Hollies Get Prickly for a Reason
Festive trees make spiny leaves to ward off nibblers.
Christy Ullrich
National Geographic News
Published December 20, 2012
With shiny evergreen leaves and bright red berries, holly trees are a naturally festive decoration seen throughout the Christmas season.
They're famously sharp. But not all holly leaves are prickly, even on the same tree. And scientists now think they know how the plants are able to make sharper leaves, seemingly at will. (Watch a video about how Christmas trees are made.)
A new study published in the Botanical Journal of the Linnean Society suggests leaf variations on a single tree are the combined result of animals browsing on them and the trees' swift molecular response to that sort of environmental pressure.
Carlos Herrera of the National Research Council of Spain led the study in southeastern Spain. He and his team investigated the European holly tree, Ilex aquifolium. Hollies, like other plants, can make different types of leaves at the same time. This is called heterophylly. Out of the 40 holly trees they studied, 39 trees displayed different kinds of leaves, both prickly and smooth.
Five holly leaves from the same tree.
Photographs by Emmanuel Lattes, Alamy
Some trees looked like they had been browsed upon by wild goats and deer. On those trees, the lower 8 feet (2.5 meters) had more prickly leaves, while higher up the leaves tended to be smooth. Scientists wanted to figure out how the holly trees could make the change in leaf shape so quickly.
complete story:
http://news.nationalgeographic.com/news/2012/121220-holly-leaves-prickly-plants-science/ 
