Taxonomy of Flowering Plants - LECTURE NOTES - Fall, 1998
Hugh D. Wilson - rm. 306, Butler Hall
23 September 1998
The Hamamelidae
 
Subclass Hamamelidae, the smallest of the dicot Subclasses, includes 11 orders, 24 families and about 3,400 species.  About 90% of the species occur in two orders, the Urticales and Fagales.  The group is weighted in taxa at the higher ranks (Orders with few families and families with few species) and its fossil record extends to the lower Cretaceous (100 million years ago).  Some believe that the group is either marked by high levels of extinction, thereby producing an array taxa well separated, more or less relictual taxa or, perhaps, polyphyletic (an artifical assemblage with elements drawn from several ancestral types.  Families included by Cronquist in this Subclass are typically woody (Urticales an exception) and characterized by a suite of structural adaptations associated with wind pollination or anemophily.  These features include:

    unisexual (imperfect) flowers with taxa either monoecious or dioecious

    perianth reduced, weakly developed or absent (apetalous)

    inflorescences with numerous, small (reduced), often apetalous flowers with the most specialized type the catkin or ament (older name for the group is 'Amentiferae')

 
 

The Order Trochodendrales is placed at the base of the Hamamelidae and the Genus Casuarina (Australian Pine - Casuarinaceae, Casuarinales) takes the most derived or specialized position.  Elements of the Trochodendrales (two, monotypic families) lack vessel elements and tend toward apocarpy whereas Casuarina shows multiple (fruiting pistillate inflorescence) fruiting structures. Our coverage of the Subclass Hamamelidae will include:

Urticales
     Ulmaceae
     Moraceae
    Cannabaceae
Juglandales
    Juglandaceae
Fagales
    Fagaceae

Since the two subclasses covered represent two, distinct 'models' for the primitive or archaic flowering plant, we will divert from the survey to take a quick look at the history or evolution of flowering plant classification systems.

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