Organisms come in all shapes and
sizes, even within groups of closely related species. The processes of evolution –
selection and drift – have produced wonderful variation in the morphology
(shape) of plant and animal organs, whether they are wings, leaves, flowers, or
exoskeletons.
The Proteaceae are a striking
example of what evolution has accomplished with morphological variability.
These plants sometimes look like species out of an imaginary world in a
children’s book. Flowers in the family are especially interesting and are
popular for cultivation. Some flowers are shaped like bottle brushes, others
like giant dishes of radiating color. Leaves can be shaped like little lances,
or deeply dissected, or jagged like a saw tooth. The genus from which the
family derives its name, Protea, was
given its name by Linnaeus who invoked the Greek god Proteus, a deity capable
of changing his shape.
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| Banksia menziesii, native to Australia. Photos taken at UC Santa Cruz Arboretum. |
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| Protea scolymocephala. Photo taken by Andrew Massyn, public domain. |
The Proteaceae are believed to be an
old family of flowering plants, evolving at least 140 million years ago. The
family originated on the ancient southern hemisphere super-continent of Gondwanaland.
Over time that large land mass split apart into the continents of Australia , Africa, South America, and Antarctica . Today Proteaceae are found on most of these
southern hemisphere continents, an indication of their ancient origin and the
effects of continental movement on the modern day distribution of species
across the globe.
The showy inflorescences common in
the family attract a lot of attention. Flowers are typically born in groups
arranged in a spike. In some genera like Banksia,
the inflorescence is a tall column of brightly colored flowers perched on the
plant’s branches. The individual flowers in the family are composed of 4 parts
which are fused into a perianth to varying degrees. (The perianth refers to the
combined structures of petals, sepals or tepals in a flower.) Some flowers are
tubular in shape. Most species have bisexual flowers, where both male and
female parts are present in the same flower. The flowers have four stamens,
which sometimes emerge directly from the perianth. The female part of the
flower is a long prominent style ending in a stigma that functions to capture
pollen. Nectar glands are often present too.
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| Grevillea spp at the UCSC Arboretum. Bee pollinating Grevillea sp (left); G. levis (right). |
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| Mimetes cucullatus, native to South Africa. Photo from UCSC Arboretum. |
Proteas rely on different
strategies or vectors for pollination. A few species are wind pollinated or
visited by mammals. Flowers that are red and produce nectar tend to rely on
birds for pollination. In contrast, insects typically pollinate flowers that
are white, blue, or purple in color. Conospermum
is an insect-pollinated genus that has an “explosive” pollination mechanism
that accomplishes two tasks: obtaining pollen from another flower, and
preventing its own stigma from being self-pollinated (self-pollination inhibits
exchange of genetic material between individuals). To accomplish this, the
style is initially bent backwards inside the flower keeping the stigma away
from its own pollen. When an insect visits the flower, the stigma snaps forward
hitting the insect, thereby picking up pollen on the insect’s body that it
acquired from another flower. The snapping motion of the style also ends up
dusting the insect with pollen from its own flower which can then be transferred
to another plant.
Proteas tend to share some
ecological commonalities. They are trees or shrubs and all have at least some
woody tissues. They tend to grow on sandy or gravel soils that are low in
nutrients, especially phosphorus. Species in the family often grow in drier
habitats, but they have adaptations to minimize water loss. Many have tough
leathery leaves that contain a lot of lignin but few nutrients, making them a
less favorable food choice for herbivores. Leaves tend to be long-lived since producing
them requires significant investment of resources by the plant. Some species
such as Leucospermum have leaves that
secrete nectar.
Remarkably, proteas lack the
symbiotic fungi that associate with the roots of most terrestrial plant species
(mycorrhizae). Mycorrhizae are beneficial to plants because they help with nutrient
acquisition. In place of mycorrhizae, many proteas have evolved a different
type of root adaptation to acquire soil nutrients. Plants form clumps of small
dense rootlets that grow near the soil surface called “proteoid roots”. These
roots develop in response to rain and may only last for a few months.
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| Banksia nirida at UCSC Arboretum. Leaves and inflorescence (left); close-up of inflorescence (right). |
Some proteas live in fire prone
areas and thus need adaptations to survive fires. These adaptations include
recruitment of a new generation of plants from seed following fire, or recovery
of above-ground shoots from tubers or “boles” that live below-ground. Boles are
underground stems from which new above-ground growth can occur. By having thick
bark or by growing isolated in outcrops of rock, plants can also escape fire
damage. Some paleobotanical evidence from central Australia suggests that fire-prone
biomes with Proteaceae present in the plant communities may be very old, stretching
back to the Cretaceous period (Carpenter et al. 2015).
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| Leucadendron discolor, native to South Africa. Photo taken at UCSC Arboretum. |
Estimates of species diversity in
the Proteaceae range from about 1250 to over 1500 species. About 70 genera are
recognized. The greatest diversity is found in Australia , where there are
representatives of all 5 subfamilies. In fact, because of its high modern-day
diversity, it is believed that the family evolved in the region of Gondwanaland
that would later become Australia
after fragmenting. South
Africa is the second most diverse region for
the family, with about 330 species and 14 genera. In fact, the “Cape Floral Kingdom ” in South Africa is one of the most
diverse regions on the planet for vascular plant diversity. One reason for the
high diversity may be the high degree of topographic variation (mountains and
valleys with different soil types and climates) in that region. The
distribution of some high elevation species in the area, including some
Proteaceae, may reflect species taking refuge in climatic conditions that they
once evolved in long ago (Verboom et al. 2015).
In my opinion, the Proteceae also
have some of the coolest common names. Scientists traditionally use Latin scientific
names (binomials) because they help reduce confusion and identify some of the shared
relationships of species within genera, but I certainly don’t mind the common
names of the Proteaceae. Some examples of names from South African plants
include:
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Protea (smokebushes)
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Serruria (spiderheads)
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Vexatorella (vexators)
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Mimetes (pagodas)
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Orothamnus (marsh
rose)
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Pranomus (scepters)
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Spatella (spoons)
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Diastella (silkypuffs)
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Hakea (needlebushes)
The most widespread human use of
proteas is perhaps the flower trade. Another commercial use, which I was
surprised to learn involves the family, is the fact that macadamia nuts are
produced by two species of Proteaceae: Macademia
integrifolia and M. tetraphylla.
Originating from Australia ,
these species were introduced into Hawaii
in the late 1800s for macadamia nut cultivation. Later, the industry spread to
Africa, California , central America, and then
finally developed back in Australia .
The native aboriginal people of Australia
also used some proteas for food. Some Dryandra
and Hakea species are involved in
honey production. Finally, a few species in Australia have been used for
timber. Like many groups of organisms, Proteaceae face conservation challenges.
For instance, in South
Africa , about a third of proteas are
threatened.
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| Macadamia integrifolia in cultivation on Maui, Hawaii. Left: flowering trees. Right: macadamia fruits and nuts. Images by Forest and Kim Starr, cropped and arranged by C. Janousek, under CC Attribution 3.0 Unported license. Original photos here, here, and here. |
I’ve never observed the Proteaceae
in their native habitats, but I hope that will change in the future. A
botanical trip to South Africa
or Australia
would be amazing! However, the Arboretum at UC Santa Cruz has a wonderful
collection of southern hemisphere plants including species of Banksia, Protea, and other representatives of the family. The plants grow
well in central California
presumably because of the shared Mediterranean climate with other regions where
Proteaceae grow. A walk through the paths of Banksia and other species is a pleasant journey to another
botanical world unfamiliar to us in North America .
The Arboretum also has quite a few species for sale at reasonable prices, and I
could not pass up the chance to buy a few!
Ecologically, morphologically, and
even mixed into your chocolate chip cookies, the Proteaceae are incredible
plants, a botanical treasure from the southern hemisphere!
References
Carpenter RJ, Macphail MK, Jordan
GJ, Hill RS. 2015. Fossil evidence for open, Proteaceae-dominated healthlands
and fire in the Late Cretaceous of Australia. American Journal of Botany
102:2092-2107.
George AS. 1984. An introduction
to the Proteaceae of Western Australia. Kangaroo Press, Kenthurst , Australia
Rebelo T. 1995. Proteas. A field
guide to the Proteas of southern Africa .
Fernwood Press.
Verboom GA, Bergh NG, Haiden SA,
Hoffman V, Britton MN. 2015. Topography as a driver of diversification in the
Cape Floristic Region of South Africa. New Phytologist 207:368-376.
Wrigley JW. 1989. Banksias,
Waratahs and Grevilleas and all other plants in the Australian Proteaceae
family. Collins Publishers Australia .
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| Banksia victoriae. Inflorescences and saw-tooth leaves (left); close-up of inflorescence (right). Photos from UCSC Arboretum. |