The xylem and the phloem are the vascular system of a tree; they transport fluid to and from every cell in the plant, via structures that branch, divide and become very small – small enough to be able to reach every cell. But xylem and phloem don’t transport the same substance, like our blood vessels do. Instead the xylem carries water and dissolved nutrients up from the roots, and the phloem carries the products of photosynthesis (‘photosynthates’) from the leaves to other parts of the tree. This post is about xylem, I’ve also written one about phloem.

As mentioned, the function of xylem is to transport water (and nutrients) throughout the tree. Although as has been the case throughout the creation of this website, I’ve learned that plants are never as simple as they seem! Recent research has found many other substances in xylem sap as well as water and nutrients, including plant growth regulators, sugars and proteins. This study^ref into poplar xylem sap found six plant growth regulators, 124 plant metabolites including salicinoids, coumarins and benzoates, and 289 proteins with major groupings including proteins related to defence, cell wall-related processes and catabolic processes (breaking down sugars).

There is evidence that xylem is used for translocating sugars from storage organs such as roots particularly when photosynthesis is not operating (eg. for deciduous species before they break bud in the early spring). For example “birch, which blooms in late winter, clearly transports hexoses in the xylem with a potential of providing nutrients to the developing tissues at rates that equal or exceed those provided through the phloem”. ^ref

The majority of water-conducting xylem in a tree is in fact mainly made up of dead cells. This makes sense because living cells have cell membranes, vacuoules and other organelles which aren’t needed for the transport of water – instead what’s needed is an open space which can store water and pass it to the next storage space. In fact xylem vessels do not work for water transportation until they are dead^ref.

Water moves through the xylem from the roots and up throughout the tree, in a continuous stream, evaporating from the leaves via the stomata. This evaporation is what pulls water up against the force of gravity. Since the process relies on continuity, if air bubbles form in the xylem this can be a problem, as explained in this post: embolisms. You can read more about this in: trees’ water system.

When new xylem cells are born, they go through a process of strengthening the cell walls with substances such as lignin, hemicellulose and pectin among others, ^ref, then the cell dies (often because the vacuoule bursts) and its contents are cleared out (digested by enzymes)^ref. This leaves a space for water to enter and occupy. In angiosperms, which use vessels for their xylem, death occurs within a couple of days, while in gymnosperms, which have tracheids instead, this happens a lot later – for trembling aspen and Norway spruce around a month.^ref

For trees – which undergo secondary thickening – there are two producers of xylem cells – the vascular cambium (located under the bark) and the procambium (located in growing tips).

The vascular cambium is a single layer of cells responsible for producing the xylem which becomes the trunk and wood of the tree. It produces new xylem cells throughout the growing season – the cells deposited at the beginning of a growing season are the ‘earlywood’ and towards the end the ‘latewood’ (and yes if you want to know, there is also ‘transitionwood’). These make up the rings in the trunk of a tree, and are what thickens the trunk year after year. You’re probably aware that these rings can be used to understand historic climate variation and for the dating of all sorts of things – covered in the excellent book by Valerie Trouet “Tree Story, The History of the World Written in Rings”.

Although technically all xylem cells are ‘dead’ in the sense they don’t contain cytoplasm or the usual organelles like a nucleus, some are more dead that others! That is, after time some xylem cells aren’t used for water transport any more either, and their main role becomes a structural one, of holding up the tree. The ‘sapwood’ is the section of xylem still actively transporting water, and the ‘heartwood’ is the section which no longer does. For example in Picea Abies (what we use in the UK for a Christmas tree), once a tree reaches a certain age, “the width of the sapwood band remains more or less constant (on average 7.8 cm for dominant and 2.0 cm for suppressed trees)”^ref. In certain angiosperms which regrow their active xylem every year (known as ‘ring porous’) there is only one active ring, and the rest is heartwood. In trees with less light which are ‘suppressed’ there is less need for water transport so the sapwood is thinner.

The structure of xylem is different between gymnosperms (conifers) and angiosperms – in fact this is one of the main differences between them. Conifers have only one type of xylem – ‘tracheids’ which are “are overlapping single-celled hollow conduits, closed at both end”^ref. These cells are relatively small – ranging between 4-80μm in diameter, no longer than 5mm in length. For water to reach the top of the tree, it enters a tracheid, travels to the top and passes out through a pit connected to its pit pair in an adjacent tracheid, in this way it zig-zags its way up the tree. Here’s an image of a pit from pinus contorta, the open area around the outside is where the water flows through and the central area can be used to close the tracheid off if an embolism occurs.

As you can probably imagine, this isn’t a super-efficient way to transport water – conifers can’t transport water as quickly as angiosperms and can’t support the high levels of transpiration that angiosperms can.

Angiosperms have two types of xylem cells – narrow fibres which are used for strength, and wide, thin-walled vessels, which are used for water transport (Ennos). The wide vessels allow for much faster water transport, which enabled the larger leaves of angiosperms (which have higher transpiration rates) to evolve.

To see the difference between conifer tracheids and angiosperm vessels, this chart is from a study^ref comparing the two. Note that both axes have a logarithmic scale, so the vessels are clearly a lot (100x or more) longer than the tracheids, and also 2-8 times the diameter.

The procambium (or ‘primary cambium’) is responsible for generating the xylem in leaves, roots and shoots. The initial xylem cells created by the procambium is called protoxylem and this in turn creates metaxylem.

In roots, the metaxylem is in the centre and the protoxylem next to it, with the phloem on the outside of the root: see the image below showing red protoxylem vessels, blue metaxylem vessels, orange procambial cells and green phloem cells (diagram from this paper). You can read more about roots and how they work in roots.

So what does it all mean for bonsai? Well the basic principle is that the tree needs water, nutrients and other substances like plant growth regulators, in every living cell and the xylem gets some of these substances there. Interrupting the xylem will slow down or stop cells accessing the inputs they need for growth, and potentially cause an embolism.

Bonsai activities which affect the xylem include wiring, carving, pruning, repotting and feeding.

Wiring too tightly may destroy the xylem on branches but there are two other layers which will be destroyed first – the bark and the phloem. Carving the sapwood will kill anything above the tree which is dependent on that sapwood. Pruning roots *may* kill the branches above if none of the rest of the root ball feeds those branches.

In general ring-porous trees are most vulnerable to xylem damage from the above because they have a small amount of sapwood, and rely on fewer wide vessels for their water transport. Popular ring-porous trees in bonsai include Oak, Ash, Black Locust, Catalpa, Chestnut, Hickory, Mulberry. Conifers aren’t ring porous, so are more tolerant of xylem cuts and interference. You can see what your tree is (diffuse or ring porous) on this website.

Because they are dead cells and no longer have a cell membrane as a barrier or the ability to create metabolites which can defend the cell, xylem cells can be (and are) populated by communities of microbes^ref. In one study of wild and cultivated olives, they found 5 phyla, 8 classes, 17 orders, 23 families, and 31 genera of bacteria, including Methylobacterium, Sphingomonas Frigoribacterium and Hymenobacter.^ref

There is actually a xylem microbiome – just like there is for roots (the rhizosphere) – it is part of the endosphere. Xylem microbes include bacteria, fungi and oomycete organisms, some of which can be beneficial and others pathogenic. On the beneficial side some species of Methylobacterium are known to assist nitrogen uptake and to produce auxins which support plant growth. Other the other side, at least ten different microbes are know to cause vascular wilt – a destructive disease which targets the xylem – detailed in this article – including Verticillium (a fungus), Ralstonia solanacaerum (a bacteria) and Pythium ultimium (an oomycete).