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

230 ORDERING THE PATHS OF DIVERSITY
Figure 5.64. Acorus.
(a)
(b)
Figure 5.65. Alismatales:
(a) Butomus; (b) Lysichiton.
5.3.6 Monocots
The monocots represent by far the most evolutionarily successful
lineage of the eumagnoliids and represent about 25% of all flowering
plants, about 50 000 species. They are very diverse ranging from
tall palm trees to tiny aquatic plants like Lemna (duckweed). Numerically
they might be considered less important than the eudicots,
but they include the grasses that provide the great majority of food
for humans, either directly (wheat, rice, millet, etc.) or by feeding
domestic grazing animals. They are clearly a monophyletic group and
most have:
single cotyledon
sympodial growth (the palms Arecales are monopodial)
linear, parallel-veined leaves in which the leaf base surrounds the
stem
primary root soon aborts and a wholly adventitious root system
develops
closed vascular bundles and lack of interfascicular cambium
flower parts in threes
pollen development (microsporogenesis) successive
monosulcate pollen
The basal monocots are aquatic or semi-aquatic plants. As the
seedling germinates the single cotyledon elongates to push the primary
embryonic root, the radicle, out of the seed and down into
the wet mud. It is because of their lack of a vascular cambium that
monocots are well able to undertake elongating growth but poor at
thickening (secondary) growth. Elongating growth adapts them for
the aquatic, climbing and epiphytic niches where they predominate,
and also permits them to re-grow rapidly after grazing. The parallel
venation of their leaves is a consequence of the extension of the
leaves from a basal meristem. Indeed it is likely that parallel-veined
monocot leaves are homologous to the petiole of leaves in other flowering
plants. Monocot trees and shrubs undergo various different and
amorphous kinds of stem thickening, which is sometimes described
as anomalous, and they are usually either unbranched or only weakly
branched.
acorus (sweet-flag or calamus)
At the base of the monocot lineage are the two species of Acorus, a
rooted aquatic. It has a tiny bisexual flowers crowded in a spadix.
The carpels are primitive, intermediate between the ascidiate of the
ANITA group and folded ones of other flowering plants. It has been
utilised for centuries as a rush for floor covering because of its spicy
scented properties.