Buds are the “small lateral or terminal protuberance on the stem of a vascular plant that may develop into a flower, leaf, or shoot.”ref Buds are responsible for primary growth, and are created by meristem tissue (a meristem is an area of stem cells which differentiates into different types of cells).
If you look inside a developing bud, you can see the starting points of the different cells which will arise – they can be vegetative buds (shoots & leaves) or reproductive buds (flowers in angiosperms or strobilus/cones in gymnosperms). Below is an image of a Jack Pine terminal bud which has many lateral vegetative buds on the sides.
When shaping your bonsai, you want to know where buds may appear, so that you can encourage the direction of growth and shape you desire. Predicting bud location is relatively easy in angiosperms, which follow a relatively reliable pattern in their growth. Bud growth is more unpredictable in gymnosperms, but many of the following guiding principles remain.
Firstly, there are different bud positions:
- The terminal bud is at the end of a stem or branch and this is the growing tip which makes the plant grow larger.
- Axillary buds develop along the stem during the annual growing season according to the architecture of the tree (see below for more); within this, preventitious buds are axillary buds which are dormant and then develop in a later season.
- Adventitious or epicormic buds are buds which do not develop according to the repeating architectural pattern – they arise spontaneously from previously non-meristematic (growing) tissue which can be anywhere on the tree. They are unpredictable as described in this post.
Below are some examples of angiosperm buds. The terminal bud is on the end of the shoot, this comes from the shoot apical meristem (SAM). Then there are axillary/lateral buds which occur along the shoot – in angiosperms these develop in the leaf axils (a position adjacent to where the leaf is attached).
Bud behaviour depends on a tree’s architecture, which is genetically determined – that is, it will be very similar for trees of the same species, albeit also affected by the environment. There is a lot of research out there about tree architectures, much of it pioneered by Halle & Olderman in the 1970s, there is even a mathematical model which can be used to represent the architecture of a given speciesref. As explained in this excellent articleref, “regular development of each plant represents the growth of repeating units – ‘phytomers’…a typical phytomer consists of a node, a subtending internode, a leaf developing at the node sites and an axillary bud (also called lateral buds) located at the base of the leaf”.
Each type represents a pattern consisting of a shoot with one or more leaves in the same arrangement. In some trees growth is repeated in a sustained way throughout the growing season (a single flush of leaves), whilst conditions are right. In others there are alternating growing and resting stages (multiple flushes of leaves). During the resting stage, new leaves and shoots are being created inside the budref. I’ve copied some of the main architectural models into this post: Tree Architectural Models
An important part of the phytomer pattern is the leaf arrangement, known as the phyllotaxis. Leaves can grow singly at one position on a stem, or they can grow in whorls where two or more leaves appear at the same position arrange around the stem. When leaves grow singly they spiral around the shoot to optimise their light capture – apparently using the ‘golden angle’ of 137.5o ref.
The leaf arrangement on your tree is important because each leaf axil (the base of the leaf) should be the location of an axillary bud (although in gymnosperms these can be missing). These are key to bonsai because they become new shoots (with leaves or flowers). They develop in the position just above where a leaf used to be; when it falls off, a scar is left and a bud generates above the scar.ref In fact what is happening is a continuous bud genesis, so when you have a bud about to burst, it already has embryonic buds developing at its base – this is why buds look like they form at the leaf axil (in fact they formed on the previous bud). Your new branches and leaves will generate from these positions, and dormant buds may be located here. Read more about buds in angiosperms here, and in gymnosperms here.
The growth of an axillary bud (and its embryonic buds) can be suppressed by its neighbours – this is how ‘apical dominance’ works. It used to be thought that in apical dominance, the shoot closest to the sun emitted hormones which suppress the growth of buds lower down the plant, ensuring that it gets the most resources. This research group at Cambridge University study the development of axillary shoots and their research says “shoot apical meristems compete for common auxin transport paths to the root. High auxin in the main stem, exported from already active meristems, prevents the activation of further meristems”ref. This results in axillary buds going dormant and becoming ‘preventitious’ buds, but they are still available to grow later if conditions change. According to this article, apical dominance in trees only works on buds in the current year of growth due to the slow movement of the hormone auxin through the treeref – meaning that current year buds on a branch are suppressed by the terminal bud on that branch and not by the main leader. HOWEVER, it has recently been found that auxin does not move fast enough to have this effect, and instead it is driven by sugar flows to the apical meristem.ref The effect of apical dominance remains, however it is now thought that sugar flow drives this and not auxin directly.
Encouraging axillary bud growth is a way of increasing ramification on a bonsai, as it can create multiple shoots instead of just the terminal buds. If the terminal buds are removed, axillary buds get the chance to grow, often more than one.
Application of exogenous cytokinins (benzyladenine) has also been shown to increase bud initiationref (see my post on ramification of Branches and Foliage for some substances containing benzyladenine).
Equally, looking at how the leaves are arranged, you can work out where new shoots will arise from existing stems. By removing the buds or shoots not meeting your design, you can encourage shoots to grow in the direction and position that you want. But it’s not enough to know about bud position, you also need to know what kind of bud is present – a vegetative or a reproductive bud, and you need to know the difference between short and long shoots – more here.