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

250 ORDERING THE PATHS OF DIVERSITY
Figure 5.119. Tubular central
floret showing inferior ovary and
the calyx converted into a bristly
pappus.
Figure 5.120. Pollen
presentation mechanism. Florets
are protandrous and the piston
like style pushes pollen on to the
surface before the stigma lobes
open to become receptive.
Figure 5.121. Diverse
Asteraceae: (a) Bidens with
marginal strap florets mimicking a
5-petalled flower; (b) Centaurea
with marginal expanded disk
florets; (c) Galactites with a showy
involucre of bracts; (d) Cynara with
a head of disk florets and a spiny
involcre; (e) Solidago spikes of small
ligulate heads; (f) Echinops with a
head of capitula each with a single
floret.
the pollen onto the surface of the capitulum. Later the stigmatic lobes
open. The fruit of the Asteraceae is usually crowned by a pappus
derived from the calyx. The pappus in Asteraceae is very variable,
either absent or cup-like, or with scales, bristles, simple or feathery
hairs, which are barbed, or glandular. The fruit, called a cypsela, is a
kind of achene of an inferior ovary. The dispersal adaptations of the
fruit contribute to the success of many species as weeds.
The features described above have evolved in many groups outside
the Asteraceae. Many of the structures of a capitulate infloresecence
found in the Asteraceae are paralleled in other families in the Asterales
such as the Goodeniaceae (∼300 species), which has an indusium,
the Calyceraceae (∼55 species), Lobeliaceae (∼1200 species) and
Campanulaceae (∼600 species). Phyteuma and Jasione, in the Campanulaceae,
both have capitulate inflorescences surrounded by an involucre
of bracts. Jasione has a kind of primitive ‘pseudo-indusium’ formed
by swollen stigmatic lobes. The one species of the monotypic Brunoniaceae,
which is remarkably similar in appearance to Jasione and is
sometimes put in the Goodeniaceae, shows a further parallel in having
an involucrate head, though the head is cymose and the florets
are hypogynous. The piston-like mode of pollen presentation in the
Asteraceae also has parallels with that in the Campanulaceae (Physoplexis)
and Lobeliaceae.
The repeated evolution of these features argues strongly that
they are adaptive. One advantage of having a capitulum, for example,
is the protection given to the ovule and seed. Functionally it
provides a large showy target for pollinators and yet each ovule is
packaged separately, as a defence against predators and for dispersal.
There is a lot of diversity in the size and number of florets that
capitula contain. There are the familiar huge capitula of the sunflowers,
which have been selected by plant breeders. At the other end of
the spectrum many species have capitula containing very few florets.
(a) (b) (c)
(d) (e) (f)
(c)