Sunday, December 28, 2014

Plant Story -- European Holly - Not Always with Spines and Red Berries

European holly, Ilex aquifolium
European holly, Ilex aquifolium is widely recognized by its spiny leaves and red berries (drupes) (see post on holly folklore). Curiously, not all the leaves on European holly are spiny and not all the plants have fruit.

First, holly trees vary in the number of spiny leaves. You can see it in any of the photos--some leaves are smooth and others have spines on the edge.

All sorts of people have thought about the variation in the spines. Young plants tend to have mostly spiny leaves. (photo below) On a big tree, the lower branches have more spiny leaves than higher branches.



The explanation appears to be that the spines deter browsing by big animals, deer for example. If protected from browsing, for example inside a fence, the plants will have mostly spineless leaves. But the regrowth after a deer or cow has eaten a number of leaves will have mainly spiny leaves. The little plant in the photo below is not knee-high, so it is quite vulnerable to animals and its leaves are almost entirely very spiny. Conversely, on a big tree, the upper branches are out of reach of browsing animals and much less spiny. (see National Geographic study)

Very young holly plant.
Very young holly plant. 
Folklore acknowledged the variation in spines: it was important to bring in both smooth and prickly leaves for Christmas decoration. If prickly came in first, the husband would rule the house for the year, if smooth, then the wife would.

Despite the variation, the leaves with their points are very distinctive and are an important part of the way people around the world recognize holly. Indeed, many other plants are described as having holly-like leaves: holly-leafed cherry Prunus ilicifolia (ilic- from Ilex holly, -folia meaning leaf), holly-leafed banksia (Banksia ilicifolia), and holly-leafed mangrove (Acanthus ilicifolium) for example. All describing leaves that resemble holly leaves with spines.

However, the holly oak (Quercus ilex) of the eastern Mediterranean is not named after holly, but vice-versa: the Ilex of the oak was taken by Linneaus to be the genus name of holly. (See leaves of holly oak link).

The second easily recognizeable characteristic of hollies, the red fruit, also varies between plants.  European holly is dioecious, so only half the trees produce fruit. That is, there are "female", fruit-bearing plants and "male" plants that produce pollen but not fruit. (It's not rigorously correct to write"male" and "female" for plants, but it gets the idea across quickly). The very big holly at the Chelsea Physic Garden in London shown below is male and produces no fruit. (The cart in the lower left is about 6' (2 m.) high.)
European holly, Ilex aquifolium
European holly, Ilex aquifolium, Chelsea Physic Garden, London
One consequence of dioecy is that if you want holly for the bright red fruit, you have to plant at least two, because without a male to contribute pollen, the female tree won't be fertilized and so will not develop any fruit. And since there is no way to know from a seed whether it is male or female, planting more than two will increase the odds of having both sexes.

The white flowers (link) are pretty and attract bees. Of course, a neighbor's hollies can pollinate yours or vice versa,.
European holly, Ilex aquifolium
European holly, Ilex aquifolium
Finally, many species of holly, for example those native to cooler parts of North America, for example possum haw Ilex decidua and winterberry Ilex verticillata are deciduous not evergreen, losing their leaves in winter. They are still easy to love for the red berries (drupes) and the birds they attract, but it is a different look: LINK


Comments and corrections welcome.

References
Herrera, C. M. and P. Bazaga. 2013. Epigenetic correlates of plant phenotypic plasticity: DNA methylation differs between prickly and non prickly leaveds in heterophyllous Ilex aquifolium (Aquifoliaceae) trees. Botanical Journal of the Linnean Society 171 (3): 441-452.
Murrell, D. 2008. Superstitions, 1013 of the wackiest myths, fables and old wives' tales. Amber Press, New York.
Obeso, J. R. 1997. The induction of spinescence in European holl leaves by browsing ungulates.  Plant Ecology 129: 149-156.

Kathy Keeler




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