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.