Sunday, 23 October 2016

Spines and cyanide

I recently posted a blog bearing the title of  'Autumn Leaves', in which I considered the phenomenon of leaf fall. 'But what of plants such as holly?' I hear you cry.

Its evergreen nature has made this tree very important in ritual over the millennia, for whereas the winter appears to bring about the death of deciduous trees, to be followed by a rebirth in spring, holly appears immortal. Of course holly does lose its leaves, but at a fairly constant rate throughout the year and anyone attempting to crawl under a holly tree becomes quickly aware of this when hands and knees receive sharp prickly reminders, especially as the dead leaves are slow to rot.

By bearing its tough, waxy leaves throughout the winter holly is able to take advantage of early spring sunshine but the downside is that its leaves are available to hungry animals which can, and do, browse the trees. Interestingly, studies in south-eastern Spain, reported in the Botanical Journal of the Linnean Society, have shown that when holly is browsed there is a rapid response at molecular level, with far spinier leaves being produced. This tendency for leaves on higher branches to be free of spines has been known for centuries and a vague memory led me to a poem by Robert Southey. In it he wrote:

                          No grazing cattle, through their prickly round,
                          Can reach to wound;
                          But, as they grow where nothing is to fear,
                          Smooth and unarmed the pointless leaves appear.

                                                                   The Holly Tree, Robert Southey 1774-1843

Be that as it may, even the smooth leaves are tough and unpalatable to many animals and are avoided where better browsing is available.  Even insects tend to eschew (rather than chew!) the leaves but one exception is the fly, Phytomyza ilicis. Here its very familiar mine is photographed on a spine-free holly leaf, but they are just as happy on a spiny leaf as, for this insect, the spines are irrelevant. (The caterpillars of the Holly Blue butterfly, Celastrina argiolus, normally restrict their diet to the flowers and buds.)

Phytomyza ilicis on a holly leaf in the churchyard at Norton,
Northants. 22 October, 2016

Another tough-leaved evergreen is Cherry Laurel, Prunus laurocerasus. This is a very popular garden shrub but, unlike holly (no relative), it is not native to Britain.

Cherry Laurel is familiar in gardens, often being used for
hedging. Daventry, 21 October, 2016
Whether in gardens or in semi-wild shrubberies it is rarely nibbled by deer, rabbits and so on and yet its leaves are unarmed. The defence is more subtle yet equally effective: the leaves are laced with cyanide. To be more precise, cyanogenic glucosides in the form of prunasin and amygdalin are present. (I know: I'm not an organic chemist either.) These substances are also present in the cherry-like berries, with nasty consequences for children who might be tempted to sample them. If I am out and about looking for leaf-mining insects I give Cherry Laurel only the briefest of glances although the presence of a tiny moth, the Apple Leaf Miner (Lyonetia clerkella) is sometimes confirmed, its larvae producing characteristic sinuous mines pale against the dark leaf.

Cherry Laurel is a member of the rose family, Rosaceae, as is the Almond, Prunus dulcis. The smell of cyanide is often likened to almonds and the kernel of the almond nuts does contain cyanide. This is no more than a harmless trace in the edible varieties but the nuts of ornamental almonds are to be avoided as there the levels of cyanide compounds make them distinctly toxic.
Not to be eaten! Ornamental almond at Newnham, Northants.
22 October, 2016
Back to evergreen shrubs, and I must mention Ivy, Hedera helix. To what extent will stock eat it? What defences does it have? According to the old song:

                             Mares eat oats and does eat oats and little lambs eat ivy...

...and up to a point this is true. According to my book - admittedly rather old (it was produced by the Ministry of Agriculture, Fisheries and Food) - 'Poisoning in animals can be ... serious, as large quantities of leaves and berries are sometimes eaten'. It appears that substances called saponins are present in all parts of the plant, although this doesn't stop the ripe berries being an important late-winter food for birds such as thrushes. It seems that at one time farmers would give cattle a few ivy leaves as a 'tonic' and this practice seems to have been harmless. Incidentally ivy belongs to the Araliaceae, a family largely of tropical lianas, and is completely unrelated to the Poison Ivies of North America. These latter plants belongs to the Cashew Nut family, the Anacardiaceae.

So we should not be surprised to find that otherwise palatable plants use a variety of defences to ward off hungry animals. Across the plant kingdom a huge range of dodges are employed, some quite bizarre - but  that is another topic.
A day after publishing this blog I noticed a holly lookalike in a nearby garden and I couldn't resist giving it a mention. It is, of course, the variegated form of Osmanthus heterophyllus.

Despite spiny, evergreen leaves it is not in the Holly family, Aquifoliaceae, but is a member of the Olive family, Oleaceae.

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