Edited by Len Phillips

Understanding basic tree physiology is essential to understanding the natural growth habit of trees and the effect that these characteristics have on tree form. The three principal areas of any tree are the roots, the trunk, and the leaves. The physiology of roots was discussed in a previous Seminar article. This article will focus on the physiology of the trunk and leaf philosophy will be covered in the next Seminar.

Functions
The trunk and branches of a tree perform three functions:

1. to support the weight of the foliage and extended branch growth,
2. to store nutrients for when they are required,

3. to provide a conduit for water and nutrients to flow from one part of the tree to another.

Cambium
The cambium is a watery layer only a few cells thick and is the generative layer, giving rise to both xylem and phloem. The cambium layer is responsible for producing new growth and is just beneath the bark, and in most tree species, appears green when the bark is scraped away. The cambium layer is responsible for producing new roots, new wood, new bark, and new shoots as well as tissue that seals over wounds.

Xylem
Inside the cambium layer is the xylem which is a system of tubes and transport cells that circulates water and dissolved minerals. Xylem is made of vessels that are connected end to end for the maximum speed to move water. The xylem is formed by cells that elongate as they grow to not only thicken their sides with lignin but ultimately to break down their dividing walls where they meet tip-to-tip, so that together they form long continuous tubes through which water is able to move from the roots to the leaves.

As the cells in the leaves lose water by transpiration, a negative pressure or tension is created and it is replaced by water from the top of the tubes so the whole water column is pulled upwards from the roots and the soil. This puts an enormous strain on the sides of the tubes within a tall tree, but their woody walls are rigid enough to withstand it.

Xylem tissue dies after one year and after the cells die, the pipes that are created become inert and are full of water. The old xylem then becomes part of the heartwood of the tree.

Phloem
Between the bark and the cambium is the phloem. It is responsible for distributing organic nutrients made during photosynthesis and known as photosynthate. Photosynthate is made of sucrose sugar and is needed in all parts of the tree where energy is used and growth is occurring. Phloem is always alive.

Phloem tissue consists of sieve-tube cells and companion cells. The sieve tube is an elongated group of individual cells, arranged end to end. Sieve-tube members are joined to form a tube that conducts solute throughout the tree. The end walls of these cells have many small pores and are called sieve plates and have enlarged plasmodesmata. The companion cell is more metabolically active than a 'typical' plant cell. Companion cells collect solutes through apoplastic (cell wall) transfers, or via the symplast through the plasmodesmata connections.

While movement of water and minerals through the xylem is driven by negative pressures most of the time, movement of fluid in the phloem is driven by positive hydrostatic pressures. This is called translocation and is accomplished by a process called phloem "loading and unloading". Cells in the leaf "load" a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. The materials flow in a reverse of the xylem with surplus materials stored in the tree's roots.

Growth Rings
It's fairly common knowledge that you can tell the age of the tree by counting the growth rings. However, look a little closer at each growth ring, and you will see a wider, light-colored area to the inside of a thin dark area. The light-colored area is growth that occurs during the summer months (but is called spring-wood), when sap flows freely through the tree and growth occurs very quickly. Conversely, the darker-colored section occurs during the winter (but is called summer-wood), as the tree's genetic code sends out signals to built up a layer to protect the fresh summer growth. Together they are called the "heartwood". The heartwood is essentially dead waste material and xylem, but it acts in a structural capacity for the tree, gradually widening the trunk as the tree becomes taller.

This outer layer of the growth ring works with the cambium layer and bark to protect the tree from the cold of winter. To prepare for the winter months, a tree will pull the majority of its sap from the upper reaches of the tree to help keep it from freezing. Growth rings are typical in temperate forest trees and tropical forest trees that have regular, annual dry seasons. In a tropical humid rainforest, trees grow continually and do not have rings.

Bark
The build up of old phloem layers forms the thick corky bark that can be seen on old trees. The older the tree, the thicker the layer of dead phloem becomes. The dead layers of old phloem are not very elastic and as the live areas of the trunk continue to expand, the bark starts to split into long fissures and plates.

Bark protects the tree from external threats and has several other functions. First of all the bark will absorb tree wastes and lock them into its dead cells and resins. Bark can also be one means of identifying tree species because bark textures are relatively uniform among trees of the same species. Bark textures are divided into at least 18 types, from smooth like a beech tree to spiny like a locust.

Sap
If a tree is harvested in winter, the sap is likely out of the tree when it is cut down. This means that the tree is a bit easier to kiln dry, and is likely going to be less susceptible to twisting, cupping, bowing or checking, even though it is just as dry as wood harvested in the summer.

In sugar maple trees, the sap is collected in the fall and stored in the roots over winter. This sap is then released in late winter to early spring as it begins to flow to the buds to start a new season of growth. The sugar content is also very high in the stored sap, thus making sugar maple sap collecting a whole industry in certain regions of the world.

Sources

• Harry Harrington, "Plant and Tree Physiology", Bonsai4me website, 2009.
• Michigan Forests Forever Teachers Guide, "Tree Physiology", Michigan State University Extension, 2009.
• Taiz, L., and E. Zeiger, Plant Physiology, 2nd edition. Sinauer Associates, Inc. 1998.

• Winterborne, J., "Hydroponics", Indoor Horticulture, 2005.

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