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

234 ORDERING THE PATHS OF DIVERSITY
Figure 5.74. Palm
inflorescences.
Figure 5.75. Bromeliad
inflorescence, showing the
disticous arrangement of bracts
and flowers common in the Poales.
Commelinids
arecales (palms)
The Arecaceae palms are the most important group of monocot trees
but, like other monocot trees, they are unbranched or only weakly
branched and lack a vascular cambium (Figure 5.30). Rather, secondary
growth occurs by the expansion, and occasional dichotomous
splitting, of a large apical meristem. The leaves of palms are large and
complex, often palmate or pinnately lobed, and highly folded. Their
flowers are relatively simple, following a standard monocot pattern
with parts in threes, but often they are grouped together in massive,
profusely branched inflorescences. About six subfamilies have been
distinguished based on the form of the leaf (fan or feather) and its
folding (rib down -- induplicate, or rib up -- reduplicate).
poales (grasses, sedges and rushes)
The Poales dominate most ecosystems where the growth of trees is
limited. The Typhaceae, Juncaceae and Cyperaceae are common dominants
of semi-aquatic and water-logged conditions. In contrast the
Poaceae dominate moist or dry habitats in many climates, everywhere
tree growth is restricted for any reason. They tolerate fire and grazing,
even mowing, regrowing rapidly after they have been damaged.
Grasses have a diverse of photosynthetic mechanism; C4 photosynthesis
is common and has evolved many times over, permitting them to
grow rapidly in the tropics.
The Xyridaceae have showy flowers. The Bromeliaceae have showiness,
but it is mostly provided by colourful bracts. The bromeliads are
very important epiphytes in the Americas as tank-plants or air-plants.
Their flowers are relatively unspecialised, but commonly each flower
is produced at the base of a brightly coloured bract. Most are epiphytic
and show many adaptations for their epiphytic life-style such
as water storage tissue or water-absorbing peltate scales. The air-plants
Tillandsia, including Spanish moss, are perhaps the most remarkable
in their ability to absorb moisture from the air. Most of the ground
bromeliads have adaptations for arid environments.