A tweet from Patrick Moore to the effect that most of the pesticides present in the food we eat are produced by the crops themselves set me thinking about the “arms race” between plants to avoid being eaten and at the same time encourage herbivores to eat other competing species.
Eating or being eaten can be viewed as a very simple game. In order to survive, any plant or animal, any living organism, must eat. That is, take in from outside the substances that it needs to grow its own body and to provide energy to run its internal processes. It also must not be eaten if it is going to survive for long. Winning this leg of the game consists of convincing any predator that it cannot be eaten, to use the terminology of Stephens (1993).
Plants have developed the ‘must eat’ game virtually to its limit by now. Housing symbiotic chloroplasts that fix carbon by photosynthesis, absorbing other nutrient elements, and the metabolic pathways that produce the whole plant have been established since the Palaeozoic. Even the later innovations of CAM and C4 photosynthesis have been around for millions of years.
Dennis Stephens (1994) further suggested that the main game among plants is ‘must not be eaten’ because they have not yet evolved as far as they can go in that department. They are still in an arms race with herbivores, with pathogens and with each other. Plants may develop spines or other physically deterring outgrowths that convince hungry herbivores that they are not edible. They may use nectar to encourage ants to wander over their surface and clean up feeding insects. But most importantly, they may produce any of a vast range of chemicals (secondary metabolites) that make them bad-tasting or toxic to the particular herbivores that threaten them. These are all physical manifestations of the strategy of being inedible.
As an aside, it’s interesting that the development of toxic secondary metabolites is a speciality of the angiosperms or flowering plants that have dominated land vegetation since the Cretaceous age. Ferns can be inedible too, but they are much less rich in this kind of chemistry. And the notably toxic members of the gymnosperms are not the really ancient ones, but those that have diversified since the Cretaceous in competition with angiosperms – the cycads, Ephedra, Taxus and some related conifers.
So there is an evolutionary pressure on plants to not be eaten by convincing herbivores that they are inedible. The most obvious benefit from this – when it succeeds – is that they do not lose biomass or get killed outright by herbivory.
But there can be a second advantage for the uneatable. Consider a three-way game between two plants and a herbivore, such as sheep grazing on a pasture of grass containing thistles. A thistle’s spines make it unpalatable; either totally inedible, or so hard for the sheep to eat that it will not be nibbled as long as any edible, palatable grass remains around it.
So the harder the sheep graze, the less the grass will compete with the thistles for light, water and ground space. The combination of grazing pressure and their spiny defence against being eaten has given them a powerful strategy in their own competitive game with the grass.
It’s also interesting to consider the strategy of the grass. It might actually derive some benefit from being grazed along with broadleaf weeds that lack the thistle’s defence, since it is better equipped to regrow after grazing than they are. But that’s another story.
Stephens, D.H. (1993) Expanding on Level 5, Sex. Letter tape of 6 May 1993.
Stephens, D.H. (1994) Postulates, Self and the Obsessive IP. Letter tape of August 1994.
Watsonia schlechteri L.Bolus grows in the montane veld of the winter-rainfall parts of the Cape, South Africa. Plants grown from seed imported via Silverhill Seeds first flowered this year and are a good match for the lectotype of W. schlechteri.
The flowers are orange-vermillion with perianth lobes to 23 mm long. The buds end in a slightly downcurved point. There are no staminodal ridges in the perianth tube, a character that distinguishes it from the closely related W. pillansii. However, the fresh leaves of these specimens lack the strongly thickened margins and midvein that are used as another distinguishing character; these are only evident in dried material.
W. schlechteri is one of the smaller watsonias, usually much less than 1 metre tall with leaves about 40 cm long. It flowers in late December to January, resuming growth from offsets soon after flowering while the previous season’s shoot may still be green. Thus it has some leaves all year, or non-flowering plants may be briefly leafless before the new growth starts in late summer. Goldblatt notes that flowering in the wild is conditional on the plants not being shaded out by surrounding vegetation.
Like other watsonias native to high altitudes, it is at risk of damage in the agonisingly hot, dry summers we get here at sea level in Adelaide. The problem is to give the plants sufficient light without excess heat, a tall order on days of 42°C with northerly winds.
Goldblatt, P. (1989) The genus Watsonia. 148 pp. (National Botanic Gardens: Kirstenbosch).
The rediscovery of Mendel’s principles of heredity at the beginning of the 20th century inspired a surge of ornamental plant breeding by researchers, commercial nurserymen, and perhaps most importantly by individual gardeners.
John Cronin, Director of the Royal Botanic Gardens in Melbourne from 1909 to 1923, had a personal hobby of experimenting with the improvement of garden flowers. He aimed to demonstrate the application of Mendel’s laws to flower breeding, and encourage gardeners to make their own hybrids. He worked with Dahlia and other genera, but particularly the winter-growing South African watsonias, which he recognised as “everyone’s flower” – easy to grow, attractive, a natural for southern Australian gardens.
In a previous publication I lamented that the exact pedigrees of his Watsonia cultivars were lost with the destruction of his papers after his death in 1923. But now the National Library of Australia has come to the rescue with their wonderful resource of newspaper files at Trove. Cronin was a tireless populariser and communicator, speaking at the evening meetings of horticultural societies around the suburbs of Melbourne and giving interviews to journalists.
In the spring of 1904, while employed by William Guilfoyle at the Botanic Gardens, he crossed a pink Watsonia borbonica with W. borbonica ‘Arderne’s White’. This cross may be represented by the following formula (but note that the order is arbitrary, it is not known which was the pollen parent and which the ovule parent in any of the crosses discussed here):
borbonica × Arderne’s White
He noted that pink flowers were dominant over white in the F1 generation, as has been confirmed by other researchers. In spring 1907 he selected one F1 plant with tall stature, dense branching and large flowers. He crossed this with a purple Watsonia meriana and the widely grown red Watsonia aletroides, and also backcrossed it to W. borbonica ‘Arderne’s White’ to create three lines for further breeding:
1. meriana × (borbonica × Arderne’s White)
2. aletroides × (borbonica × Arderne’s White)
3. Arderne’s White × (borbonica × Arderne’s White)
Cronin’s appointment as Principal of Burnley Horticultural College in 1908 seems to have interrupted this work, and in the following year he succeeded Guilfoyle as Director of the Botanic Gardens. By 1913 he had time to resume his watsonia experiments, and on 20 March sowed seeds from his three 1907 crosses at the Botanic Gardens nursery. Six years is not an inordinately long time to store Watsonia seeds, but there would be some loss in viability which may have unintentionally favoured some genotypes over others. Cronin’s management of the plants was another possible source of selection pressure to produce watsonias adapted to Melbourne gardens: he left the corms in the ground over summer, and gave the plants no fertiliser or watering even though 1913-14 was a drought period.
This generation produced their first flowers in October 1914; Cronin stated that these resembled the 1907 selection in size and colour, and were inbred that year. I interpret this to mean that he produced an F2 generation in each of the three lines by cross-pollinating siblings, since selfing would have produced little or no seed due to incompatibility. Thus,
1. (meriana × (borbonica × Arderne’s White)) × (meriana × (borbonica × Arderne’s White))
2. (aletroides × (borbonica × Arderne’s White)) × (aletroides × (borbonica × Arderne’s White))
3. (Arderne’s White × (borbonica × Arderne’s White)) × (Arderne’s White × (borbonica × Arderne’s White))
Large numbers of these seedlings were raised in the main nursery of the Botanic Gardens. By October 1916 Cronin saw the first flowers of the inbreds, which had a wider range of colours than their parents. Some whites showed up, as would be expected from recombination, including some with flowers of improved size and form compared to the original ‘Arderne’s White’. The watsonias commercially released in the 1920s as the Commonwealth hybrids or “Watsonia Cronini” were selections from this generation.
Line 1 would have produced the many Cronin cultivars with a mixture of characters from W. meriana and W. borbonica. These often have subtle tertiary flower colours due to genes from both species influencing anthocyanin pigment production. Floral bracts are typically well-developed and obtuse, compared to the shorter acute bracts of W. borbonica. Examples include ‘Lilac Towers’, which is the most widely grown Watsonia in Australia today and may be the same as Cronin’s ‘Sydney’, and the one illustrated below which may be his ‘Maitland’.
Line 2 would have yielded flowers with long tubes and small lobes like Watsonia aletroides. The one illustrated here was discussed in a previous post.
It’s significant that Cronin did not use a long breeding program: the cultivars released were no more than three generations away from the original genotypes that had been imported from Africa in the 19th century. As he was working with a perennial that is normally propagated vegetatively, he could stop at the F2 with its fixed heterozygosity. I have bred watsonias four generations on from these and other old cultivars, and can attest that hybrid breakdown soon appears. Some of the resulting plants had interesting extremes of flower shape or colour, many were dwarf or weak in growth, but few were gardenable.
In the spring of 1917 Cronin presented this data to the horticultural correspondent of The Leader, and was lecturing on flower hybridisation to amateur horticultural societies with his new watsonias as exhibits. The following year he gave an interview to The Argus, repeating that his new watsonias were produced by first crossing and then inbreeding on Mendelian lines.
Anon. (1917) Melbourne Botanic Gardens – New colors in flowers – The laws of Mendel. The Leader (Melbourne), Saturday 10 November 1917 pp.13-14.
Anon. (1917) Horticultural society. The Advertiser (Footscray), Saturday 15 December 1917 p.3.
Anon. (1918) Botanic Gardens Experiments. The Argus (Melbourne), no.22,553. Monday 11 November 1918 p. 6.
Cooke, D.A. (1998) Descriptions of three cultivars in Watsonia (Iridaceae) J.Adelaide Bot. Gard. 18: 95-100.
Pescott, E.E. (1926) Bulb Growing in Australia. (Whitcombe & Tombs: Melbourne).
Watsonia humilis Mill. is endemic to the Western Cape area of South Africa. It is winter growing and summer dormant like many other South African irids, restricted to a particular fynbos association on seasonally wet clay and loamy alluvial flats.
Its original range in the Breede River Valley between Tulbagh and Worcester, and the lowlands between Malmesbury, Franschhoek and Gordon’s Bay, has been reduced by 98%. Two populations are known to remain: one of fewer than 50 plants in an urban reserve at Gordon’s Bay and a second rediscovered in 2012 in the Breede River Valley near Wolseley, more than 80 km away. A third population may still exist between Somerset West and Sir Lowry’s Pass despite bulldozing by vandals in 2011.
Although critically rare in its natural habitat, it is secure in ex situ cultivation around the world. At least one strain has been passed between gardeners in Australia since its first importation in the 1830s.
Watsonia humilis is in the section Watsonia, subsection Watsonia of the genus. It is distinguished by its small size, unbranched inflorescence, strongly keeled bracts with outcurved tips and stamen filaments no longer than the perianth tube. Its pale coloured perianth (pale pink to white, never red) suggests that it is adapted to insect pollination unlike its larger and usually red-flowered relatives.
This watsonia can be grown in the open garden here but to keep track of the corms I grow it in 20 cm pots, lifting and dividing each summer. It multiplies vegetatively and I have used it as the ovule parent in hybridization.
The common form in Australian gardens has white perianth lobes and a tube shading to deep pink at the base. The F1 hybrids with bright red flowered W. meriana Mill. have uniformly pale pink flowers, the intensity and hue of the pink (anthocyanin) pigmentation varying slightly between individuals. This result shows that white flowers in this species have a different genetic basis to the recessive acyanic mutants of other species: it suggests incomplete dominance, possibly with more than one genetic locus involved. This is to be expected if white flowers were a stabilised local adaptation in the source population, rather than a passing mutation as in W. borbonica ‘Arderne’s White’. These hybrids lack the outcurved bract tips of W. humilis, which may be the most useful diagnostic character for recognising it.
Goldblatt, P. (1989) The genus Watsonia. (National Botanic Gardens: Kirstenbosch) ISBN 062012517
Goldblatt, P., Manning, J.C., Raimondo, D. & von Staden, L. (2013) Watsonia humilis Mill. National Assessment: Red List of South African Plants version 2013.1. Accessed on 2014/1/28.
Watsonia hysterantha Mathews & L.Bolus is endemic to granite outcrops on a small section of the western Cape coast, South Africa.
It is winter growing and summer dormant like many other South African irids, but unusual in flowering in autumn at the beginning of the growing season. This not hysteranthy in the strict sense of producing pre-formed flowers direct from a bulb before the leaves like Amaryllis and Nerine. Instead, corms that have reached a critical size in the previous year “bolt” when they start growth, producing a flowering stem with about 4 short sheathing leaves. Smaller corms produce a vegetative shoot with about 6 leaves to 70 cm long and no stem above the ground.. As in all the watsonias, growth is sympodial – meaning that growth is terminated by the inflorescence. Each corm therefore dies after flowering, but not before producing offsets below ground as well as seed.
A practical consequence in a warm autumn like we had in Adelaide this year is that, while the hysteranthous amaryllids were all late in coming into leaf and many missed flowering this year, Watsonia hysterantha did not wait for low soil temperatures but started growth in early March as normal.
Per the Royal Horticultural Society colour charts, the flower colour is RHS 40B; it’s quite literally miniate, or the colour of red lead oxide.
This watsonia can be grown in the open garden but to keep track of the corms I grow it in 300 cm pots, lifting and dividing each summer. It multiplies vegetatively, and the only difficulty I have found with its management is in maintaining a high proportion of flowering-size corms. My stock came from Bruce Knight of Sydney, who imported seed in about 1995. As it is endangered in its natural habitat and further importations may no longer be possible, it’s important that existing stocks in Australia are maintained. Another Watsonia species, W. humilis, is even more critically endangered but at least one strain of it has been passed between gardeners in this country since its importation in the 1830s.
Goldblatt, P. (1989) The genus Watsonia. (National Botanic Gardens: Kirstenbosch) ISBN 062012517
The graph below plots the date on which the first flower has opened on each of six Moraea species in my garden over 18 years. The site is the Adelaide suburb of Warradale, close to sea level at 35° S latitude. All six species are summer dormant, spring flowering corms from South Africa where they evolved in Mediterranean-type climates similar to Adelaide.
This kind of “experiment” is rather in the tradition of Gilbert White, or those other English country parsons who competed to report the first cuckoo of spring in letters to The Times. Among its many limitations,
- The date of first anthesis may not be the best way to characterise flowering period; a measure of peak flowering may be more meaningful
- Microclimate was not controllable, although all plants were grown in the open in the same garden
- Each species was represented by only one genotype
- There are gaps in the records when a species either failed to flower or was not observed in a particular year
However, the graph seems to justify some comments:
- The sequence of flowering shows some constancy, with a ‘late’ group of M. bellendenii, M. ochroleuca and M. setifolia always flowering after the ‘early’ M. aristata, M. flaccida and M. vegeta.
- Flowering time did not change in the same direction or to the same extent for all species in each year. Each species has its own response to the weather that it experiences. For example, M. aristata and M. vegeta moved in contrary motion much of the time, but in near unison from 2007 to 2010 (yes, that’s a musical metaphor!).
- There may have been a general trend to earlier flowering in the “global warming” period of the late 1990s. Subsequently, flowering times may reflect the southern oscillation with late flowering in the notably cool, wet years of 2010 and 2011.
The mechanism controlling flowering has not been investigated in these Moraea species, but experience suggests it is not determined by photoperiod but by the accumulation of sufficient shoot biomass to support flowering and fruiting.
A second graph gives the corresponding results for seven species of Watsonia with similar phenology to the Moraea.
Again, the sequence of flowering shows some constancy, with W. aletroides consistently the first to flower and W. angusta or W. marginata usually the last. Again, each species reacted in its own way to seasonal conditions. Again, some of them seem to show the effect of a run of hot dry years in the late 1990s but more complex responses to the alternation of La Nina and El Nino periods. The response of W. laccata was especially variable, possibly as (unlike the other six), this species was represented by a population of mixed genotypes from seed.
The combined effects of temperature, insolation and rainfall may be complex, and biomass accumulation may not be easily modelled by the day-degrees method that is useful in higher latitudes where plant growth is closely correlated with temperature. The effects of rainfall and temperature may vary greatly according to the part of the growing cycle that they impact, and cannot be understood by integrating them across the whole growing season.
A cool, overcast winter may delay or inhibit flowering in these irids by slowing their biomass accumulation. As commercial Gladiolus growers have found, flowering fails if winter light intensities are too low. Drought in the period of vegetative growth has the same result. The first elongation of the flowering stem in Moraea flaccida and Watsonia meriana has been shown to coincide with the exhaustion of the old corm from which the plant has grown and the beginning of resource allocation to the formation of the next season’s corm. This is the most vulnerable point in the annual cycle of these irids. Once flowering has been initiated, even a few days of hot dry weather in spring can goad the plants into completing their flowering rapidly by producing fewer and smaller flowers.
(Please click on the graphs if you want to see them full size)
The South African irid Moraea ochroleuca is in flower here at the moment, with its characteristic buttercup-yellow bowl-shaped flowers. The shiny brown veins that radiate from the base of the perianth secrete nectar, and each flower lasts for just two days.
Goldblatt et al. (2005) described its smell as putrid and the pollination syndrome as sapromyophily by carrion flies in the families Calliphoridae, Muscidae, and Sarcophagidae. However, the M. ochroleuca in my garden has a faint smell of yeast, just like the medium used to culture Drosophila in genetics labs. And, sure enough, a species of Drosophila is attracted to these flowers where it remains for long periods apparently feeding on the nectar. I doubt that Drosophila could pollinate this flower, since it stays on the nectaries and would only contact the stigmas, which are held far above the nectaries, by accident. In any case, my M. ochroleuca have never set seed in 12 years despite an attempt at hand pollination, and this is probably due to self-incompatibility as they are all one clone.
Maybe this clone is atypical in its scent. Or maybe we’re describing the same scent in different ways; it’s probably a subjective judgement whether a smell should be called putrid. Humans don’t perceive smells the same way that insects do, and there is lot of variation among humans in judging a perfume. The scent of another sapromyophilous irid, Ferraria crispa, has sometimes been compared to carrion. But to me, it has more of a pungent chemical smell reminiscent of iodine; and my wife says it’s like very stale spices. Neither the Moraea or the Ferraria has the pervasive rotting-corpse stench of Stapelia (Apocynaceae) or Dracunculus (Araceae); it’s necessary to get up close to detect their scent at all.
The presence of nectar is unusual in a sapromyophilous flower; it may be that, having only recently evolved this pollination strategy, Moraea ochroleuca still needs to provide a food reward to the pollinator. Goldblatt et al. also recorded a range of other Diptera including syrphids (hoverflies) as well as honeybees as occasional visitors.
The history of Moraea ochroleuca in Australian gardens is obscure. Unlike many other showy South African irids, the species formerly placed in Homeria were not promoted in 20th century gardening literature or advertised by retail nurseries. However, a few of them were early introductions into botanic gardens, and there may even be some truth in the anecdotes about Boer war veterans bringing back “Cape tulips” as souvenirs. The plant growing at the Adelaide Botanic Garden in 1859 under the name of ‘Moraea grandiflora’ may have been M. ochroleuca. The flower pictured above is heritage garden stock from the Adelaide area, and is a close match to a feral specimen collected by Ray Alcock at Yallunda Flat, AD96452089, that Peter Goldblatt determined as M. ochroleuca.
Francis, G.W. (1859) Catalogue of the Plants under Cultivation in the Government Botanic Gardens, Adelaide South Australia.
Goldblatt, P., Bernhardt, P. & Manning, J.C. (2005) Pollination mechanisms in the African genus Moraea (Iridaceae, Iridoideae): floral divergence and adaptation for pollinators. Adansonia 27: 21-46.