Flat leaves are found across the conifer families including Taxus, Cephalotaxus, and even in Pinus (check out Pinus krempfii) as well as many of the Araucariaceae and Podocarpaceae families. Commonly known trees with this leaf shape include all varieties of yew, swamp cypress, dawn redwood and coast redwood. Here are some examples:
Conifer flat leaves are relatively inefficient from a photosynthesis point of view, since water and photosynthates have to travel further to reach the vascular bundle/s.ref As a result, they have better photosynthetic efficiency in humid, low light environmentsref and are more common where it is wet or tropical. In these areas their greater leaf surface can help them attract the energy they need without drying out due to having more stomatal openings on their surface.
The primarily southern hemisphere family Podocarpaceae is super interesting and not one that I have worked with much from a bonsai perspective, although buddhist pine (Podocarpus macrophyllus) is a species commonly found as an indoor bonsai in the UK. (It is hardy down to just below -10°C so could be an outdoor bonsai as well). Podocarpaceae are interesting from a foliage perspective, as they have evolved a wide range of different leaf shapes which in some cases have become similar to angiosperms and take the forms of flattened leaves with modifications that allow them to grow larger. Below are some examples from which it’s clear that this family has found a workaround for the venation constraints of others in the conifers.
One study mapped the structure of Podocarpaceae leaves and created a cross-sectional image for each sub-family, their results are shown below (apologies for the poor resolution but the original paper wasn’t great to start with). You can see that although most species only have one vascular bundle, they also have various mechanisms to get substances to and from it – including the orange ‘organised accessory transfusion’ cells and ‘pitted thick-wall mesophyll’ cells. Interestingly leaf (f) from the image above – the largest leaf – is also shown below and it’s the only one to have multiple vascular bundles (N and O) which is similar to angiosperm leaves.
Whilst the majority of Podocarpaceae are flat-leaved, quite a few members of Araucariaceae have flat leaves as well. The famous Wollemi Pine or ‘pinosaurus’ Wollemia nobilis has flat leaves, as does the equally well-known monkey puzzle tree Araucaria araucana and various others in the family. These trees don’t tend to be common bonsai subjects as their leaves are quite large and observing the Wollemi pine in my garden, they don’t have much variation in their form so wouldn’t be that easy to style.
Also unusual among conifers is the Phyllocladus genus. Plants in Phyllocladus don’t have true leaves at all – any leaves they develop are non-synthetic and ephemeral – ie. they quickly drop off. Instead Phyllocladus use their stems, which have developed a leaf-like flattened form, to photosynthesise.ref These are called phylloclades – see below for some examples from New Zealand:
I know they look identical to leaves – and they contain all the same components including vascular bundles and photosynthesising palisade and mesophyll cells. The nuances of why they are not leaves but flattened branches instead are based on the way they develop and branch, and the relationships between organs – if you want to read the details refer to this excellent paper. But sometimes they may get lumped in with flat leaved conifers.
Which brings me to Taxus or yew. This is a very common bonsai subject, with its own family, Taxaceae. Yew leaves are famous for two things. Firstly, along with every other part of the yew except for its aril (the red ‘berry’), yew leaves are renowned for being extremely toxic. They contain ‘taxine alkaloids’ of which only a small amount is needed to bring on “dizziness, nausea, vomiting, diffuse abdominal pain, cardiac arrest, respiratory paralysis and death” in all animals including humansref. Secondly, they are one of the few conifers which are known to do just fine in shady positions, although they are also happy in full sun as their leaves adjust to different light levels.ref
Taxus have a unique stomatal feature called ‘papillose’ cellsref which can only be described as ‘pimply’ (see below) – basically the entire surface of the leaf has tiny protrusions as part of the cuticle structure. The presence of these pimples is one of the ways of identifying a Taxus leaf. I couldn’t find any clear explanation as to the role of these papillae, except perhaps to provide some level of water-repellence.ref Taxus are known to harbour endophytes (microbes) in their leaves which help combat pathogensref so it could be that the nooks and crannies created by papillae are a nice home for endophytes.
Anyway, what of all this is relevant for bonsai? It sort of depends on which flat-leaved species that you have. Many will be suited to humid and/or low-light environments, so keep an eye on your watering and don’t let them dry out – they will probably appreciate a misting every now and then. Full sun may not be necessary – or may even be over the top depending on your location – but as always find out what a given species needs as there are lots of quirky members of this club with their own unique requirements.