Taxonomy
of Flowering Plants - LECTURE
NOTES - Spring, 1998
Hugh
D. Wilson - rm. 306,
Butler Hall
23, 25, 30 March 1998
The Rosidae
The Subclass Dilleniidae, includes 18 orders, 116 families
and over 60,000 species. The largest subclass with regard to number
of families and more or less tied with the Asteridae with regard to species
divrsity. As was the case with the Dilleniidae, about 75% of the
species occur in five orders; in this case the Fabales (18,000),
Myrtales (9,000), Euphorbiales (8,000), Rosales (6,600)
and Sapindales (5,400).
A diverse set of flowering plant
orders that, like the Dilleniidae, lacks a distinct set of key characters.
General trends or features of the Subclass include:
-
syncarpy is the rule, with the exception
of the high frequency of apocarpy in the Rosales and monocarpy in
the Fabales and Proteales.
-
Leaves are often simple or, if compound,
often pinnately compound
-
petals usually distinct, sometimes
wanting, sometimes connate at the base, rarely sympetalous.
-
nectary disks of various types are
frequently encountered, many stamodial in origin and positioned at the
base of the ovary.
-
placentation is various, but
most often axile with, generally, fewer ovules per locule than found in
the Dilleniidae.
Cronquist: "In the last analysis, the Rosidae and
Dilleniidae are kept apart as subclasses because each seems to constitute
a natural group separately derived from the ancentral Magnoliidae,
rather than because of any definitive distinguishing characters...it is
conceptually more useful to hold the two as separate subclasses than to
combine them into one or to abandon any attempt at organization of the
Magnoliopsida into subclasses." THEREFORE: Focus here is
on distinctive characters at the order and family level.
Our coverage of the Rosidae will include:
Rosales
Crassulaceae
Rosaceae
Fabales
(Leguminosae)
Mimosaceae
Caesalpiniaceae
Fabaceae
Myrtales
Onagraceae
Euphorbiales
Euphorbiaceae
Apiales
Apiaceae
(Umbelliferae)
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