Yew tree pollination

 February is the month when the tiny male cones of yew trees begin to shed their yellow pollen. These were photographed in Teesdale a couple of days ago. 

Yew trees are dioecious - either male of female - and the male pollen is carried on the wind to single female ovules hidden amongst the foliage on another tree. These are tiny and can be hard to spot, but here is one, ready to receive pollen.

After fertilisation the outer wall of the ovule swells to become a soft, fleshy, aril that surrounds the single hard seed inside. It turns bright red in autumn, to attract the attention of birds. 

Every part of a yew tree, except the aril, is poisonous to humans but thrushes, fieldfares and redwings quickly devour them. Nuthatches extract the hard seed, wedge it into crevices in yew trunks and hammer it open to extract the kernel. There's more about them in this Guardian Country Diary.

Yew

This is the time of year when yew 'berries' begin to become conspicuous. Yew is one of only three British native species belonging to that division of the plant kingdom known as the gymnosperms - seed-producing plants that don't have flowers and don't enclose their seeds in an ovary to form a fruit. The other two are juniper (whose seed has a succulent outer coating - seed picture at the bottom of this post) and Scots pine (which carries its seeds in woody cones - see picture below). 

In this picture you can seed the fully formed yew seed on the right, half covered by a pale green cup. This is the tissue that quickly expands to form the scarlet succulent aril, that you can see half-formed on the seed on the left. This squashy cup is attractive to birds that disperse the seeds, effectively judging from the numerous seedlings that appear in places where birds perch, such as crevices in old stone walls after the seed passes unharmed through the bird's gut. The seed itself is deadly poisonous to mammals but I've watched nuthatches wedge it in a bark crevice, hammer it open and eat it.

Here's a fully formed aril, which almost completely encloses the seed, like a scarlet doughnut.

Juniper 'berries'

Scots pine cone

Yew Midge Gall

This year some of the yew trees in Teesdale are heavily infested with this 'artichoke' gall, caused by a tiny midge called Taxomyia taxi. This insect has a strange two-speed life cycle. Eggs are laid on the yew shoot tip foliage in late spring, inducing the formation of a swollen terminal bud where they spend the winter. Some emerge as adults in the following year but others, like his one, have slower development and spend a second winter inside an enlarged and more conspicuous gall before emerging as midges two years after the eggs were laid.

Margaret Redfern's recent New Naturalist book, Plant Galls, is a mine of information on the strange world of these plant-insect interactions.

Yew

These strange objects are the pollen-producing structures of a yew tree. There are separate male and female trees and at this time of year these little pollen-producing cones, known botanically as strobili, are very conspicuous on the male trees, releasing pollen that's carried on the wind to the females. Yew belongs in the division of the plant kingdom known as the gymnosperms that includes the cone-bearing conifers, but unusually it doesn't produce its seeds within a woody cone. Instead each seed is carried in a succulent red cup, which you can see by clicking here.