What is a Tree?

What is a Tree?

So I've just completed the first official day of the Tree House Construction Workshop, but first I'd like to cover a subject we touched on during the day: 

What is a tree? 

This question can be answered in many different ways. Trees are both biological wonders as well as natural curiosities. They have been revered for centuries and their impact on mythology and folklore cannot be understated. Often seen as having mystical and magical powers, trees have captured our collective imaginations for centuries. For the traditional woodworker, the connection between material and craftsman has long been documented (check out the Cabinet Maker's Notebook by James Krenov). During the time I spent working in the forest, I am now beginning to understand this relationship for myself. I have come to feel more connected with the material I have used so often and never have I had the chance to work so closely with its source. One can buy and work with wood without giving much thought for the tree from which it came. Much like how our supermarket meat comes cleanly wrapped and packaged in plastic, saving us from thinking about how or where it came from. This zombie-like way of mindless consumption is worrying and allows us to live a life disconnected from reality and responsibility. I am so glad I've had this experience which has set me down a new path of deeper understanding. The feeling of knowing, of understanding is liberating, to master a subject is just that. I now feel I am at the beginning of a new way of seeing, a new perspective is opening up and a broader, more wholesome connection between me and my work is possible. Now I must continue my research and continue on my path of learning. A higher lever of understating and connection can always be achieved.  

    

So to answer this question scientifically - 

Scientists still can't completely decide how a tree should be defined - Here's a few definitions: 

- "The generally accepted definition of a tree is that it is a plant with a more or less permanent shoot system that is supported by a single woody fibrous trunk."

-  “a woody perennial plant, typically having a single stem or trunk growing to a considerable height (usually at least 13 feet) and bearing lateral branches at some distance from the ground.”

The difference between a tree and a shrub?

Shrubs normally branch near the ground and sometimes have several narrow stems rather than one single trunk. Having several stems means a shrub can support many leaves but their overall structure is less rigid than one single, thick tree trunk.

Trees and Fungi - A symbiotic relationship:

Roots provide structural anchorage to keep trees from toppling over. They also have a massive system for harvesting the enormous quantities of water and the mineral resources of the soil required by trees. In some cases, roots supplement the nutrition of the tree through symbiotic associations, such as with nitrogen-fixing microorganisms and fungal symbionts called mycorrhizae, which are known to increase phosphorous uptake. 

What are mycorrhiza? Mycorrhiza is the term given to the symbiotic association between a plant and a fungus. The plant makes organic molecules such as sugars by photosynthesis and supplies them to the fungus, and the fungus supplies to the plant water and mineral nutrients, such as phosphorus, taken from the soil. Mycorrhizae exist as very tiny, almost entirely microscopic threads called hyphae. The hyphae are all interconnected into a net-like web called a mycelium, which measures hundreds or thousands of miles which are all packed into a tiny area around the plant. The mycelium of a single mycorrhiza can then extend outward and connect multiple plants and even connect with other mycorrhizae to form a Frankenstein-like underground mash-up called a common mycorrhizal network. There are two different types of mycorrhizal connection. One is called ectomycorrhizae and simply surrounds the outside of the roots. The other is called endomycorrhizae and actually grows inside of the plant—their hyphae squeeze in between the cell wall and the cell membranes of the roots. 

Plants are able to get nutrients themselves through their roots, but they have a limited ability to do so. Their roots need to be in direct contact with the soil to absorb the nutrients, and plant roots only grow so small. Fungi, on the other hand, can get much smaller. Fungal hyphae can wedge in between individual bits of soil to cover almost every available cubic millimeter of soil. This increases surface area and allows the plants much greater access to nutrients than they could get by themselves. For many plants living under difficult conditions, they wouldn’t be able to survive at all without mycorrhizae. 

More info on mycelium and mycorrhizal connection:

Around 90% of land plants are in mutually-beneficial relationships with fungi. The 19th-century German biologist Albert Bernard Frank coined the word "mycorrhiza" to describe these partnerships, in which the fungus colonises the roots of the plant. Fungal networks also boost their host plants' immune systems. That's because, when a fungus colonises the roots of a plant, it triggers the production of defense-related chemicals. These make later immune system responses quicker and more efficient, a phenomenon called "priming". Simply plugging in to mycelial networks makes plants more resistant to disease. Researchers have found evidence that plants can go one better, and communicate through the mycelia. In 2010, Ren Sen Zeng of South China Agricultural University in Guangzhou found that when plants are attached by harmful fungi, they release chemical signals into the mycelia that warn their neighbours. Zeng's team grew pairs of tomato plants in pots. Some of the plants were allowed to form mycorrhizae.

Once the fungal networks had formed, the leaves of one plant in each pair were sprayed with Alternaria solani, a fungus that causes early blight disease. Air-tight plastic bags were used to prevent any above-ground chemical signalling between the plants.

After 65 hours, Zeng tried to infect the second plant in each pair. He found they were much less likely to get blight, and had significantly lower levels of damage when they did, if they had mycelia. "We suggest that tomato plants can 'eavesdrop' on defense responses and increase their disease resistance against potential pathogen," Zeng and his colleagues wrote. So not only do the mycorrhizae allow plants to share food, they help them defend themselves. However sometimes the mycelial networks can also bring harm to plants. There are plants that don't have chlorophyll, so unlike most plants they cannot produce their own energy through photosynthesis. Some of these plants, such as the phantom orchid, extract the carbon they need from nearby trees, via the mycelia of fungi that both are connected to.

How Does a Tree Grow?

In humans and other animals growth can occur in most parts of the body.  As we mature, our bones, skin and muscle all increase in size.  Trees do not grow like this.  Trees grow by producing new cells in a very limited number of places.  These places of cell division are called meristems.  Meristems are zones of intense activity.  They are where all new cells are formed and where they expand. 

• Trees grow in height as a result of meristems that are located at their branch tips.  These meristems are called apical meristems. Roots also expand through the soil by growing at their tips as a result of apical meristems. All buds that you see on a tree contain apical meristems. Apical growth is called primary growth.
• Trunk diameter growth occurs as a result of axillary or lateral meristems in the cambium. The cambium area produces new xylem  and phloem each year. Axillary growth is called secondary growth. 
• Xylem is the woody tissue, it transports water and minerals, provides structure  and stores carbohydrates. Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves.  
• Pholem is the inner bark cells. 

In every growing season the cells of the cambium divide to produce layers of new cells: On the outer side of the cambium new cells are added to the phloem; the part of the tree responsible for transporting sugars, produced by the leaves to other parts of the tree. On the inner side of the cambium new woody cells are added to the xylem. The xylem cells made in spring are wide with thin walls. These wide cells help the tree transport large volumes of water from the roots to the trunk and branches to support the growth of new leaves and flowers.

Parts of a tree

Roots

The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree. The radicle or embryonic root is the first part of a seedling to emerge from the seed during the process of germination. This develops into a taproot which goes straight downwards. Within a few weeks lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the taproot eventually withers away and the wide-spreading laterals remain. The tree acquires minerals such as phosphorus from the soil and fungus.  Many large trees have buttress roots which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface.

Trunk

The trunk transports water and nutrients from the roots to the aerial parts of the tree, and distributes the food produced by the leaves to all other parts, including the roots. In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the bark. The bark is perforated by a large number of fine breathing pores called lenticels, through which oxygen diffuses. The innermost layer of bark is known as the phloem and this is involved in the transport of the sap containing the sugars made by photosynthesis to other parts of the tree. Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. The cells are continually dividing, creating phloem cells on the outside and wood cells known as xylem on the inside. The newly created xylem is the sapwood. It is composed of water-conducting cells and associated cells which are often living, and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The oldest, inner part of the sapwood is progressively converted into heartwood as new sapwood is formed at the cambium.

Buds

Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to climatic conditions; growth normally ceases when conditions are either too cold or too dry. In readiness for the inactive period, trees form buds to protect the meristem, the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales. These are thick, small and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives.

Leaves

Leaves are structures specialised for photosynthesis. They are an important investment by the tree and may be thorny or contain phytoliths, lignins, tannins or poisons to discourage herbivory. Photosynthesis provides most of the oxygen that humans and animals breathe. Here’s how it works: Tree and plant roots absorb water, as well as minerals and nutrients, from the soil. At the same time, the leaves or needles absorb carbon dioxide from the air. These raw materials flow to plant cells containing chlorophyll. Chlorophyll uses sunlight energy to transform the carbon dioxide and water into oxygen and carbon-based compounds such as glucose, a sugar that helps plants grow. In the process, the plant or tree produces excess oxygen, which it releases into the atmosphere.

Seeds

Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.

Tree Reproduction

Seed plants have special structures on them where male and female cells join together through a process called fertilisation. After fertilisation, a tiny plant called an embryo is formed inside a seed. The seed protects the embryo and stores food for it. The parent plant disperses or releases the seed. If the seed lands where the conditions are right, the embryo germinates and grows into a new plant.

Angiosperms vs. Gymnosperms?

Angiosperms, also called flowering plants, have seeds that are enclosed within an ovary (usually a fruit), while gymnosperms have no flowers or fruits, and have unenclosed or “naked” seeds on the surface of scales or leaves. Gymnosperm seeds are often configured as cones. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, Gnetales (Gnetum, Ephedra and Welwitschia), and Ginkgo (a single living species).

Angiosperms – seed plants with flowers: Angiosperms have flowers. The flowers are special structures for reproduction. They contain male parts that make pollen and female parts that contain ovules. Some plants have these male and female parts in different flowers. Pollen is carried from a male part to a female part by wind, insects or other animals (a process called pollination), where it releases male gametes that fertilise the female gametes in the ovules. The ovules develop into seeds from which new plants will grow. In most angiosperms, part of the flower develops into fruit, which protects the seeds inside them. Fruit can be soft like oranges or hard like nuts.

Gymnosperms – seed plants with cones:

Gymnosperms are seed plants but their seeds are held in cones. Male cones make pollen, which is carried to female cones by the wind. After the female gametes are fertilised by male gametes from the pollen, the female cones produce seeds, which are then scattered away from the plant by wind or animals.

Evergreen or Deciduous?

Trees are either evergreen and have foliage that persists and remains green throughout the year, or deciduous and shed their leaves at the end of the growing season and then have a dormant period without foliage.

What is Sap?

Saps may be broadly divided into two types: xylem sap and phloem sap. Xylem sap transports water, minerals and hormones from the bottom to the top of the tree.

Phloem sap is the sticky sugary stuff we tend to come into contact with whether by accident or on purpose. This is made up of the sugars created by photosynthesis, which is then fed back into the tree and leaves as much-needed food during the growth period.  Often the reason we encounter sap on the outside of a tree is because something is wrong. Leaking sap on the bark can be a sign of disease, pests or damage. A common pest that causes damage to trees, and forces protective action, is the bark beetle. These burrow into the tree to lay their eggs under the bark layer. When the eggs hatch the larvae burrow start to carve out a network of deep burrows, and it usually requires chemical treatment to save the tree. However the tree’s own sap can often serve as an effective defense mechanism – the holes made by the burrowing adult beetle fill up with the sticky fluid, which can prevent it laying the eggs and even sometimes trap it.

Tree Facts:

• Leaves clean the air and water by filtering out dust, particles, and pollutants. They also release oxygen into the air as a by-product of photosynthesis. Leaves cool the surrounding air temperature by evaporating water, lessening the heating effect of pavement and buildings in cities. The bigger the trees, the greater the effect.

• The number of trees in the world, according to a 2015 estimate, is 3.04 trillion, of which 1.39 trillion (46%) are in the tropics or sub-tropics, 0.61 trillion (20%) in the temperate zones, and 0.74 trillion (24%) in the coniferous boreal forests. 

• There are around 80,000 species growing worldwide

• Trees are the tallest free-standing organisms in the world. They live longer and become larger than any other living organism on earth

• An estimated 15 billion trees are cut down annually and about 5 billion are planted. In the 12,000 years since the start of human agriculture, the number of trees worldwide has decreased by 46%.

Where to find botanical illustrations online? Check out this site

https://www.botanicalartandartists.com/botanical-and-herbal-art-online.html 

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x - Thanks for reading - x