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Thursday 20 September 2018

Ash flowers don't need to be showy to attract wind

Ash trees flower before their leaves open
Ash trees flower before their leaves open

Jim Hurley - Nature Trail

The Ash is Ireland's most common, tall native tree. It is not in leaf yet but is coming into flower. The Ash is wind-pollinated, so it needs to flower before the leaf buds open so that its pollen can blow about unimpeded through its bare branches.

Many wind-pollinated trees bear catkins like the Alder, Hazel and willows. The Ash is wind-pollinated, but it does not bear catkins.

When most people think of flowers, they think of showy structures bearing brightly-coloured petals like the recently enjoyed Daffodils and Tulips. Ash flowers don't have either petals or sepals; they are stripped down to their essentials and since they don't need colour or showiness to attract the wind to pollinate their flowers they are certainly not eye-catching.

Ash trees are nominally dioecious meaning that, like people, there are separate male and female sexes. One tree is a male individual; another tree is a female individual. So far, so good. However, things get complicated in that some individuals can change sex and do regularly, even on an annual basis.

Some trees change sex completely, others do so partially, so that they may be mainly male but have one or two females branches or vice versa. Some trees are hermaphrodite being equally male-flowered and female-flowered. Some ever have dual-sex flowers.

However, when both sex organs are present together they do not normally ripen at the same time thereby preventing self-pollination and self-fertilisation.

Male and female Ash flowers look superficially alike. Both are borne in dense bunches and both are dark purple in colour.

In the image above, the pale grey shoots and twigs of the Ash can be seen bearing the familiar, mitre-shaped, sooty black winter buds. The terminal buds on the right are beginning to burst and the young green leaves are about to emerge.

The flowers may be seen sticking out in spiked clusters near the ends of the twigs. These are all female flowers catching pollen grains blowing in the air; male flowers also stick out in spiked clusters but are smaller, darker and more tightly bunched.

Once the female flowers have been fertilised by wind-borne pollen, they develop into fruits bearing conspicuous wings in late summer and autumn. Because of their appearance, bunches of these fruits are popularly known as 'keys'. In winter and early spring, the keys fall from the trees and are dispersed by birds and mammals providing a welcome food source for birds, mice and squirrels.

Wexford People

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