Primrose, Primula vulgaris, Primulaceae

With the possible exception of the bluebell, the primrose Primula vulgaris is probably Britain's most popular spring flower. Even now, on a freezing mid-February day, new leaves and flower buds are beginning to form in primroses in woodlands here in north-east England.  Primroses have been grown in gardens for centuries - probably since people first cultivated gardens - giving rise to numerous varieties and, through hybridisation with cowslips, the garden polyanthus. The primrose also excited Charles Darwin's curiosity, in his struggle to understand and define the nature of species. Like many before him, he was aware that ...

........ primroses, with their large single flowers, grew along woodland edges and hedgebanks....

... while cowslips, with their numerous small flowers on a common stalk, grew in pastures, but...

... wherever the two coincided they hybridised to produce false oxlips, with large flowers on a common stalk. Although every field guide to wild flowers describes primroses and cowslips as distinct species, in evolutionary terms they are really one - a genetically diverse species with a wide range of variation which, at its extremes, produces distinctive plants that are adapted to life in woodlands or grasslands. Primroses and cowslips are on the way to becoming two separate species, where they would satisfy the modern evolutionary biologist's absolute definition of a species, based on an inability to interbreed with other related species, but they haven't reached that point yet. No wonder this example of evolution-in-progress attracted Darwin's inquiring eye.

Ever since primroses and the primrose-cowslip false oxlips hybrids were introduced into gardens they have been exchanging their genes with other cultivated Primula species from the European mainland, introducing new flower colour genes that give us the range of brilliant hues that are available in garden centres today. Sometimes the gene exchange extends beyond the garden and back into the wild - as, for example, in the primrose you can see here.

Darwin was fascinated by primroses for another reason. His Cambridge University botany teacher and mentor, John Stevens Henslow, drew his attention to the fact that there are two kinds of flowers in any wild population of primroses...

....so-called pin-eyed flowers, with the stigma at the end of a long style, level with the top of the corolla tube...

... and with the stamens located way down in the corolla tube, as you can see in this sectioned flower...

... and thrum-eyed flowers, like this, where the stamens are at the top of the corolla tube...

... and where the stigma, at the end of a much shorter style, is located way down in the corolla tube.

The presence of two forms of flowers in the population is known as heterostyly and Darwin made numerous self- and cross-pollinations between the two forms, demonstrating that self-pollination failed and that crossing beween the two forms was necessary for seed set, and also that the differential placement of the stigma and stamens in the two forms aided cross pollination between them by insects. In his autobiography he remarked "No little discovery of mine ever gave me so much pleasure as the making out of the meaning of heterostyled flowers"

The two forms were illustrated in diagrammatic form in this publication: Darwin, C. R. 1862. On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations. [Read 21 November 1861] Journal of the Proceedings of the Linnean Society of London (Botany) 6: 77-96.

The scientific study of this floral arrangement has yet to run its course and, 150 years after Darwin first described the mechanism, the genes that control the development of the two primrose flower types are currently under investigation by Professor Phil Gilmartin at Durham University.

Darwin wasn't the only eminent Victorian to be beguiled by this flower. Primroses were the favourite flower of Benjamin Disraeli, Prime Minister in two Conservative governments during Queen Victoria's reign. Such was her affection for him that she sent a wreath of primroses to his funeral on 19th. April 1881. Thereafter that date was celebrated annually as Primrose Day.

Disraeli image source: http://upload.wikimedia.org/wikipedia/commons/0/0c/Benjamin_Disraeli_by_Cornelius_Jabez_Hughes%2C_1878.jpg

Passion Flower, Passiflora citrina, Passifloraceae

Passion flowers Passiflora spp. are amongst the most elaborate of all flowers but I particularly like the relative simplicity of this diminutive species, Passiflora citrina which hails from the hills of western Honduras and eastern Guatemala. Its mountain origins mean that it does well in a cool conservatory. The flowers are only a few centimetres long and the whole plant will happily grow in a large pot on a windowsill, trained up slender canes, where it will flower for months on end.

The structure of Passiflora blooms is unusual, with the ovary and stamens held aloft from the centre of the flower on a long column. It has been suggested that this arrangement evolved to avoid damage from visiting pollinators (usually bees, although large red-flowered species are visited by hummingbirds) that probe for nectar at the base of the petals. Keeping the ovary - all important for seed production - out of harm's way will making sure that the visiting pollinator contacts those stamens and stigmas on the way in and out is a win-win strategy from the plant's perspective. The flowers are protandrous, meaning that the anthers shed their pollen first and then, once it has all been removed by pollinators, those three nail-shaped stigmas on top of the ovary move outwards and become receptive, so promoting cross pollination. 

Passion flowers are loaded with religious symbolism, with the number of floral parts being taken by the faithful to symbolised various episodes in the life of Christ. For example, the ring of floral filaments is considered to represent the crown of thorns, the three stamens represent the nails used in the crucifixion and the five stamens represent the five wounds - usually with this species Passiflora caerulea being the 'typical' symbolic passion flower.

The edible passion flower P.edulis, like the one above, is easy to grow from the dimpled seeds in commercial passion fruits, although the flowers are quite small and lack the boldness and symmetry of P.caerulea. I've found that the best way to germinate these seeds, and those of many other tropical and subtropical plants like lychees, is to clean the seeds then put them in a polythene bag of moist peat (or a substitute like coir) and keep them in the airing cupboard near the hot water tank - remembering to check regularly for germination.

Butterwort, Pinguicula moranensis, Lentibulariaceae

Carnivorous plants have a problem. On the one hand, they must catch and digest insects to supplement the meagre soil nitrogen supplies in the boggy habitats that they inhabit. On the other hand, they depend on insects to pollinate their flowers, so there is no future in 'biting the hands that feed them' to the extend that they produce no seed. So most tend to have flowers that are well separated from their lethal, carnivorous leaves by a long flower stem - and this attractive butterwort from Mexico, Pinguicula moranensis, is no exception.

This species makes an excellent plant for the cool conservatory, for two reasons. One is that it produces these attractive magenta blooms from late autumn right through until early summer.

The second reason is that it's a formidable consumer of several annoying greenhouse pests, including whitefly, greenfly and those annoying little mushroom gnats that tend to come with many brands of today's potting composts. The way in which small insects are attracted to the glutinous glands on the leaves and are then trapped and digested is very satisying - and you can see some examples of butterwort's insect catching prowess here. This plant is flowering flypaper.

This species is very easy plant to propagate - just pull the leaves away from the leaf rosette and insert each to a depth of about 5mm. in a moist compost. Buds will develop at each leaf base, to produce a new rosette, much as they do when you propagate an African violet or Streptocarpus species.

Place butterworts amongst your conservatory plants and they will act as a form of botanical biological insect control; they won't eliminate your pest problems but they will produce tangible evidence that they are making inroads into the insect pest population. This plant comes with a feelgood factor if you are troubled by tiny winged insect pests.

Seen from the side, it's clear that this butterwort's flowers must be pollinated by long-tongued butterflies, landing in the flat-faced flower and probing with a long proboscis to the bottom of that long nectar spur behind the flower. The flower is the only part of the plant without adhsive and digestive glands - look closely at the flower stalk (double-click) and you can see here that it's coated with adhesive glands that have caught a few tiny greenfly.

Golden Hop, Humulus lupulus aureus

Hops have been cultivated in Britain for flavouring beer since the sixteenth century – and for much longer in continental Europe – thanks to the resins produced at the base of the bracts in the female flowers (seen in the picture here), which are converted to bitter isohumulones during the brewing process. There are separate male and female hop plants but in modern times only female hops (with the cone-like flowers in the photo here) have been cultivated, as the seeds are undesirable in the brewing process.



My reason for growing hops (the golden-leaved variety) is rather more prosaic. Hops are rapid climbers with incredibly tough stems and for about a decade it was only the covering of hops woven around the fence posts that kept the fence at the bottom of our garden upright, after the posts had rotted at the base.


Charles Darwin was fascinated by climbing plants and during one of his bouts of (probably psychosomatic) illness took to his bed and had hops in pots brought into his bedroom, so he could wile away the hours studying the climbing process in this plant. He recorded his observations in detail in his book The Movements and Habits of Climbing Plants , published in 1875, in his usual precise prose.

“ When the shoot of a Hop (Humulus lupulus) rises from the ground," he wrote,"the two or three first-formed joints or internodes are straight and remain stationary; but the next- formed, whilst very young, may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun. The movement very soon acquires its full ordinary velocity. From seven observations made during August on shoots proceeding from a plant which had been cut down, and on another plant during April, the average rate during hot weather and during the day is 2 hrs. 8 m. for each revolution; and none of the revolutions varied much from this rate. The revolving movement continues as long as the plant continues to grow; but each separate internode, as it becomes old, ceases to move.”


Darwin was describing the process that botanists refer to as circumnutation, where the shoot tip describes a wide circle until it touches something that it can coil around (clockwise, in the case of hops). He went on to time the rotations of a hop shoot over no less than thirty seven rotations, testament to Darwin’s incredibly meticulous approach to making scientific observations that can, regrettably, make some of his books somewhat tedious to the modern reader. Still, the mental picture of Darwin laying in bed, carefully timing the rotations of hop shoot tips with his watch, does have a considerable charm.

You can find some further information on hop epidermal hairs used in climbing and also hop resin glands by clicking here.

Iris spp., Iridaceae

Until last year we had a large patch of this Iris unguicularis growing outside our back door, between the house wall and the path - a sunny, well drained, south-facing spot that suited it perfectly and guarateed plenty of flowers from Christmas onwards. This photo was taken in January last year, after a sleet shower, when the plant was blooming in the thawing snow. Soon after that I dug it up so that I could relay the path, and divided the plant, and there's no sign of flowers yet this year, which is a shame. It comes from North Africa.

You can appreciate the primrose fragrance if you bring the flowers indoors, although they only last for a day. Iris unguicularis (also known as Iris stylosa) is a delightfully delicate winter-flowering Iris but it was always hard to find blooms as perfect as this one to photograph. Snails hibernated in the dry leaves that accumulated at the base of the plant and often began to nibble the flowers even before they opened. The dry leaves were also favourite nest material for field mice.

Every year I plant pots of Iris danfordiae so that we can enjoy their vivid but short-lived flowers in the conservatory in late winter. Then I transfer them into the garden, when the bulbs produce numerous smaller bulbs that take years to reach maturity and only flower sporadically. I must have transplanted a couple of hundred into the garden, but never see more than a handful of flowers every year. It comes from Turkey.



When it comes to spring-flowering Irises, this is my personal favourite - Iris bucharica which comes from Central Asia, growing in high altitude areas of Afghanistan, for example. I have tried it outside in the garden a few times but it never thrives here, perhaps because the garden is too shady, but in pots it produces a fine display of beautifully fragrant blooms in March, and for my money it's a far superior container-grown Iris to the ubiquitous I. danfordiae.

Hydrangea spp., Hydrangeaceae

Season's Greetings and Best Wishes for 2011 to all readers of this blog.

I know some people who think that Hydrangea flowers are more attractive when dead that alive. It's a sentiment that I have some sympathy with...

... because I don't find the heavy, long-lasting infloresences of mop-headed Hydrangea macrophylla cultivars particularly attractive, but when winter has done its work on the dead flower heads the skeletonised florets, like those in the first photograph, are transformed into something rather delicate and beautiful.

Hydrangea infloresences are an interesting example of division of labour, where the central mass of short-lived fertile flowers, bearing stamens, stigmas and ovaries, have no particular visual advertising to attract insects but devolve that job to the large sterile florets, that surround the edge of the inflorescence like orbitting satellites. Petals wilt as soon as a flower is fertilised but since that outer ring of sterile advertising florets can never be fertilised they last indefinitely, throughout the whole flowering period and often right through the winter as skeletonised petals.






















The arrangement is delightful in some of the larger species like H. aspera (also known as H. villosa) when it's grown in an open, woodland garden.

Advertising all those fertile florets with just a handful of sterile florets is a very efficient way for the plant to attract its pollinators, but it's hardly surprising that plant breeders have hybridised and selected species for large numbers of sterile, long-lasting florets for maximum impact in a shrub border, reversing nature's economical use of advertising in the quest for maximum visual impact.

Tephrocactus articulatus var. papyracanthus, Paper-spined Cholla

This must surely be one of the most intimidating of all plants - the Paper-spined Cholla Tephrocactus articulatus var. papyracanthus. It comes from Argentina and those sabre-shaped spines that look so lethal are about eight centimetres long, but they're not as dangerous as they look. They really are like paper and simply bend under the slightest pressure; you'd be hard-put to draw blood with them. The tufts of tiny bristle-like hairs called glochids that you can see at their base are far more of a problem. They have barbed tips and are intensely irritating when they embed themselves in your fingers.The easiest way to remove them from flesh is to use a piece of adhesvive tape to pull them out.

Paper-spine cholla ( or the 'Edward Scissorhands' cactus, as my kids used to call it) is a slow-growing relative of the prickly pear cactus and is easily propagated by breaking off one of the jointed stem segments and rooting it in well-drained, gritty compost - but be sure to watch out for those nasty little glochids.