It’s useful for bonsai enthusiasts to understand how a leaf is structured, as this answers some questions about how water/air/nutrients/sugars get in and out of the leaf and therefore also the rest of the tree. Of course there are many different leaf types belonging to different trees in different environments, so there will be many differences between them. What’s important to know is the main structures which are common to most leaves. Below is a diagram of a leaf cross-section:
The outside of the leaf is covered by the ‘cuticle’, which is the first line of cells between the leaf and the outside environment. This is not a passive line of cells, but instead “waxes, fatty acids, and aromatic components build chemically and structurally diverse layers with different functionality.”ref Not only that, the cuticle changes as the leaf develops – building up its layers and constituents over time until the leaf has fully extended.ref The above diagram only shows a cuticle on one side of the leaf – but apparently a cuticle “covers the outer epidermal surface of most above-ground tissues, such as leaves, fruit, and floral organs.”ref
The main function of the cuticle is as a barrier. It protects the tissue beneath from mechanical damage by the elements, or from insects, and acts as the primary defence against pathogens.ref It is composed of “the polyester cutin, containing oxygenated and unsubstituted fatty acids, glycerol, and phenolic acids, that is impregnated by waxes of very-long chain fatty acids (VLCFAs) and their derivatives.”ref In another study the top layer of the cuticle was found to contain Kaempferol. This is a flavonol which is known to be an antifungal, antibacterial and antioxodant (see this article about HB-101).
Since the waxy cuticle is impermeable to water and CO2, leaves have specially controlled holes distributed across itref – these are known as stomata (described below).
Underneath the cuticle is the epidermis – the upper epidermis at the top of the leaf and the lower epidermis at the bottom. Humans have an epidermis too – it’s the top layer of skin. Everything you could want to know about the plant epidermis is covered in an excellent article in ‘The Plant Cell’ journal from January 2022. The authors say “the epidermis plays many important roles including regulating the exchange of gases, water, and nutrients with the surroundings, responding to external threats such as pathogens, herbivores and abiotic stresses, resisting mechanical strain, detoxifying xenobiotics, and contributing to mechanical strength while allowing the flat and flexible shape necessary for maximum light capture.”
There are three main types of cells in the epidermis, and these develop into their final form starting from the leaf tip and gradually moving back towards the petiole until all of the cells are formed.
The first cell types are ‘pavement’ cells – so named because they interlock with one another and look like paving (sometimes it’s crazy paving – other times it’s very neat). Among the paving are stomatal guard cells – these “form microscopic valves in the leaf surface” so that gas can get in and out for photosynthesis. You may have heard of stomata – this is the name for the hole that is created and controlled by the stomatal guard cells. Basically plants breathe through their stomata – air comes in, oxygen from photosynthesis and CO2 from respiration come out, and water vapour comes in and out as well. It’s actually reasonably easy to ‘see’ the stomata even with the naked eye – depending on the species of tree and the shape of the leaf. If you put adhesive tape on a leaf, and pull it off, you pull off some of the cuticle which shows the outlines of the stomata. There is quite a lot to say about these guys – see this post: Stomata.
Aside from the stomatal guard cells and the pavement cells, the epidermis can also have ‘trichomes’ which are hair-like protrusions from the surface. These can appear in lots of different forms, and can be ‘glandular’ (or not). If a trichome is glandular, it can “biosynthesize, store and secrete a large diversity of specialized metabolites including terpenoids, alkaloids, polysaccharides, and polyphenols” – such as the terpenoids that conifers exude to defend against insectsref. This image from the journal nature shows the trichomes on white spruce:
Anyway moving on past the cuticle and the epidermis, you come to the mesophyll. The mesophyll is “the parenchyma between the epidermal layers of a foliage leaf”ref – OK great Merriam-Webster dictionary, now what is ‘parenchyma’? Parenchyma is “the essential and distinctive tissue of an organ”ref which in the case of leaves means the cells which photosynthesise and store the products of photosynthesis. So the mesophyll is the engine room of the leaf.
Referring to the image at the top of this post, you will see there are two types of cell in the mesophyll – palisade cells and spongy mesophyll cells. The palisade cells face the light, and are located on the top of the leafref. They are columnar cells (with the end of the column facing the light) and they are supposed to contain the majority of chloroplasts, which are the organelles responsible for photosynthesis. The spongy mesophyll cells are arranged in a lattice, with air gaps (like a sponge) to allow for the absorption of CO2 – they also contain chloroplasts, but apparently not as many. Good luck trying to find a research paper which actually counts them! The best I could find was this dataref looking at five species living in different light conditions, and the number of spongy mesophyll cells ranged from 40-50% of the total chloroplast count. Which isn’t exactly a minority.
The shape of these cells has evolved to improve photosynthesis. The palisade cells which are long and columnar, “act as light conduits”ref distributing collimated (parallel) light to chloroplasts within the leaf. Internal light scattering also takes place, allowing photons of light to reach the chloroplasts in the spongy mesophyll cells. When a leaf has a different (usually lighter) colour on one side, this can keep light inside the leaf by reflection.
Not to forget, leaves have their own microbiome, just like the roots. This is called the phyllosphere and contains many bacterial and fungal species in symbiotic relationships with the host plant.
You can dive even deeper into the structure of leaves by going into the plant cells themselves, looking at mitochondria, chloroplasts, vacuoles and the thousands of chemical reactions going on, but that’s a post for another day.