The history behind the classes.
Traditionally, the
flowering plants
have been divided into two major groups, or classes,: the
Dicots (Magnoliopsida) and the
Monocots
(Liliopsida). Many people take this separation into two
classes for granted, because it is "plainly obvious", but botanists have
not always recognized these as the two fundamental groups of angiosperms.
Although Theophrastus (circa 370 BC) is credited with first recognizing
differences between the two groups, classification of plants was based upon
overall growth form -- trees, herbs, vines -- until the 1600s.
In 1682, John Ray published his Methodus Plantarum Nova, in which Dicotyledones and Monocotyledones were first given formal taxonomic standing. This system was popularized by the French botanist Antoine Laurent de Jussieu in his Genera Plantarum of 1789, a work which improved upon, and gradually replaced, the system of plant classification devised by Linnaeus.
Even after the general acceptance of Monocots and Dicots as the primary
groups of flowering plants, botanists did not always agree upon the
placement of families into one or the other class. Even in this century some
plants called paleoherbs
have left problems for taxonomy of
angiosperms. These plants have a mix of characters which do not occur
together in most other flowering plants. For instance, the
Nymphaeales,
or water lilies, have reticulate venation in their
leaves, and what may be a single cotyledon
in the embryo. It is not clear whether it is a single lobed cotyledon, or two
which have been fused. The water lilies also have a vascular arrangement in
their stem similar to that of monocots.
There are also monocots which posses characters more typical of dicots.
The Dioscoreales
and Smilacaceae have broad reticulate-veined leaves;
the Alismataceae have acropetal leaf development; and
Potamogeton is one of several monocots to have floral parts
in multiples of four.
This "fuzziness" in the definitions of Monocotyledonae and Dicotyledonae
is not simply the result of poor botany. Rather, it is a real phenomenon
resulting from the shared ancestry of the two groups. It is now believed that
some of the dicots are more closely related to monocots than to the other
dicots, and that the angiosperms do not all fit neatly into two clades. In
other words, the dicots include a basal paraphyletic group from which
the monocots evolved.
Click here
for a cladogram which illustrates our current understanding of basal
angiosperm relations.
Despite the problems in recognizing basal angiosperm taxa, the standard
distinctions between dicots and monocots are still quite useful. It must be
pointed out, however, that there are many exceptions to these characters in
both groups, and that no single character in the list below will
infallibly identify a flowering plant as a monocot or dicot.
The table summarizes the major morphological differences between
monocots and dicots; each character is dicussed in more detail below.
For more information, refer to the page on
monocot
morphology.
Having taught in introductory botany for more than five years, I have fielded
many questions from students, and present below some of the more common
questions and misconceptions. Thanks go to my students for taking an
active role in their own education, and asking these questions
Q: Are pine trees monocots or dicots?
A: Pines are conifers, and are neither monocots nor dicots. Only
flowering plants are considered to be members of these two classes. This
question is similar to asking whether a chicken is a monocot or a dicot; it is neither.
Q: Do all dicots produce flowers?
A: Yes, sort of. All dicots and monocots are flowering plants, and so are
descended from flower-producing plants. However, the flowers are not
always large and showy the way we expect flowers to be. Oaks, maples, and
sycamore are all dicot trees, but they do not produce obvious flowers.
Grasses and cattails are monocots whose flowers are often overlooked
because they do not have sepals or petals.
There are also some flowering plants which flower only rarely. Duckweeds
are tiny flowering plants which reproduce and spread primarily by
vegetative growth; they grow by cellular division, and the resulting
cluster will then break apart.
Q: If monocots don't have wood, then what supports palm trees?
A: Palms rely on overlapping leaf bases, thickened enlarged cells, and prop
roots to stay up. This strategy is also used by cycads and tree ferns. We
hope to have a special exhibit soon expanding on the architecture of trees
which will explain this in more detail.
The fuzzy distinction between the classes.
The characters which distinguish the classes.
MONOCOTS
DICOTS
Embryo with single cotyledon
Embryo with two cotyledons
Pollen with single furrow or pore
Pollen with three furrows or pores
Flower parts in multiples of three
Flower parts in multiples of four or five
Major leaf veins parallel
Major leaf veins reticulated
Stem vacular bundles scattered
Stem vascular bundles in a ring
Roots are adventitious
Roots develop from radicle
Secondary growth absent
Secondary growth often present
Common questions about the classes.