This series of posts will highlight the plants that help you celebrate the Yuletide season.
Did you have a Holly Jolly Christmas? Holly was used in Pagan and Roman celebrations prior to its association with Christian traditions. Its characteristic colors and shape offered a fitting juxtaposition of symbolism for the celebration of Christ (refer to lyrics below) leading to its integration into our Christmas traditions. This evergreen tree has been important for winter celebrations for millennia, but what about the biology behind the boughs and berries?
Of all the trees that are in the wood, The holly bears the crown
The holly bears a blossom as white as lily flower,
And Mary bore sweet Jesus Christ to be our sweet saviour
The holly bears a berry as red as any blood,
And Mary bore sweet Jesus Christ to do poor sinners good.
The holly bears a prickle as sharp as any thorn,
And Mary bore sweet Jesus Christ on Christmas Day in the morn.
The holly bears a bark as bitter as any gall,
And Mary bore sweet Jesus Christ for to redeem us all.
From the carol The Holly and the Ivy
Holly of the American (Ilex opaca) or European (Ilex aquifolium) variety may have bright red berries in the winter, but in the spring they display delicate white flowers. These flowers differ among individual trees from either species because they are dioecious plants. That’s your plant biology word of the day. It means that there are separate male and female plants with slightly different-looking flowers. Insects like bees, wasps and moths are responsible for holly pollination. Only the female trees will produce the crimson berries in the winter. While we’re on the subject- they may be commonly referred to as berries, but really they are drupes (like pecans). Within each berry are four small seeds enclosed together in a hard shell that is surrounded by the soft red flesh.
Unlike the poinsettias that I wrote about yesterday, holly berries are poisonous to humans and pets. So, if you use any real holly boughs and berries in your winter decorations, make sure they don’t get eaten. Ingesting a couple of berries won’t cause ill effects, but pets and small children can be killed if twenty or more are eaten. The toxic ingredient is said to be theobromine (the same ingredient in chocolate that is poisonous for dogs). Nevertheless, the berries are an important winter food source for birds and other woodland mammals because the holly berries persist long into the winter after many other food sources are exhausted. Ingestion of the berries by birds is key for seed dispersal for hollies, but I couldn’t find any research as to why birds (or certain bird species) were impervious to the effects of theobromine or other toxins in the berries.*
The holly is a symbol of life in the dead of winter because of its vibrant green color. It’s able to protect its leaves from the harsh winter conditions with a thick cuticle covered by a layer of wax, which gives the leaves their shiny appearance. Production of these hydrocarbons requires a significant energy investment, but it allows for the retention of leaves beyond a single growing season (as is the case for deciduous trees that lose their leaves every fall). While transient warmer temperatures during the winter may give evergreens an opportunity for primary productivity that leafless deciduous wouldn’t have, the evergreen advantage is really in the net energetic returns on persistent leaves/needles beyond a single growing season. Despite their productive appearance, there actually isn’t a lot of photosynthesis occurring in evergreens during the winter. As temperatures drop down to freezing, photosynthesis slows and stops. The chlorophyll-containing machinery of the light reactions persists in the thylakoid membranes, but it is in quenching mode only and not driving electron transfer. These evergreen plants put up a great façade of vitality during the cold winter months, but internally, they are working very hard to protect their photosynthetic machinery by keeping it inactive.
So, holly isn’t just beautiful, but its biology is also consistent with the imagery it adds to our winter celebrations as an all-in-one conundrum of life and death.
*If there are any ornithologists that come across this post, please feel free to comment below.
References and Links: