It is a prickly subject for ilex-ologists. 

Ilex, the genus of plants we know as holly, is a winter favorite and has many Yuletide uses. Ubiquitous as decorations and known as a beautiful symbol of the season, holly has hustled into our holiday habits. 

For botanists, though, these plants have been the subject of pointed debate. A look at the leaves leads to a lesson and a question. Why is there so much diversity in the spikiness of the leaves? Observers have noted that leaves can appear rounded, with a few sharp edges, or highly prickly around the entirety of the leaf. 

Diversity doesn’t happen in a vacuum and the diversity of holly leaves one wondering about its heterophylly. This wonderful word described the aforementioned quirkiness of character of the leaves on a single plant that occurs as a result of environmental factors. 

A few explanations exist as to why leaf morphology is inconsistent, though they are not all agreed upon. One intriguing study shows that spinescence, meaning tending toward spininess, is determined by herbivory. What a rush; I have waited a lifetime to learn and use the word spinescence in a sentence.  

The theory is that the prickliest leaves occur at herbivore level where animals browse, and is an epigenetic, or gene modification, in response to that stimulus. After leaves are eaten, they grow back with more spikes to resist further eating. This change in leaf is not generational and won’t change the plant’s DNA. 

One would think that the smoothest leaves would thus be at the top of the plant, away from the munching level of most animals. However, other scientists have observed that the location of the smooth leaves isn’t always at the top of the plant and they can be noticed at the bottom of the tree at a wholly munchable level.

This suggests another possible reason for the diversity: age. After a leaf is eaten, it will grow back spikier to deter continued consumption. Older leaves that were never browsed are rounded and not as prickly.   

No matter how sharp holly leaves are or aren’t, they still are a favorite. Our native species is American holly — ilex opaca — and has been revered for more reasons than festive finery.

Indigenous people made tea from its leaves and used this tisane as a cure for cough, pneumonia, flu and measles. The bark was also said to be steeped to treat malaria and epilepsy. The sap was considered an herbal tonic.   

The berries, though, pose a risk as they are considered poisonous to people, dogs and cats. Even just a few can cause illness, including dehydration, vomiting and diarrhea.

Remember that when using holly for decorations. Berries — which are only found on the female plant and only when there is a male plant nearby — will dry and drop from your wreaths and decorations. Animals and small children can find and inadvertently consume them. Poison control should be your first call if this happens. 

If you go out and look at the holly in your yard to observe leaf morphology, mission accomplished. Enjoy this terrific tree now with its greenery, after the holidays for its berries and the birds that they bring, and always for its sturdy and strong stance in our woods. 

Suzan Bellincampi is Islands director for Felix Neck Wildlife Sanctuary in Edgartown and the Nantucket Wildlife Sanctuaries. She is also the author of Martha’s Vineyard: A Field Guide to Island Nature and The Nature of Martha’s Vineyard.