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

236 ORDERING THE PATHS OF DIVERSITY
Figure 5.79. Commelinaceae:
Tradescantia gigantia, an upright
form (Enchanted Rock, Texas).
Figure 5.80. Zingiberaceae: (a)
Zingiber; (b) Heliconia.
commelinales (spiderworts)
The Commelinales are mainly herbs like the Haemodoraceae and
Commelinaceae (Figure 5.79) but include the aquatic Pontederiaceae.
Frequently the flowers are zygomorphic (monosymmetric). Heteromorphic
flowers are found in the Commelinaceae (enantiostyly) and
Pontederiaceae (heterostyly).
zingiberales (gingers)
The Zingiberales may be regarded as plants that do not produce an
aerial stem except when flowering (Figure 5.80). They have showy,
often strongly zygomorphic flowers especially adapted to bird pollination.
The form of the flower with a large lip (labellum) is only
exceeded in complexity in the monocots by the orchids. Sepals
and petals are fused (connate) and sterile stamens (staminodes) are
petaloid. The different families illustrate a great variety of specialisations
for pollination in the tropics. The bananas, Musaceae, are
the largest of all herbs, though they look like trees. The gingers,
Zingiberaceae, include more than a thousand species of tropical
herbs. In some ways the flower is analogous to that of the orchids
with its pronounced zygomorphy and adnation of a single stamen to
the perianth with other stamens converted into tepals and the anther
supporting a slender style. However, the filaments and style are long
and the anthers and stigma are exposed so that the pollinator is not
as effectively ‘controlled’ as in the orchids. The Costaceae have five
staminoids connate as a labellum and the stamen is broadly petaloid.
Pollinators include hummingbirds and large bees. As in the gingers,
the anther supports the slender style. The arrowroots (Marantaceae)
have an asymmetrical flower with a single median stamen, which is
half petaloid and all others are staminoidal and petaloid. The style is
under tension and triggered to scoop pollen from the bee pollinator.
Canna (Cannaceae), or Indian shot, has two whorls of three tepals, and
up to five petaloid staminoids, which are showier than the perianth.
In addition the fertile stamen and style are petaloid.
(a) (b)