5.3 Class Magnoliopsida – flowering plants - Cambridge University ...

(all Faboideae/Popilionoideae). All these large genera are important in
open habitats, especially in the arid and semi-arid tropics.
rosales (roses and allies)
The Rosales include several interesting families such as the figs
and mulberries (Moraceae ∼1200 species), the elms (Ulmaceae ∼140
species), the nettles (Urticaceae ∼1050 species) and buckthorns
(Rhamnaceae ∼880 species) but Rosaceae (∼3000 species) is the largest
family. It is central to the evolution of many other families in the
Rosales but relatively difficult to circumscribe. For example the ovary
may be hypogynous, perigynous or epigynous and perhaps the only
widespread feature is the possession of a hypanthium, a floral cup
that is primitively small and saucer- or cup-shaped, or has evolved to
become large and ultimately connate with the carpels. Traditionally
the family has been divided into four subfamiles differing in the form
of the fruit (the Spiroideae -- an aggregate of follicles, the Rosoideae -an
aggregate of achenes, the Prunoideae -- a drupe, the Maloideae -a
pome). One of the smallest families in the order, with only three
species, but certainly not the least significant is the Cannabaceae
because it contains both hops and cannabis.
cucurbitales (gourds and begonias)
The Cucurbitales include three families interesting for different reasons.
The Coriariaceae have only one widely distributed genus, Coriaria
(Mexico to Chile; Mediterranean to Himalayas/Japan; New Guinea
to New Zealand and W Pacific) but it is interesting because it has a
nitrogen-fixing association with Frankia in root nodules. The Begoniaceae
have only two genera, Symbegonia with 12 species from New
Guinea and Begonia with over a thousand. Symbegonia differs from
Begonia in having a corolla fused in a tube. Begonia are succulent
herbs and shrubs. Water conservation is aided by Crassulacean Acid
Metabolism (CAM) and they have stomata in clusters. The Cucurbitaceae
are climbers with tendrils and are important commercially
as gourds, squash, melons, etc. These families share a tendency to
fleshy or juicy tissues.
fagales (oaks and beeches)
The Fagales include many familiar wind-pollinated trees of temperate
regions. The related families of trees, Fagaceae (beech, hornbeam
and oak), Betulaceae (birch and alder), Casuarinaceae (she-oaks) and
Juglandaceae (walnuts) exhibit a wide range of adaptations for windpollination.
Different genera show different degrees of reduction of
the flower and its aggregation into unisexual catkins. Wind-pollinated
species are concentrated in the temperate regions but some genera
like the oaks and hornbeams have insect-pollinated species in the
tropics and here have stiff erect catkins. Walnut (Juglans) and wingnut
(Pterocarya) have a tiny but well-formed perianth. In Oak (Quercus robur)
male florets have a six-lobed perianth and seven to eight stamens but
female florets have a minute perianth but a scaly cupule. Hazel (Corylus)
has male florets consisting of two bracteoles and four stamens
only with a bract at the base and female flowers that are surrounded
5.3 CLASS MAGNOLIOPSIDA – FLOWERING PLANTS 243
Figure 5.98. Rosaceae: Rosa.
Figure 5.99. Cucurbitaceae:
Cucurbita.
Figure 5.100. Fagaceae: Alnus.