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

242 ORDERING THE PATHS OF DIVERSITY
(a)
(b)
(c)
Figure 5.95. Cesalpinioideae:
(a) Delonix; (b) Cassia, half-flower;
and (c) side and front view.
(a)
Figure 5.96. Faboideae: (a)
Trifolium (keeled flowers in a head);
(b) Vicia (half-flower and dissected
flower).
Figure 5.97. Mimosoideae;
Acacia: (a) florets in spherical
heads; (b) single floret and a pistil.
fabales (legumes)
The Fabales include the Fabaceae, sometimes recognised as a single
family, with three subfamilies, or as three families (Figure 5.95). They
all share the characteristic of a kind of fruit, the legume that gives
them their alternative name, the Leguminosae. The Caesalpinioideae
(∼2000 species) have large showy, more or less regular flowers. They
are basal in the family and the other two families show different
patterns of specialisation.
The Mimosoideae (∼3100 species) have small regular flowers in
dense spikes or heads, brush blossoms with numerous exserted stamens.
Two evolutionary trends are observed in the Mimosoideae. One
trend is for an increase of the number of stamens although each
has a tiny anther. The filaments are long and the stigma is small
and cup-shaped. Pollen is released as a polyad. In Acacia only one
polyad can fit on each stigma and the number of seeds produced in
the legume is directly related to the number of pollen grains in the
polyad. Another evolutionary trend shows a reduction of the number
of stamens but specialisation of the flowers in the head to form a
kind of pseudanthemum. For example, in Parkia the lower florets are
showy and scent-producing but sterile, the intermediate ones sterile
but nectar producing and the upper fertile.
The Caesalpinioideae and Mimosoideae are mainly trees and
shrubs but most of the third subfamily, the Faboideae (∼11 300
species) are herbs. They have the zygomorphic flower that gives them
their alternative name (Papilionoideae). The flag blossom is pollinated
by large bees which land on the keel. The nectary is at the base of
a staminal tube. In forcing its proboscis into the staminal tube the
heavy insect pushes the keel petals (alae) down and the stamens and
stigma rub against its ventral surface. A similar type of papilionate
flower has evolved in parallel in the Polygalaceae, also in the Fabales.
The Faboideae includes many agriculturally important species, peas
and beans of all sorts. Clovers are especially important in pasture.
(b) 5.97 (a) (b)
The stereotypical nature of the Fabales/Fabaceae is emphasised
because about one third of all species in the order/family belong
tooneofafewverylargegenera:Acacia (1200 species), Mimosa (400
species) (both Mimosoideae), Cassia (540 species) (Caesalpinioideae),
Astragalus (2000 species), Crotalaria (600 species), Infigofera (700 species)