Generally speaking, leaves are quite small, but collectively they present a massive surface area to the environment.  The total surface of leaves is significantly greater than the surface area of the Earth.  Leaves are the organs of photosynthesis, not only do they make the basic chemicals for the plant, but they replenish the oxygen in the atmosphere. Generally speaking, a leaf has two surfaces.  One surface (the adaxial) is presented to the sun, the other (the abaxial surface) is shielded from the light to some extent.  Both surfaces are covered with a waxy layer (the cuticle), which helps restrict water loss. The underside of the leaf has many tiny pores, the stomates, which can open and close to allow the exchange of gases and water vapour.  The lower surface may be covered in minute 'hairs'.  These create an unstirred layer, so that air next to leaf has a high water content and thus water loss through the stomates is reduced.  The veins of a leaf contain xylem tissue which delivers water and mineral salts, and phloem which transports sugars and other organic compounds from the leaf. [caption id="attachment_43706" align="alignleft" width="300"] adaxial surface[/caption] [caption id="attachment_43705" align="alignright" width="300"] abaxial surface[/caption]   The upper  surface and lower surface  of a foxglove leaf.  Whilst the veins can be seen on the top surface, they are ridged and covered in hairs on the lower surface. [caption id="attachment_13486" align="alignleft" width="240"] galls on sycamore[/caption]   Once a leaf has grown then its size, shape and the veins remain constant, but during the course of the year other things can and will change.  For example, in autumn the coloured pigments in the leaf break down and many compounds are transported out of the leaf as it undergoes senescence. The leaf may be subjected to attack, caterpillars and leaf miners may eat its tissues.  Certain wasps may lay eggs within the leaf so that a gall develops.  Greenfly use their piercing mouthparts to access sugars and other compounds from the phloem.  Bacteria may infect the leaf either through wounds inflicted on the leaf, or by penetrating the stomates. To some micro-organisms, a leaf is ‘home’; it is a habitat.  Rather like it is now common to talk about our gut microbiome or  the microbiome of the skin, so each leaf has a microbiome. Recently, microbiologists from China and Germany have studied the microbiome of oak leaves.  Specifically, they examined the microbiome of the leaves of the pedunculate oak.  They examined how the organisms present changed during the course of a growing season.  They found that the upper and lower sides of the leaves had different communities of micro-organisms.  Both surfaces  shared the same sorts of bacteria, but they varied in the amounts / proportions of the bacteria on the two surfaces.  Furthermore the differences between the upper and lower surfaces changed as the seasons changed.  The diversity of organisms on the lower surface of the leaf fell as the year progressed.  Some of the bacterial DNA on the lower surface was that of bacteria which could be found on / in aphids and whitefly.  Some of the bacteria found were associated with fermentation of sugars.   The study did not investigate fungal species on the leaves, but it does indicate that there are complex microbial communities present on leaves, and that the two surfaces offer different ‘opportunities’ for micro-organisms.  Similarly, it has been established that there is a root microbiome. The root microbiome is a complex community of bacteria, fungi, archaea, and viruses living in and around plant roots. This microbiome helps the plant take in nutrients, fight diseases, and cope with environmental stress. [caption id="attachment_43724" align="alignright" width="350"] Hornbeam - abaxial surface[/caption]   [caption id="attachment_43723" align="alignleft" width="350"] Hornbeam - adaxial surface[/caption]

The grey—brown skeletal branches of trees are being cloaked in fresh green leaves as they unfurl from the buds that protected them through the winter months.  Their bright green colour is due to large amounts of chlorophyll.  The chlorophylls are pigments that can absorb many of the wavelengths of visible light, but not green.  Green wavelengths are reflected back into the environment, which is why our eyes perceive both young and mature leaves as green. Each leaf is made up of a variety of cells and tissues.  The top and bottom of the leaf are covered with a layer of cells  termed the epidermis.  It consists of many interlocking cells (rather like jigsaw pieces), sometimes called pavement cells.  Their function is to protect the underlying cells and also produce the waxy, waterproofing layer — the cuticle.   The lower epidermis is ‘pierced’ by the stomates.  These are the ‘breathing pores’ of the leaf, allowing the exchange of gases and water vapour.   The epidermis may also bear trichomes.  These are small ‘hair—like’ projections.  If there are many of them they can give the leaf a white or silvery appearance, helping to trap moist air near to the leaf surface to reduce water loss.  They  may also help to reflect sunlight, so that the leaf does not get too hot and on cold days can serve to protect the leaf from frost damage. Some trichomes have a protective function in that they may physically restrict the feeding of insects and other herbivores, and some contain a cocktail of toxic chemicals [e.g. nettles]. Under the upper epidermis and within the leaf, there is one or more layers of cells packed with chloroplasts - the palisade layer.  This is the principal site of photosynthesis within the leaf, where carbon dioxide is fixed into sugars and other vital nutrients.  The ‘by-product’ of photosynthesis is oxygen, which is not only essential for plant respiration but needed by the vast majority of animals on this planet.  It diffuses out of the leaf through the intercellular spaces of the next layer of the leaf - the mesophyll layer. The stomates allow gases in and out, but can close through the movement of their guard cells.  Stomates tend to close up at night or when the leaf experiences water stress.  Running throughout the body of the leaf is the xylem and phloem tissues, which conduct water, minerals and sugars etc around the plant. The sheer abundance of chlorophyll in many leaves masks the presence of other pigments, which only become visible when the leaf begins to senesce and the chlorophylls break down.  The leaf turns a yellow / orange colour due to the presence of carotenoid pigments.  Autumnal leaves can display a variety of colours due to other pigments such as the anthocyanins and xanthophylls.  Some leaves take protection very seriously   Curious fact : the leaf with the largest surface area is that of the Amazonian water lily, which can be 10 feet in diameter.

Leaves have three main parts:  The petiole, a stalk-like structure that connects the blade of the leaf to the stem of the plant. Some leaves don’t have petioles,  and are known as sessile leaves. The blade or lamina, usually the largest part of the leaf.  The edge of the leaf or the leaf margin may be described as entire, toothed, or lobed. The oak leaf, for example, is clearly lobed. The blade has many veins, forming a network, carrying water and nutrients, The base, the base is the region of the blade that attaches to the petiole. A leaf is said to be simple if its blade / lamina is undivided, if the ‘teeth’ or lobes do not reach down to the main vein of the leaf.  A compound leaf has several leaflets, which join up with a single leaf stalk or petiole. When identifying tree leaves, it is always important to look for the petiole,   as a single leaflet of a compound leaf can look like a simple leaf.  More details of leaf and tree structure can be found on this link on our website. Now for a challenge.  Can you or your children find a leaf (and name the tree it came from), that Has a serrated / toothed edge Has a lobed margin Has a smooth edge / margin Is a compound, palmate leaf Is a compound, pinnate leaf Is hairy Is not green, but red or a mixture of colours Is more or less circular Is fleshy / succulent Has spines on its edges Is needle shaped Has a thick (waxy?) cuticle or is very shiny Has net venation  is marcescent (might keep you hanging around) Go forage!

Honey bees face many threats - such viruses, mites and pesticides, but also threats from their own kind - robber bees.   When nectar becomes scarce at the end of the summer and into autumn, then bees will seek food wherever they can find it - like honey from other colonies.  From the robber bees ‘point of view’, this is simply another form of foraging behaviour.  This robbing of honey deprives a colony of an important winter resource.   To limit this behaviour, hives have guard or gatekeeper bees, who ‘inspect’ all arriving bees.  But how do they know who is friend and who is foe?  Not being able to tell the difference could mean a long and lean winter, with little honey in the hive.  Honey bees can recognise members of their community / hive by detecting waxy chemicals present in their exoskeleton (cuticle) known as cuticular hydrocarbons (CHC’s). Read more...

Plants need to protect themselves from the vagaries of the environment - they need to prevent or resist the entry of disease pathogens (bacteria, fungi and viruses) and also restrict the loss of water from their tissues.   The flow of water through a plant (the transpiration stream) is important in the supply of minerals to the growing regions and the ’control' of leaf temperature (as water evaporates it uses heat energy -  exerting a cooling effect). However, uncontrolled water loss would soon result in wilting and death, so the outer layer of plants - the epidermis, has a special layer, the cuticle.  The cuticle is made from waxes, which often form complex and intricate patterns (when viewed with an electron microscope).   Read more...

According the United Nations FAO, some two million hectares of forest were burnt in the Mediterranean region, between 2006 and 2010.  Most of these fires were 'human induced'; they (the fires) are the most frequent cause of degradation / loss of forest / woodland in this region.  In 2012, a fire swept through some 20,000 hectares of forest near Andilla (Valencia).   After the fire, it was found that though oaks, holm oaks, pines and junipers had been completely burnt, the vast majority (98% +) of the Mediterranean Cypresses were still standing, tall and green.   There followed a three year study of the fire resilience of the Mediterranean Cypress (Cupressus sempervirens var horizontalis) to see if stands of these trees might function as buffer zones to hinder or prevent the spread of forest fires. 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...

Plants take up essential nutrients (nitrates, phosphates, potassium, magnesium etc) through their roots, but some also take up other, more unusual substances – such as silicon, nickel, cadmium, copper, cobalt. Grasses, in particular, are known to take up silicon and then deposit it in their leaves.  The silicon that they absorb is converted into tiny granules called phytoliths, which are often located under the waxy cuticle.   Read more...