The Holly and the Ivy... and the Ivy

The two leaf forms of ivy (Hedera helix), cordate (left) and palmate (right)

Its Christmas time, and all around the house... people have placed a variety of plants to add to create a festive spirit about. Poinsettias have become quite popular in this part of the world (and make a convenient, low risk present for neighbours), but the traditional Christmas trees and garlands of holly and ivy are still the most popular. All three of these now Christian traditions arose from older traditions (1): ivy (Hedera helix) in particular was considered a sybol of female fertility because of its late flowering period (September to November) and production of berries (around this time of the year) (2). Although it can be quite an invasive pest in some parts of the world, these two facts make it an important source of nectar and pollen for insects in late autumn/early winter and an equally important source of food for birds later in the year in its native range.

However, one of the most striking features of holly is that two distinct, different leaf shapes will be seen on the one plant - a five lobed, palmate form and a cordate form that shows little to no lobing. The lobed leaf is found on the climbing, juvenile stems of the plant, with the cordate form on the flowering stems. This is known as heteroblasty, a phenomenon that is found in many plant species, but that is most famously illustrated in ivy. It was first described by Karl Goebel in 1898, who noted that as plants grow they add new modules (stem with attached leaf) which show gradual changes of form in most cases (3). However in some species, such as ivy, the changes are more dramatic. The reasons for this are still unclear but defence against herbivory and nutrient and water supply differences have been suggested as causes (3). Indeed, Ivy has been shown to produce palmate leaves in low light conditions and cordate leaves in high light (4).
 

References:

1. Miles, 2008. Christmas in Ritual and Tradition, Christian and Pagan p. 275
2. Phillips, 1977. Wild Flowers of Britain p. 172
3. Zotz et al., 2011. Botannical Reviews 77 pp. 109–151
4. Rogler and Hackett, 1975. Physiologia Plantarum 34 pp. 141–147