The energy source for each plant to live is sunlight. Through chlorophyll this energy is captured and transferred into an energy carrier (ATP) and through a series of steps -the photosynthetic pathway- this creates from water and carbon dioxide, sugars. Without sufficient light the plant is litterally starving, as it cannot take up energy in any other way.
A plant needs a range of elements to form cells and systain health and growth. Think of nitrogen, phosphorous, magnesium etc. In the roots these elements are absorbed from the soil. By doing this, the plant also pulls in water. This causes pressure in the roots, pushing the sap up in the stems. In order to get the thus created sap up to the leaves, a plant has sap-transporting wood, called phloem. The phloem is the part of the wood directly under the layer of bark
In the leaves the water, together with CO2 taken up from the air, are transformed into sugers and oxygen under the influence of sunlight. This process is called photosynthesis. These sugars and nutrient are redictributed in the tree and used to create new branches, leaves etc. In order to transport this sugar-rich sap to the other parts of the plant, trees have sap transporting cells in the bark, called the xylem
The layer between the bark and the wood is effectively only one layer of cells thick, and is called the Cambium. This is the most important layer of cells in the entire trunk: It causes the growth in girth of the plant. As cells in the Cambium divide, they specialize: On the inside of the tree it becomes part of the sapwood. On the outside it becomes part of the bark. As such, the cambium causes the bark to be pushed out from the tree, and adds layers to the wood of the tree.
When a plant is healthy and not dormant (which would happen during winter for instance) roots continuously grow. There are 2 main reasons for this growth:
- Nutrients close to the root are absorbed, leaving less nutrients to be absorbed: The plant actively seeks for nutrients in the soil
- Roots harden when aging. This means that the roots form a layer of bark, which is impenetrable for water and nutrients: Only the tip of the root, in other words, the youngest cells, actuelly absorb moisture and nutrients. The rest of the roots are just means of transport and anchoring
As roots only use the tips for absorption, it is clear why in bonsai one can root-prune without a lot of problems AND why we want as fine a root network as possible: Only the tips of the roots are of any use in keeping the plant provided with nutrients and moisture!