Incredible plants: The Proteaceae

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.

Banksia menziesii, native to Australia. Photos taken at UC Santa Cruz Arboretum.

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.

Grevillea spp at the UCSC Arboretum. Bee pollinating Grevillea sp (left); G. levis (right).

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.

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).

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:

-          Protea (smokebushes)

-          Serruria (spiderheads)

-          Vexatorella (vexators)

-          Mimetes (pagodas)

-          Orothamnus (marsh rose)

-          Pranomus (scepters)

-          Spatella (spoons)

-          Diastella (silkypuffs)

-          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.

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.

Sainsbury RM. 1991. A field guide to smokebushes and honeysuckles (Conospermum and Lambertia). University of Western Australia 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.

Banksia victoriae. Inflorescences and saw-tooth leaves (left); close-up of inflorescence (right). Photos from UCSC Arboretum.