Plant of the month, (©Pierre Laszlo, all rights reserved)
Strelitia genus (Strelitiaceae).
Commonly known as bird of paradise for its flower, it is a familiar sight. It is widely cultivated as an ornemental plant. The beak-like sheath for the flower (a modified leaf known as the spathe) is perpendicular to the stem, which gives it the appearance of a bird’s head. The flowers emerge one at a time from the spathe. They consist of three brilliant orange sepals and three bright blue petals. Two petals fuse into an arrow-like nectar-producing gland, or nectary.
The best-known plants, such as grown by horticulturists and having become household names, can still surprise us with fascinating features. This is the case of Strelitzia. It holds interest to historians of culture and to chemists, besides natural historians.
Any plant, I believe, is a source of wonder: provided that, however seductive its flowers may look, one goes beyond the mere appearance and one investigates in some depth its make-up. In other words, scientific facts lead to the re-enchantment of nature. As we shall see, Strelitzia plants illustrate this axiom aplenty.
The five known species of Strelitzia currently grow along the eastern seaboard and/or mountains of South Africa. Their closest relatives live afar. Those are the genus, Ravenala, the traveller’s palm, found in rainforests of Madagascar; and Phenakospermum, which grows in transitional or secondary rainforests in tropical northern central South America.
This dispersal stemmed from the breakup of the Gondwana landmass. These three genus had a common ancestor during the Eocene, about 50 million years ago. After the divergence, the three ancestors had to survive the harsh climatic change, the EoceneAOligocene transition, presumably due to the impact of two meteorites. The climate became cooler and drier as the atmospheric concentration of the greenhouse gas CO2 decreased, 34 million years ago.
Bird of paradise flowers are spectacular, from their large size and colorful appearance, mimicking their avian namesake. Their aristocratic bearing called for distinguished dedicatories, starting with Queen CharloZe, including Grand duke Nikolai Nikolaievich the elder, third son of Czar Nicholas I of Russia; and, of late, Nelson Mandela.
The genus is named Strelitzia, after Queen Charlotte of England, who had earlier been Duchess Sophie Charlotte of Mecklembourg-Strelitz’.)This passionate amateur botanist was honored for her big support of the budding botanical gardens at Kew, near London. Strelitzia reginae, «bird of paradise of the queen» refers to her specifically. But it was also renamed recently «Mandela’s Gold». As for Strelitzia nicolai, it was named after the Czar’s son. As a final note on the names of these plants, let me venture a personal accent: my wife Valerie suggested that I write this piece. She was born in Los Angeles, the city whose official flower, as befits the City of Angels, is precisely the Bird of Paradise.
The five Strelitzia species are notesworthy for being large, even very large, herbaceous plants, not trees. Consider S. nicolai: growing more than six meters tall, its leaves resemble those of a Musa (banana tree), indeed a distant relative. The stem however almost entirely lacks lignin fibers.
To enlarge upon the already mentioned stepwise opening of the flowers, they emerge from a large and thick boat-shaped bract. Six to eight flowers come out of the bract, following one another. With time, the flower and its blue petal arc upward, becoming vertical relative to the horizontal bract. On the third day an unpollinated flower dies. Within the next 48 hours, the next flower emerges from the bract, and so on until the last flower emerges.
In a mature S. nicolai, the blue petal stands erect and the female organs, consisting of a thin, three-part stigma about four cm long attached to a style over nine cm long, reach beyond the tip of the petal. The stigma has a sticky surface, perfect for catching and holding pollen grains.
Which brings up pollination, predominantly effected in South Africa by four bird species, known as sunbirds, small, colorful, long-billed birds of the Nectariniidae family. Strelitzias share this feature with the other seven families in the monocotyledon order Zingiberales (gingers), that also includes banana trees. Co-evolution presented these plants with half-a-dozen impressive adaptative traits to attract birds.
The nectaries, with their secretory surfaces greatly increased by folding of the epithelium and by transfer cells, are three pockets held between the carpels. They exude copious nectar, favoring quantity over quality: the liquid is relatively poor in sugar but overabundant.
A large receptacle collects it at the bottom of the bract. Sometimes, the nectar may even overflow the bract. Why such a plentiful secretion? The flow rate of the nectar translates into the flower angle, a visual cue to the sunbirds.
Strelitzias have a tough, robust construction of their flowers, with the sturdy boat-shaped bract, to make them survive even rough foraging by the birds. When a bird comes to feed, it perches on the stiff inflorescence bract. The flower opens and exposes the anthers. The sticky pollen adheres to the feet of the bird. The bird then transfers this pollen to the stigma of the next flower it visits. Which implies proximity of these plants to one another.
The very colors of the flowers, orange and blue, partake of the attraction of Strelitzias to sunbirds. Our eyes are very sensitive to the contrasting colors of orange and blue. Birds have eyes similar to ours in this respect, they perceive color over wavelengths ranging between 300 nm and 660 nm.
In short, what makes these plants beautiful to us, humans, is nothing but a set of utilitarian devices meant not at all for us, but for the birds! Petals are blue from anthocyanin pigment in the vacuoles of epidermal cells. The brilliance comes about from their papillae, i.e., microscopic hair. The orange sepals show under the microscope elongated color-containing vesicles with numerous carotenoid tubules parallel to the long axis.
The color orange is shared by the arils in S. nicolai. Arils are a growth, like a tuft of hair, adorning the seeds and, presumably, assisting in their dispersal. Strelitzia arils owe their orange color, it was discovered just a few years back, to the bilirubin pigment. Yes, a bile pigment which, it was previously thought, exists only in animals. Strelitzia flowers, in S. reginae in particular, owe their orange color in part to bilirubin.
Chemists at the Lonza company in Swi;erland were prompt to seize on those findings. They devised a cosmetic, VivillumeTM, from extracts of Strelizia arils: with age, skin’s luminosity and uniformity of tone tend to degenerate. This cream, with ingredients degrading bilirubin, improves skin tone to yield an illuminated complexion.
Yet another fascinating feature of Streli’ia I shall mention is the superhydrophobicity of their leaves. This long word simply means that they are not wettable, and that they are self-cleaning. Rain drops just sit on the leaves without penetrating them. This is due, not only to secretion of wax particles, also to a nanoscopic texturing, an arrangement of periodic microridges/microgrooves that discourages wetting. This leads to current engineering applications for various materials, in electronics for instance.
All in all, Strelitzia are plants that challenge mankind to understand nature, not only admire it in awe!