart, because of their shortened internodes and fre- 

 quent bulbous, cormose, or rhizomatous form, linear leaves, and hypo- 

 geal germination. Rather few dicotyledons have seeds in which the 

 cotyledons remain underground and serve for food storage. Peperomia 

 shows stages in the development of this character; in some species, one 

 cotyledon remains underground (Fig. 136). 



The monocotyledon commonly has many adventitious roots that 

 largely take over the function of the primary root and sometimes replace 

 it, structurally, in the seedling. The dicotyledon has few adventitious 

 roots in the seedling, and a primary root is present, except in parasites, 

 saprophytes, and some other highly specialized taxa. 



There are fairly constant differences in flower structure. Spiral ar- 

 rangement of floral appendages is prominent in many of the lower 

 dicotyledonous families; it is present in only the lowest monocotyle- 



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