Repeated evolution of hollyleaf form, e.g. in foliar-spinescent Grevillea
(writing in progress)
One form of spinescent leaf is the ‘holly-leaf’ form, in which a sclerophyllous leaf has not only a ‘pungent’ apex but also several similarly spinescent lobes on its margins.
The form of the ‘holly leaf’ is widely recognised in European culture, its association with Christmas being partly explained by the fact that Ilex aquifolium (see https://en.wikipedia.org/wiki/Ilex_aquifolium ) is evergreen, with leaves that (by virtue of their sclerophylly) remain unwilted when a spray of foliage is picked as a decoration. (The persistent freshness of the red fleshy fruits adds to the decorative value of holly.)
The holly-leaf form has evolved independently many times in angiosperms. In various families and genera around the world, we see again and again the specific epithet ‘ilicifolium/a’.
A prime example is a genus in the Proteaceae, namely Grevillea.
I focus here on Grevillea for several reasons, as follows:
- Grevillea (about 360 spp.) varies greatly among its species in leaf-form, ranging from non-sclerophyllous to sclerophyllous, and from non-spinescent to spinescent.
- Grevillea is one of the most widespread genera of Proteaceae in Australasia, ranging as far as the Indonesian island of Sulawesi, plus New Caledonia (although, interestingly, not New Zealand).
- The holly-leaf form crops up across the length and breadth of Australia in various spp. of Grevillea, apparently having evolved again and again within the genus.
Although the holly-leaf form occurs also in the genus Banksia in the same family (e.g. Banksia sessilis), it is in Grevillea that this leaf form has a particularly impressive tally of species.
By the way, three spp. of Macadamia (Proteaceae) have leaf-margins with similarly rigid spines, but the overall leaf-shape is different because the leaf in Macadamia is relatively long and the marginal lobes do not project much, the spines being effective more because the leaf-margin is wavy than because the lobes project out to the sides of the leaf.
As far as I know, all of the spp. of Grevillea shown below are foliar-spinescent.
On the sandstone substrates of Kakadu and elsewhere in the Northern Territory, there are several spp. of Grevillea with the holly-leaf form of leaf-spinescence. One of the better-known is G. angulata.
Grevillea angulata:
Grevillea angulata:
Grevillea angulata:
Grevillea angulata:
Another from the same area is Grevillea aurea.
Grevillea aurea:
Grevillea aurea:
A similar leaf-form in the same genus, but this time in sandstone substrates in the Grampians of western Victoria, right across the continent from the above spp., is Grevillea aquifolium. The specific epithet ‘aquifolium’ is that of European holly, so this species is named after European holly just as species called ‘ilicifolium/a’ are named after European holly.
Grevillea aquifolium:
Even in southwestern Australia, near Perth, the genus Grevillea has again produced the holly-leaf form. An example if G. monticola.
Grevillea monticola:
Grevillea monticola:
Another sp. along similar lines is G. wickhamii. This species is widespread in tropical, semi-arid northern Australia.
Grevillea wickhamii:
Grevillea wickhamii:
In southeastern Australia, we have G. ilicifolia, which is leaf-polymorphic.
Grevillea ilicifolia:
Grevillea ilicifolia:
(writing in progress)
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