Monocots versus Dicots

The Two Classes of Flowering Plants


Contents


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.


The fuzzy distinction between the classes.

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.


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 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

  • 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.