Mahonia x media 'Charity'

I posted some pictures a couple of weeks ago of Sparmannia africana, which has remarkable stamens that spread outwards after they are touched. Here's another genus with touch-sensitive stamens - Mahonia - but in this case the stamens move in the opposite direction when they've been stimulated. The plant portrayed here is the hybrid between M.japonica and M. lomariifolia known as M. x media 'Charity', which has long racemes of lily-of-the-valley scented flowers that open in autumn and bloom well into winter, producing purple-blue berries that blackbirds love in early spring. In my garden I've watched over-wintering blackcaps pecking the flowers in this species and there a suspicion that they may be after the nectar, as the flowers don't attract any insects at this time of the year.

When the flowers first open the six stamens are spread wide, pressed against the petals, but if you touch the base of the stamen filament there's a slight delay and then they quickly move inwards, eventually coming to rest against the stigma.

Here's a flower where I poked the base of four of the six stamens with a pine needle. Three have closed on the stigma and the fourth is well on the way. The speed of movement depends on the ambient temperature - it was a cold day when I triggered these - but under warm conditions it takes about one second. Why do they do this? Hard to be sure, but I guess that it's likely that the foot of a visiting insect triggers the stamen movement, which forces pollen onto the insect's leg.

Mahonia and the closely related Berberis species have touch-sensitive stamens. Some botanists argue that species in these two genera should be amalgamated under the single genus Berberis, because they are so similar and some Mahonia and Berberis can form inter-generic hybrids (x Mahoberberis).

There's one other startling feature of these two genera that you can't see unless you prune off large branches. Their wood is bright yellow, due to the presence of a natural dye called berberine. The brilliance of the yellow wood is startling - even more so if you view it under ultra-violet light, because the berberine then fluoresces even brighter yellow.

Cucumis metuliferus, Cucurbitaceae

Each year I try to grow something a little bit unusual that's also edible and this year it was the turn of this weird fruit, the African horned melon Cucumis metuliferus. The plant resembled a small-leaved melon that rambled across the greenhouse staging, producing numerous small unisexual yellow flowers. Whenever a female flower appeared I carefully pollinated it but none seemed to be setting any fruit. It was only when I had given up and was about to throw the plant away that I parted the leaves and found this fruit, about 12cm. long and looking like something from another planet, revealing that one pollination had been successful. It looks like a vegetable version of a medieval mace and would probably make a good weapon, because those spines on the tips of the horns are very hard and sharp.

So, what does it taste like? Rather like cucumber, but with lemon juice added - quite refreshing - but the texture is strange. Each seed is surrounded by a semi-solid gelatinous coating and in the mouth it feels rather like eating frog spawn (not that I've actually eaten frog spawn, but this is what it must be like). Definitely an acquired taste. I spat out the seeds and will sow some next year, to see if it's any better second time around.

The plant is a native of Africa, although I gather that it's now cultivated in several countries and is known as kiwano on the US West Coast.

Cercidiphyllum japonicum, Katsura tree

For most of the year you could walk right past a Cercidiphyllum japonicum tree without a second glance but in autumn it always stops people in their tracks - not just because of its autumn colour but because of its wonderful aroma. As the leaves turn yellow they produce a compund called maltol, which makes the whole tree smell of burnt toffee, rather like a toffee apple or candyfloss stall. Maltol is used as a flavour enhancer in the food industry, where it's known by the E number E636.

This specimen is in Durham University Botanic Garden but about five years ago I planted one in my own garden - and it has grown remarkably quickly. It's rather a large tree for a small garden - reaching 45m. in the wild, but it will be a while before mine becomes a problem. Katsuras, which are native to China and Japan, are supposedly frost-sensitive but mine came through last winter's prolonged snow and ice without any sign of damage. It will be interesting to see how the tree copes with a hot, dry summer (if we ever get another one here), as under those conditions it sheds all its leaves as a drought-avoidance mechanism, then refoliates when the rain returns - so can have two 'springs' in a single season.

This autumn year I collected a bag of those toffee-scented leaves to keep through the winter and sniff occasionally - as a reminder of that wonderful aroma on a warm, sunny afternoon.

Sparmannia africana, African Hemp

All plants are capable of gradual movement as a result of slow growth, but relatively few species can make the kind of rapid movements that we tend to associate with animals. The Venus fly trap Dionaea muscipula and the sensitive plant Mimosa pudica are the two species with rapid leaf movements that are most familiar but another fast-mover is African hemp Sparmannia africana. If you gently brush the cluster of golden stamens in the centre of the flower nothing happens for a second, than they move outwards, away from the stigma. The time delay between the photo above and that of the same flower whose stamens have been brushed, below, is about five seconds and you can easily see how the stamens have spread apart.

Presumably this is some kind of mechanism to aid pollination, although how it would do that isn't obvious. Similar stamen movements occur in Mahonia and Berberis flowers, although in those cases the stamens move rapidly inwards when they are stimulated.

Sparmannia africana comes from South Africa and makes a very fine house plant - if you have room for it. My plant grew too big for our conservatory and is now in Durham University Botanic Garden, where it has reached about three metres in height and is currently flowering and setting seed.

Mutants and Monstrosities

You don't need to be a gardener for very long before you discover that within every species there are always a few plants that don't conform to the norm. They're either mutants (where changes in their genes produce a different shape, form or colour of plant) or they're monstrosities (where there some external agency, like environmental stress or disease has led to abnormal growth).

Only mutants produce heritable changes, that will be passed on to at least some of their progeny. I would guess that the abnormal ox-eye daisy (Leucanthemum vulgare) above is a mutant. Some of its florets are spoon-shaped and since this abnormal plant turned up in my garden a few years ago others have appeared in subseqient years with the same characteristic, probably arising from seeds produced from those abnormal florets. Cultivars of a number of daisy-like species that have flowers composed entirely of spoon-shaped petals have been bred, so it's highly likely that this is a genetically controlled change in the pattern of petal development.

I'm pretty certain, on the other hand, that this two-headed ox-eye daisy is a monstrosity, the result of some agency that has interfered with the normal development of the flower and produced twin heads. Temperate-shock (i.e. frost) can sometimes do this.

This monstrous spear thistle Cirsium vulgare is the result of several flower heads becoming joined, side by side into a cock'scomb-like inflorescence, probably as a result of either stress during a critical phase of flower bud development or perhaps infection with a bacterium - Corynebacterium is known to cause a similar phenomenon, known as fasciation, in ...



Linaria purpurea...

Forsythia x intermedia, where it causes a switch in symmetry of the stem, from radial symmetry to flat, plank-like growth...

... and occasionally in herbaceous species like candelabra primulas ......

Some mutant forms of growth are widely cultivated for human consumption, like the cauliflower - which is a mutant inflorescence where the flower buds proliferate to the extent that they can no longer open properly.

My favourite monstrosity, though, is to be found in foxgloves Digitalis purpurea, where occasionally the last flower to form at the top of the floral axis results from fasciated growth of several flower buds. If you compare the monstrous flower on the left with the normal one on the right you can see that it has four stigmas, styles and ovaries, within a single tubular corolla - four flower buds fused to form a single flower.

Seeds

Since this is the time of year when most plants in my part of the world are setting seeds, I thought I'd post a few images of these wonderful objects. This is one of the ripening seeds from my runner beans Phaseolus coccineus, with the testa and one of the cotyledons removed, revealing the embryonic plant inside. The cotyledons contain the store of proteins, carbohydrates and lipids that will provide sustanance for the germinating seedling until its first true leaves unfurl and begin to photosynthesise. In this image you can see that the rudiments of the venation in those first true leaves, that will transport sugars to other part of the young seedling, have already formed in the embryo. The embryo is attached to the cotyledons at its hypocotyl and the embryonic root points downwards and to the left. 

There's a heavy crop of oak (Quercus spp.) acorns in this part of Durham this year - it's a mast year. Oak acorns - technically nuts - germinate soon after they fall to the ground, anchoring themselves via their root but then suspending further growth until spring, when the shoot forms. If you are planning to plant oak trees, it's best to sow the fresh acorns soon after they fall.

Once seeds germinate their root needs to spear into the soil quickly, driven by the hydraulic force inside expanding cells. Here you can see the glistening root cap of a maize (corn) Zea mays seedling, producing the mucilage that lubricates its path between the abrasive soil particles. Further back, you can see the forest of root hairs, each a single elongated epidermal cell that makes intimate contact with the surface of soil particles and absorbs water and soluble minerals. Each root hair has a short life span, of perhaps a day, and new hairs are formed constantly behind the advancing root tip; collectively they constutive a vast absorptive surface.

Cyphostemma juttae, Vitaceae

About 20 years ago a South Africa botanist gave me a few seeds of this strange succulent, Cyphostemma juttae, which grows in rocky, arid parts of Namibia. It has taken all that time for the seedling to reach this size - about 70 cm. tall - on my windowledge and since it can grow to 2.5 metres tall in the wild it's still got some way to go, although it's under less than ideal conditions, in a flower pot.. Every winter it sheds those succulent leaves completely, leaving only the swollen stem - known botanically as a caudex - then regrows leaves in late spring. It first flowered about five years ago, but this is the first year that the flowers have produced fruits.

The stem constantly sloughs off layers of papery epidermis, which I suspect might have some role to play in reflecting intense sunlight.

The thick, lobed succulent leaves have a saw-tooth margin and each leaf lobe is about 15cm. long.

These small, glassy beads appear on the underside of the leaves, soon after they've fully expanded.

I've yet to discover whether they are crystalline sugar or resin - I suspect they are the latter.

One of the most remarkable aspects of the plant is that it is in the grape family and is commonly known as the Namibian grape or tree grape. You'd never suspect that until you take a look at the fruits, which turn purple when they ripen, but unlike the edible grape they are poisonous.