Across the world there may be three trillion trees. A mature tree may have 200,000 leaves, so there are a lot of leaves in the world - not counting those on herbaceous plants, grasses and shrubs.  The broad structure of a leaf is outlined here in woodlands.co.uk Tree ID. The leaf is the site of photosynthesis, providing food for the tree, and oxygen for us.  As the leaf is rich in nutrients, it is a source of nutrition for many organisms - other than the tree.  Oak trees are said to support over 2000 species, ranging from mammals, birds, beetles, spiders, fungi - through leaf-based food chains.  Leaves also support many micro-organisms through the detrital food chain (the decomposition of leaves in the litter layer and the soil). We do not eat many tree leaves, though some do make their way into our diet.  For example, the evergreen shrub Camellia sinensis is widely grown in many parts of the world for the production of tea.  The young leaves can be picked in spring and dried to make tea.  Leaves of other plants are used in various herbal infusions or for flavouring such as bay, sage, oregano, thyme etc. The fact that leaves are attractive to so many herbivores means that trees (and other plants) take measures to protect themselves. Some measures are physical - such as spines, thorns, prickles etc.   But when is a thorn a thorn, rather than a spine or a prickle?  These terms are used casually and interchangeably.  Botanically speaking, they are all ‘spinose structures’ that is hard, rigid extensions or modifications of leaves, roots, or stems - all of which have sharp, stiff ends and all have the same role - to deter animals from eating the plant that bears them.  Plants that bear sharp structures that deter herbivory are termed spinescent.  There are differences between these various ‘structures’. thorns are derived from shoots (they may be branched or not, may or may not have leaves). The thorns of Hawthorn (Crataegus monogyna) can bear leaves. spines are derived from leaves (they may be formed from all of the leaf or just part of it and like thorns they have vascular tissue*) prickles are derived from the epidermis (the outer layer of cells of a stem, root or a leaf).   Prickles may be found almost anywhere on a plant and they do not have vascular tissue inside.  Wild lemon and lime trees (Genus: Citrus) have spines, which protect young plants and indeed the fruits. The defences on roses are often described as thorns, but they are prickles, as they do not have vascular tissue (xylem and phloem) inside them. Sometimes, the leaf epidermis forms smaller, ‘simpler’ physical barriers called trichomes.  These are outgrowths of epidermal tissue but generally consist of only a few cells which form a defence against small insects.  Equally, a thick,  waxy cuticle on a leaf may be something of a deterrent to smaller insects. Leaves sometimes form ‘teeth’ on the leaf margins and leaf apices.  A classic example of this is seen in Holly.  Holly leaves that develop at ground level are wavy, with large triangular ‘teeth’, bearing spines.   As the tree grows and holly can reach up to 80 feet,  the leaves become less spiny. The spines offer protection against grazing animals at the lower levels but are no longer needed when the trees reach a certain height. While physical defences such as spines, prickles and trichomes can deter various herbivores,  chemical defences may also be deployed.  Chemical defences can take different ‘forms’, such as  [caption id="attachment_28705" align="alignright" width="300"] Oozing latex - Euphorbia[/caption] tannins and phenolics. These create an bitter taste, they are complex polyphenols built from several phenolic molecules. Tannins are common in leaf tissues - particularly in the cells on the top surface of a leaf.  Scale leaves of buds are often particularly rich in tannins, reducing  the palatability or "tastiness" of the tissue thereby offering protection from herbivores.  Alkaloids are again usually bitter tasting compounds -, many of them derived from amino acids. Glycosides, as the name suggests, contain a sugar that is joined to another chemical, such as cyanide (as seen in bitter almonds (amygdalin). Another possibility is that leaves may emit chemicals (aka VOC’ volatile organic compounds, scents, aromas) that deter insect visitors, or if a leaf is under attack by a insect pest then a leaf may release a VOC to ‘warn’ nearby plants of the attack so that they produce chemicals that make the leaves distasteful. How long a leaf lives is incredibly variable, it may be eaten within days of its formation, it may last till autumn or it may last for years.  Many trees of temperate climes are deciduous, that is they shed their leaves come the shorter days of autumn.  The advantage of this is that the tree offers less resistance to the winds of winter, so is less likely to suffer physical damage (also true of snowfall).  The tree enters a state of dormancy until spring.  If in spring the tree produces flowers before the leaves (like Blackthorn) this  can facilitate wind dispersal of the pollen.  However, losing leaves each year means that their nutrients are either lost or have to be moved out and stored somewhere else.  Having longer lasting leaves means that nutrients are retained, which is a distinct advantage in a nutrient poor, harsh environment.  The longest lived leaves are found in a plant of the Namib Desert : Welwitschia.  This plant has two leaves throughout its life of some two thousand years.  The leaves may reach a length of 4 metres, the ends die or get worn away but the base generates new tissue.   Welwitschia is a type of Gymnosperm. Image (with thanks) by Nhelia from Pixabay  

Spring is in the air and it feels as if the whole of the outside world has been painted over with a fresh coat of green. A new cycle of living plant matter means a new cycle of fungi that grow on and reproduce from this living plant matter. The few members of the kingdom that are conspicuous around this time of year can feel like an intrusion into this pristine world. I am talking about the rusts, the subject of previous postings on Blackberry Leaf Rust (Phragmidium violaceum) and Bluebell Rust (Uromyces muscari). The rusts aren’t a particularly loved group, even among mycologists. Gardeners curse at the appearance of Puccinia allii on leeks, onions or garlic, or Puccinia menthae on mint, and as mentioned in my earlier pieces, Puccinia graminis, or Wheat Stem Rust or simply Stem Rust, has caused havoc with cereal crop yields. Read more...

The autumnal fall of leaves in deciduous trees is a well recognised event; their changing colours prior to being shed often make for spectacular displays - the New England Fall. Evergreens (with certain exceptions) do not undergo a similar loss of leaves but that is not to say that their leaves are forever green or permanent.  Indeed, each year, evergreens have a seasonal drop of their needle-shaped leaves, it is normal part of the tree’s cycle.  The leaves / needles of conifers have varying life spans; they are not a ‘permanent fixture’.  Many conifer needles will turn yellow then as they age, falling off the tree after one to several years. This change can be gradual or in some species quite rapid.  White Pines (Pinus strobus) typically retain their leaves for 2 to 3 years, whereas Scots Pines (Pinus sylvestris) usually retain their needles for three years. Larches, which are conifers (Larix sp) are somewhat unusual in that they shed their leaves every autumn.  The stress that a tree experiences through drought may result in more rapid browning and greater loss of leaves. Read more...

We have now entered those glorious few weeks in the foragers’ yearbook when the proliferation of brambles across the country is at last yielding its fruits. For the bulk of the seasons, however, Rubus fruticosus can be viewed as little more than an annoyance, an invasive native with barbed, snaking canes that spread across pathways and woodland floors to form impenetrable thickets, snare up by passers-by and crowd out surrounding biodiversity (See ‘Native dominants or botanical ‘thugs’ in woodlands).  Or so it might seem. You will have probably noticed the tracks left by moth caterpillars munching on bramble leaves. Any blackberry-picker will also be well aware of the close association brambles seem to have with stinging nettles. Both attract a wide variety of insect life during their flowering months.  Deer and small mammals such as dormice, not to mention numerous bird species, are also the beneficiaries in terms of food and shelter of the bramble’s vigorous growth . Read more...

The leaf miner moth (Cameraria ohridella) that arrived in the UK about a decade ago has spread widely, and many horse chestnut trees have been ravaged by its activities (see related posts / links). The brown, lifeless leaves that dangle from the branches in late summer are a clear sign of the activities of this insect.  It had been hoped that certain parasitoid wasps might help control the leaf miner, but this is not the case. There is now the suggestion that birds, like blue tits, might help. Read more...

Phytomyza ilicis is a (dipteran) fly that lays its eggs in holly leaves.  It is one of the few insects that is able to make use of holly leaves as a food source and somewhere to live (when a larva).  The female fly lays eggs in the holly leaf (near the main veins or midrib – on the underside) using a thin tube or ovipositor.  The eggs are usually laid in early Spring when there are young and ‘soft’ leaves.  Older leaves have a thick and tough cuticle that is far more difficult to penetrate.  The larvae or maggots emerge from the eggs and tunnel their way along the midrib / veins emerging some time later into the lamina or blade of the leaf.  Here they feed on the photosynthetic tissues of the leaf – the palisade and mesophyll layers, creating a leaf mine (see featured image above).  The number of leaf mines per leaf is a maximum of three and often just 1 or two. Read more...

Many people have been following the various blogs (and contributing comments) on the problems that Horse Chestnut trees have faced over these last few years.  These problems include :- bleeding canker leaf blotch fungus leaf miner moth The horse chestnut leaf miner moth, which originated in the Balkans, first appeared here in 2002/3 in the South East, but it has since rapidly spread to many areas of the U.K. Read more...

The ‘progress’ of the horse chestnut leaf miner (Cameraria ohridella) has been reported in the blog from summer 2006 to more recently, when a national survey was under way.  This small, but highly efficient parasitic moth was first ‘discovered’ in trees bordering Lake Ohrid in Macedonia in the mid-Eighties.  It was later described as species new to Europe, and since that time has managed to spread through almost all of Europe. Read more...