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

Insect pollination is closely associated with the origin and subsequent
diversification of flowers. However, it is important to remember
that insect pollination is associated with several other groups of seed
plants, both living and extinct: Bennettitales, Gnetales, Cheirolepidaceae
(extinct conifers), Cycadales and Medullosales (seed ferns).
Insects grew in diversity with the origin of seed plants in the Late
Devonian and this increase of diversity with the origin of flowers
is just part of a continuous trend. Indeed there is some evidence
to indicate a temporary decline in insect diversity as flowering plants
became more abundant in the Cretaceous. Nevertheless, flowers diversified
in parallel with particular groups of pollinator specialists,
the bees (Apoidea/Apidae), the pollen wasps (Vespidae: Masarinae),
brachyceran flies (Acroceridae, Apioceridae, Bombyliidae, Empididae,
Nemestrinidae, Stratiomyidae and Syrphidae) and the moths and butterflies
(Lepidoptera). The evolution of a bisexual reproductive axis
was a crucial event.
Several trends in floral evolution can be discerned. Primitive flowerseitherlackaperianthorhaveoneinwhichthereisasingle
whorl of tepals. In the perianth there has been a trend from having
a perianth in which distinct whorls are not clearly differentiated to
clear specialisation of a distinct calyx and corolla. The calyx protects
against drought, temperature shock and predatory insects and the
corolla attracts and controls pollinators. Both calyx and corolla may
have been derived from tepals, but it is likely that in some groups,
such as the buttercups, the petals originated from stamens, to which
they are anatomically similar. In the peony, Paeonia, there is a gradual
transition from leaves, through modified leaves on the flowering stem
called bracts, into the perianth with parts at first sepal-like and then
petal-like. Generally there has been a trend for the greater integration
of the floral parts with greater precision in number and placement as
flowers have become specialised to particular patterns of pollination.
An important aspect of the diversification of flowering plants and
their evolutionary success has been their vegetative flexibility. They
have evolved into a bewildering range of forms through the activity
of sub-apical and intercalary meristems. Flowering plants also have
a greater capacity for elongation of cells, including root hairs and
trichomes. One of the most important vegetative specialisations has
been their possession of vessels in their wood, permitting more efficient
water transport and hence greater photosynthetic rates, fast
growth rates and earlier maturation. There have been significant
adaptations in seasonal habitats; for example, although a few nonflowering
plants are deciduous this has been a particular floweringplant
trait that has adapted them to high latitudes or seasonally-dry
environments.
One aspect to the burgeoning biotic diversity of flowering plants
from the Cretaceous onwards was their great expansion of chemical
diversity. New ranges of what have been called secondary compounds
provided attractants for pollinators or seed dispersers, and repellants
and toxic compounds to inhibit herbivores.
5.3 CLASS MAGNOLIOPSIDA – FLOWERING PLANTS 225
(a)
(b)
Figure 5.55. Specialisation in
the perianth in the Asparagales:
(a) Belamcanda; (b) Kniphofia.