How does a tree grow?

Growth is the common characteristic of all living things. All living things begin their life in a single cell called zygote. Unicellular plant or animal will stay as one cell. But multicellular plants and animals will add more cells to form tissues and tissues will form organelles and so on. Growth increases size and mass of the living things.

Example: A newly born baby growing into an adult man or woman is called growth. Similarly, when we plant a seed it grows into a plant and then into a tree and it is also called growth.

Growth in plants and Growth in animals:

Plants grow throughout their life whereas animals grow only for a fixed period in their life cycle.

Plants grow at the tip of the stem and the root but animals grow all round their body.

Plants grow through a process called cell division. Cell division produces new cells that enable a plant to grow longer. These cells make up tissues called meristems, which all plants have. A meristem consists of unspecialized cells that divide and generate new cells.

Meristems that are found at the tips of roots and buds are called apical meristems, and these are the meristems that are responsible for primary growth in plants.
Animals do not have any meristems.

Like all plants, trees also start with a seed. The shoot grows towards sunlight and the root grows towards soil. The green leaves help the tree to make its food with the help of chlorophyll present in them. Photosynthesis takes place with the help of sun’s energy, water and carbon dioxide. The green leaves prepare glucose or sugar and release oxygen and water. The tree uses the sugar to produce all of its parts – leaves, wood, bark, roots, flowers and fruits and to grow larger.

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 taller when new cells are produced at the tips of twigs, causing the twigs to grow longer. Trees grow in height as a result of meristems that are located at their branch tips.  These meristems are called apical meristems.  All buds that we see on a tree contain apical meristems.

Tree roots grow from specialized tissue at their tips. Roots anchor a tree in the soil and absorb the water and nutrients a tree needs. Most of a tree’s roots are found on the top of the soil, even when the tree is very large. Root hairs near the growing tips take in water from the soil.  We will need a microscope to see this tiny, tube like hairs; yet, in a large tree, they can absorb hundreds of gallons of water each day. Surprisingly, only a small amount of this water is used in photosynthesis. The rest is released from the leaves in a process called transpiration.  Roots expand through the soil by growing at their tips as a result of apical meristems.

Tree trunks and branches grow thicker as new cells are added beneath the bark. These cells make up vessels, called xylem and phloem that carry water and food throughout the tree.

Xylem carries water and nutrients from the roots up to the leaves. Active xylem is called sapwood. Phloem, also called inner bark, carries food from the leaves to the branches, trunk and roots. Outside the phloem is the outer bark, which protects the tree from injury. 

The cambium is found between the phloem and xylem. If we look at a tree stump, we will not see the cambium, because it is only one cell layer thick. The cambium’s function is to make new xylem and phloem cells. 

Growth of the trunk diameter occurs as a result of another meristem called the vascular cambium.  The vascular cambium produces new xylem and phloem each year and as a result the trunk, branches and roots continue to increase in diameter.

When a tree is cut we can see light and dark rings. These rings tell the age of the tree. These rings are called cambial rings.

The cambial rings are wider when the tree is growing fast, such as when it has plenty of light, space and water.

The cambial rings are narrower during drought or other times of environmental stress.

Injuries to a tree, such as fire scars or insect attacks, are seen as scars in the cambial rings.

One light plus one dark ring equals one year of growth. By counting the dark and light rings inward from the bark, age of the tree can be determined at the time when it was cut.

New branch growth is inhibited by various plant hormones such as auxin. Branches and branchlets that get the maximum sunlight produce this hormone. The strength of this hormonal effect varies from species to species. Growth is positively affected by the quantity of carbohydrates produced by photosynthesis, so branches with lot of leaves that receive more sunlight also tend to grow faster.

The apical meristem holds a lot of stem cells within the growing plant. It is generally at the top of the tree. The trunk of a tree is the remnants of the apical meristem as the tree grows in height.

When auxin is released into the vascular system of the plant, it prevents any branches growing from below.  The concentrations of auxin are high, when the apical meristem moves higher. When the concentration of auxin becomes lower it triggers the formation of secondary meristems to branch out. Basically, when the tree gets taller, the auxin concentration at the secondary meristems is lower. 

If we cut the apical meristem in a growing tree, then the secondary meristems on the side will start growing. This way the tree can grow horizontally instead of vertically, though they will eventually begin to grow vertically again as one of the branch tips assumes the role of the apical meristem.

The cycle of growth of trees depends on seasons. Growth of trees also depends on the region where they are growing. During summer there is bright sunlight and good supply of water. The green leaves, which are the food factories, make more food that are required by the plant and store the excess food for the season when there is no sunlight.

Therefore during summer season, the trees give out many branches with numerous leaves. The trees increase their size. The crown of the trees will have abundant leaves, flowers and fruits. Therefore trees are busy throughout summer.

During winter there is not much of sunlight or water for photosynthesis. The trees will rest, and will start depending on the food they stored during the summer. The leaves will start losing the green colour chlorophyll.

In the autumn season, the green leaves turn into yellow or orange colour. Small amounts of these colours have been in the leaves all along even during summer. But we were not able to see them in the summer, because they were covered up by the green chlorophyll. In this season some amount of glucose will be left in the leaves. Sunlight turns this left over glucose into red colour pigment called anthocyanin.

The leaves turn brown or sometimes purple because of the presence waste generated during winter due to lack of sunlight. Orange, purple, yellow and brown foliage of trees present a beautiful display during this season. The bright red and purple fall foliage colours come from anthocyanin pigment. These pigments are formed in the autumn from the glucose that was trapped during autumn season.

The leaves will start falling from the tree leaving the tree bear.  The leaf has actually been preparing for autumn since it started growing in the spring. At the base of each leaf, there is a special layer of cells called the "abscission" or separation layer. All summer, small tubes which pass through this layer carry water into the leaf, and food back to the tree. In the fall, the cells of the abscission layer begin to swell and form a cork-like material, reducing and finally cutting off flow between leaf and tree. Glucose and waste products are trapped in the leaf. Without fresh water to renew it, chlorophyll begins to disappear and the leaves will begin to fall.

Now the trees look bear and will start surviving on the food that was stored during summer. This is the period of dormancy for a tree.

The next season will be the rainy season. During the rains, bud will start sprouting on the trees. The bear tree will begin a new life. Leaves start growing. Once the leaves emerge the tree becomes very active and leaves will start manufacturing of food. Trees will become a power house and can generate energy to light up our lives.