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

apiales (umbels and ivy)
There are two families in the order (Apiaceae ∼3100 species and Araliaceae
∼800 species). The Apiaceae is a family that has many species
cultivated for food (carrot) or more frequently for spice (coriander,
cumin, etc.). The compound umbel is so characteristic of the Apiaceae
that the family was one of the first flowering plant families to be
clearly recognised as the Umbelliferae (Figure 5.117). However, the
compound umbel is confined to subfamily Apoideae. In some genera
like Hydrocotyle, in subfamily Hydrocotyloideae, the compound umbel
is reduced to a single floret; and in Eryngium, subfamily Saniculoideae,
the inflorescence is rounded rather than flat. The umbel is normally
visited by a range of different insects, which move freely over the
platform of the umbel. In each floret, the ovary has a disc at the
base of the paired styles. The styles are swollen, together forming a
nectariferous stylopodium. Flowers in different parts of the umbel
may be specialised. Those in lateral umbels are sometimes female
and sterile with abortive ovaries and shorter stylopodia. The ovary
has two carpels and matures into a dry fruit, a schizocarp, which
splits into two mericarps joined by a carpophore. Some species have
fruits with thick corky walls so that they float, have wings for wind
dispersal, or have hooks for animal dispersal. The Apiaceae have a
particularly effective combination of anti-herbivore chemical repellants
such as polycetylenes and sesquiterpene lactones. The Araliaceae
share many similarities with the Apiaceae but are mainly tropical and
rarely form regular compound umbels. They also produce a fleshy
fruit.
asterales (daisies, lobelias and bellflowers)
Most species of Asterales are in one family, the Asteraceae, which is
widely recognised as one of the most advanced families of flowering
plants (Figure 5.121). Its origin is relatively recent but it has 1100 genera
and 20 000 species. All species have a capitulum, a head with many
florets on a common and usually flattened receptacle, and surrounded
by an involucre of bracts. The involucre is particularly obvious in
‘everlasting’ flowers like Helichrysum, where it is showy. Florets are
tubular and epigynous with a single ovule. They are protandrous and
mature centripetally (i.e. the capitulum is racemose). The florets are
of different sorts, either polysymmetric or monosymmetric. The latter
have either three corolla lobes that are connate and greatly expanded
to form a strap, or all five corolla lobes extending in one direction.
Florets may be pistillate, hermaphrodite or functionally staminate.
Heads are made of a single kind of floret or a combination of kinds,
commonly with central disk of hermaphrodite, polysymmetric florets
and a margin of showy, monosymmetric ray florets, with the latter
pistillate or sterile, as in the daisy and sunflower. One group, which
includes lettuce and dandelion, have all florets with all five corolla
lobes forming the strap.
Florets are protandrous. Anthers form a tube. Pollen is shed into
the anther tube and then the immature style elongates and pushes
5.3 CLASS MAGNOLIOPSIDA – FLOWERING PLANTS 249
(a)
(b)
Figure 5.117. Apiaceae: (a)
Heracleum; (b) floret in section
showing stylidium and schizocarp.
(a)
(b)
Figure 5.118. Structure of a
capitulum (a) capitulum t.s.
showing central and marginal
florets; (b) florets of diverse sorts.