Monocot vs. Dicot: A Tale of Two Tree Trunks in Tucson

My wife and I vacationed in Tucson, Arizona a few weeks ago to get away from the Wisconsin winter.

While wandering around the parking lot at one of our hotels, I noticed that a couple of trees had been cut down—guessing, but probably a mesquite tree (dicot) and a Mexican fan palm (monocot).  Take a look at the cross sections in the two photos and see how different they look.

As you no doubt recall, flowering plants are split into two groups based on how many seed leaves or cotyledons they have, one or two. Corn has one, so it is a monocot, and peas have two, so a dicot.  (As in “split pea soup”—two cotyledons.)  When the seed germinates, the stored starches and sugars in the cotyledons provide energy for the seedling so it can produce roots and leaves.

Monocots include grasses, lilies, sedges, orchids, onions and palms. Most shrubs and trees are dicots. (Pines, spruces and most other needle evergreens are not flowering plants; they are in the more primitive group of gymnosperms. So, neither dicot nor monocot.)

One of the ways dicots differ from monocots is how they move water and food around the plant. Dicots have xylem for water and phloem for food (e.g., starch molecules) evenly distributed around the outer edge of the stem or trunk, under the outer protective covering—bark in the case of trees. Palms have what are called vascular bundles distributed throughout the interior of the stem or trunk. In dicots, wood is the accumulation of each previous year’s now dead phloem and xylem, and you can see each year’s growth as tree rings.  Palms don’t make tree rings, they just add more vascular bundles as the tree trunk diameter increases with age.

Hence, the visible difference in the cross sections.