Hawthorn – Sunday 17th September, 2023

It is autumn, or as, they say in the US, fall, an apt name as the big signifier is falling leaves. Meteorological autumn began on September 1st, astrological autumn in a few days with the equinox on September 21st.  Deciduous trees will lose all their leaves between now and December. This is very noticeable along Earlham Grove where curled, brown, dead plane tree leaves are being scattered on the pavement. Daily, they are picked up by the roadsweepers and bagged.

 I look around our garden for such signs. The willow by the greenhouse has leaves that are turning yellow,  and those in the willow hedge near the Dare to Dream stage too. The hawthorn near the large cherry tree has some yellow leaves; the change is unavoidable. In these leaves, chlorophyll is no longer being produced and that already there is dead. Soon the leaves will fall.

 This is due to the hormone auxin in the leaves and in the rest of the tree. In autumn, as the days grow cooler and daylight lessens, the auxin level in the leaves falls, becoming less than that in the bulk of the tree. Where the stem of the leaf joins to the branch is call the abscission layer. The falling off of auxin causes the cells in the abscission layer to elongate and fracture, weakening the connection. It will just take a breeze to blow the leaf away.

 The vessels that bring nutrients and water to the leaf have been closed off. This has caused the chlorophyll to die, and reveals the yellow which was always there but has been masked by the green of chlorophyll. The yellow is formed by the pigment carotene and red by anthocyanin.

 Most conifer trees are evergreen, and their leaves are different. They are either needles as in the pines, or stringy as in the cypresses. The resin in the tree stops the leaves from freezing in a cold winter.

 A tree that is a little puzzling is the privet. We don’t normally see it is as a tree as we use it as a hedge. Privet is popular because it doesn’t lose its leaves in autumn. But the leaves are not like conifers, but very like those of deciduous trees, flat and paddle like. The privet is regarded as semi-deciduous, which means it will lose some of its leaves but we’ll hardly notice. Unless we have a very cold winter, and then it will give up and shed many of them.

 Leaves can be regarded as factories for making sugar out of water, CO2 and sunlight, enabled by chlorophyll, a process known as photosynthesis. But as the days shorten and grow cooler, there is too little sunlight for this to work. Deciduous trees have developed the system where they shed their leaves, rather than keep them through the winter. But already on the stems are the buds for new leaves that will grow in spring.

 Holly leaves are waxy and retain their leaves, just shedding older ones from time to time, but not having an autumn leaf fall. The abscission layer is not activated by autumn light and chill. The prickly leaves protect the bark from being foraged by larger animals like deer, losing them would end this barrier.

 Our most attractive leaf for autumn colour is the cherry. At a certain point, you get the last of the green along with yellow and red. The plane trees on Earlham Grove can be quite glorious too in their autumn colour. I sometimes pick one off the pavement to admire it, but there is no point keeping the leaf, as the remnant of life will ooze out of it, leaving it brown, crinkly and very dead.

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