Eudicots

Most gardeners are familiar with monocots and dicots, but what are eudicots? Let’s find out!

Flowering plants (angiosperms) are often categorized by the type of seed they make. You can see these differences with the naked eye. Seeds that come in a single body, such as corn, are classified as monocotyledons, or monocots. Seeds that split into halves, such as peas and beans, have been called dicotyledons, or dicots. Cotyledons are seed leaves. They rarely look like the other leaves produced by a plant. Monocots generally have a single seed leaf, while dicots have two seed leaves. So, how do eudicots fit in?

High tech botany

Electron microscopes and genetic mapping are drastically changing the way we look at plants. Superficial similarities can no longer be used to classify them. In 1991, an evolutionary botanist, James A. Doyle, and a paleobotanist, Carol L. Hotton, created the term ‘eudicot’ to differentiate between simple, primitive dicots and more modern tricolpate dicots. [How’s that for a word?] Tricolpate is another word for eudicot. Molecular research demonstrated that dicots are not what we thought they were. In fact, dicots are not even included in the new taxonomy! This is because dicots are not all descended from a single ancestor.

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[Did you know that the study of pollen grains and other spores is called palynology?
I didn’t, either.]

Scanning electron microscope image (500x) of pollen grains from sunflower, morning glory, prairie hollyhock, oriental lily, evening primrose, and castor bean. (Dartmouth) Public Domain

You will now begin hearing people talk about monocots, eudicots, and basal angiosperms. I would love to tell you that basal angiosperms are those primitive dicots, but it’s not that simple, either. For now, we will simply say that basal angiosperms are an “everything else” collection of flowering plants. If you want to get really technical, angiosperms are now divided into eight orders, instead of two, or three. Here’s the list and any examples I could find:

• Amborellales - a single species of shrub native to New Caledonia
• Nymphaeales - three families of aquatic plants, including water lilies
• Austrobaileyales - 100 species of trees, shrubs, and lianas, including star anise
• Chloranthales - [I couldn’t find any examples, sorry]
• Magnoliids - laurels, magnolias
• Monocots - grasses, grains, corn, bamboo
• Ceratophyllales - pond and marsh plants, such as hornworts
• Eudicots - everything else!

Eudicots are further separated into two groups: core eudicots and basal eudicots. Core eudicots include members of the sunflower family and the rose family. The basal eudicots are a more eclectic group. All this new information is being resorted using something called the APG IV system. We will discuss all of this in more detail another day. The reason behind much of this reclassification lies in pollen grain grooves.

Pollen grooves

Using an electron microscope, one can see that pollen grains have distinct patterns or grooves. Eudicots have three grooves, called colpi, that run parallel to the polar axis of the pollen grain. At the base of each of these colpi there are three or more openings, called germination pores. Most other seed-bearing plants, including gymnosperms and monocots, have only one germination pore. This pore is found in a groove called the sulcus. The sulcus is pointed in a different direction from the more visible grooves.

Eudicot pollen grain (Marie Majaura) CC BY-SA 3.0

Monocot pollen grain (Wikipedia) Public Domain

Using these new classification tools, we learn that eudicots make up 75% of all flowering plants and 50% of all plant species. At this point, that means there are over 280,000 eudicot species on Earth! Those species include apples, cannabis, figs, olives, oranges, peaches, peas, and plums, along with oaks, maples, and many others.

So, the next time someone starts talking about monocots and dicots, you can set them straight with the latest botanical discoveries!