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]

We can communicate in many ways, other animals may  message each by vocalising or gesturing.  Some release air-borne chemicals (pheromones) to attract a mate.  Plants also communicate by chemical signals that they release into the environment. The most obvious example is the release of scents to attract pollinators.   These floral scents are usually oils, made from volatile organic compounds (VOCs).  VOCs are carbon based compounds, such as the terpenes. They usually have a strong and pleasant odour.  They form partt of the scent of many plants, for example, honeysuckle, roses, jasmine, lilac, and mock orange.  Different plants produce different scents.  These different chemical signatures help pollinators identify particular species, and locate the nectar on offer. The VOCs produced by ripe fruit help attract animals that assist in seed dispersal. [caption id="attachment_43370" align="aligncenter" width="675"] Buff tailed bumblebee visiting a foxglove[/caption] VOCs are also produced by leaves when plants are under attack.  These may deter the attacking organism directly, e.g. caterpillars, or they may attract an organism that predates upon the herbivore. When pollen beetles feed on oil seed rape, the plants release VOCs which attract the attention of insects (a type of wasp). The wasps lay their eggs in the larvae of the pollen beetles. The pollen beetle larvae are then ‘eaten alive’, by the developing wasp larvae.  Sometimes, VOCs are produced by stressed or attacked plants,  these VOCs ‘warn’ nearby plants of the same species. These plants can then activate their defences before they are attacked. This helps ensure the survival of some of the population. Recently, it has been suggested that mycorrhizal systems act as a physical and chemical ‘means of communication’ between the trees in a woodland, in what has been termed the ‘wood wide web’. The term was coined by Dr. Suzanne Simard, a forest ecologist at the University of British Columbia to describe the complex relationships between fungi and plants in woodland ecosystems.  Interestingly, VOCs can influence air quality and atmospheric processes.  In a woodland,  the scent of pine or other conifers may permeate the air.  Pines release volatile compounds, such as alpha-pinene.  Recent research has established that these volatile compounds / vapours are not only responsible for the characteristic scent, but also contribute to the formation of aerosols in the atmosphere* both in and around such woodlands and forests.  In the presence of sunlight, VOCs can interact with the gas nitric oxide to form ground level ozone, which contributes to smog formation. Forests can be a significant source of VOC emissions. The production of VOCs is affected by environmental  conditions.  VOCs release generally goes up with an increase in temperature, as the plants metabolism increase.  An increase in temperature also helps in the evaporation / volatilisation of plant oils and scents. Water availability and light intensity also affect VOC release.  * An aerosol is a ‘mixture’ of very small particles (solid or liquid) in air; other examples of aerosols include mist, cigarette smoke, or car exhaust fumes Addendum : Heavier and far rarer VOCs are the sesquiterpenes, larger molecules that trees in the Amazon release when exposed to drought.  These reactive molecules help form tiny particles that seed clouds and scatter sunlight. These emissions could change the weather and climate over the whole region.  For full details see : https://www.earth.com/news/amazon-trees-emitted-never-before-seen-chemicals-during-drought/