Download The characters which distinguish the classes

The characters which distinguish the classes.
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.
MONOCOTS
Embryo with single cotyledon
Pollen with single furrow or pore
Flower parts in multiples of three
Major leaf veins parallel
Stem vacular bundles scattered
Roots are adventitious
Secondary growth absent
DICOTS
Embryo with two cotyledons
Pollen with three furrows or pores
Flower parts in multiples of four or
five
Major leaf veins reticulated
Stem vascular bundles in a ring
Roots develop from radicle
Secondary growth often present
 Number of cotyledons -- The number of cotyledons found in the embryo is the actual basis for
distinguishing the two classes of angiosperms, and is the source of the names Monocotyledonae ("one
cotyledon") and Dicotyledonae ("two cotyledons"). The cotyledons are the "seed leaves" produced by the
embryo. They serve to absorb nutrients packaged in the seed, until the seedling is able to produce its first
true leaves and begin photosynthesis.
 Pollen structure -- The first angiosperms had pollen with a single furrow or pore through the outer layer
(monosulcate). This feature is retained in the monocots, but most dicots are descended from a plant which
developed three furrows or pores in its pollen (triporate).
 Number of flower parts -- If you count the number of petals, stamens, or other floral parts, you will find
that monocot flowers tend to have a number of parts that is divisible by three, usually three or six. Dicot
flowers on the other hand, tend to have parts in multiples of four or five (four, five, ten, etc.). This character
is not always reliable, however, and is not easy to use in some flowers with reduced or numerous parts.
 Leaf veins -- In monocots, there are usually a number of major leaf veins which run parallel the length of
the leaf; in dicots, there are usually numerous auxillary veins which reticulate between the major ones. As
with the number of floral parts, this character is not always reliable, as there are many monocots with
reticulate venation, notably the aroids and Dioscoreales.
 Stem vascular arrangement -- Vascular tissue occurs in long strands called vascular bundles. These
bundles are arranged within the stem of dicots to form a cylinder, appearing as a ring of spots when you cut
across the stem. In monocots, these bundles appear scattered through the stem, with more of the bundles
located toward the stem periphery than in the center. This arrangement is unique to monocots and some of
their closest relatives among the dicots.
 Root development -- In most dicots (and in most seed plants) the root develops from the lower end of
the embryo, from a region known as the radicle. The radicle gives rise to an apical meristem which
continues to produce root tissue for much of the plant's life. By contrast, the radicle aborts in monocots, and
new roots arise adventitiously from nodes in the stem. These roots may be called prop roots when they are
clustered near the bottom of the stem.
 Secondary growth -- Most seed plants increase their diameter through secondary growth, producing
wood and bark. Monocots (and some dicots) have lost this ability, and so do not produce wood. Some
monocots can produce a substitute however, as in the palms and agaves.
Common questions about the classes.
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.
Monocots differ from other flowering plants in a number of features.
Almost every basic botany course includes a unit in which dicot and monocot characters
are compared. If you have not been through such a course, or if it has been a while since
then, you may wish to view our page comparing monocots and dicots. The difficulty is
that most of these characters are neither unique to monocots nor universally present
among the members. This is because of the simple fact that plants do not evolve to fit the
neat categories we create for them. This does not mean that such characters cannot be
useful in recognizing monocots, but it does mean that no one character will always work - it is best to compare on a number of features.
The three characters which are most closely congruent with the usual definition of
monocots are the embryo morphology, stem vascular structure, and the plastid protein
inclusions. The embryo of each seed plant possesses one or more leaf-like structures
called cotyledons, which absorb nutrients and may function as the first leaves in some
plants. Monocots have a single such cotyledon, while other flowering plants usually have
two. The exception to this is the water lily Nymphaea, which has what appears to be a
single cotyledon with two lobes, but which has usually been interpreted as two fused
cotyledons. Since the water lilies are quite closely related to the monocots, this may
represent a transitional form. The image below at right shows a monocot embryo with
cotyledon.
Monocot characteristics : At far left, a cross section through the stem of a corn stalk.
The small dark circles visible in the stem are strands of vascular tissue; imagine you are
looking down on the end of pipes that run up and down through the corn stem. The
vascular strands in monocots are not arranged in a neat ring, as they are in dicots, but are
scattered within the stem. At right, images of a corn kernel. The kernel is the fruit
produced from the corn flower, and each kernel contains an embryo that may develop
into a new corn plant. The embryo has only one cotyledon, which is a part of the embryo
used to absorb the nutrients stored in the endosperm, a food reserve stored for the young
plant. At center above, are corn kernels stained to show chemical composition. The top
central image is an unstained kernel sliced open. Below it is a kernel stained for protein
(orange), showing that the protein is concentrated in the embryo and its cotyledon. Below
that is a kernel stained for starch (black), showing that the endosperm is mostly starch.
Compare with the image at right.
The second character common to all monocots is the atactostele. This refers to the
arrangement of vascular strands in the stem, which in the case of monocots are scattered
in the stem, though more densely packed near the periphery of the stem. This feature is
not unique to monocots, but is found also in the paleoherbs and certain magnolia-like
dicots.
The third feature common to all monocots is the structure of certain plastid inclusions.
All plant cells contain plastids, some of which become photosynthetic, but those which
are found in the phloem tissue do not do so. When examined in properly prepared
material, the plastids in the phloem are shown to contain crystalloid proteins, the
shape of which is distinctive for different major groups of plants. In the monocots
and a few paleoherbs, these crystalloids have a triangular shape, unlike all other
flowering plants. This is an obscure character, but it is reliable.
Monocots exhibit a wide range of growth forms.
In addition to the variability of habitat, monocots also exhibit a wide range of growth
forms. Growth form, or more technically habit, refers to the overall architecture of a
plant, whether it grows upright from a trunk as do trees, or creeps along the ground like a
vine, or any number of other possibilities, only some of which will be mentioned here.
At least four times in their history, monocots have evolved tree-like growth forms. Most
monocots are herbs, and none have true wood. This might be expected to make the
evolution of tree-like growth forms difficult, but it obviously has not been an
insurmountable obstacle. In fact many groups of plants have evolved tree forms without
wood -- ferns, lycophytes, and sphenophytes to name a few. In the monocots, palms,
grass-trees, bamboo, pandanes, and yuccas are among the groups which grow as trees.
Each develops a massive central trunk which may bear leafy branches, as in yuccas and
pandanes, or may support the leaves directly, as in palms. An example of a monocot tree,
Dracaena draco, may be seen above in a photo taken in the Canary Islands.
A number of monocots have developed tree-like forms in which the leaves attach only at
the base of the plant. These include the massive agaves as well as bananas and the socalled traveller's palm. In these cases, the leaves may occur only near the base of the
plant, forming a rosette, or they may wrap around the stem, lending it support, as in
bananas. Some of the rosette forms have evolved succulent leaves, which are fleshy and
store water. These include the agaves, aloes, and many bromeliads.