Branches and Trunk
 
The branches and trunk obviously create the visible structure of a tree but we should now also be able to start to understand the reason why it creates shapes and branch formations that it does. By understanding the role a leaf plays in producing food for the tree during photosynthesis, we now know one of trees basic needs is to provide light to the leaf. It is therefore no surprise that a tree will form a structure to maximise this and adapt as necessary to maximise available light.
 
If we look at trees that grow in dense woodland they will be taller with most of the foliage at a high level where light is available, conversely, an individual tree in an open space will form a broad lower structure as it does not have to compete with other trees for light. This characteristic is more pronounced with deciduous trees but is also applicable to evergreens.
 

As well as the structural element branches also perform other essential basic functions; now we must start to look under the bark. Under the bark a continuous layer of cells (cambium) divide as the tree grows. The outside of these cells become the phloem carrying sugars that are produced during photosynthesis around the tree. The inside of the cambium forms wood fibres and vascular elements known as the xylem which transports water and minerals up from the roots. The xylem is commonly known as a growth ring with each ring representing one year of growth (this can appear as a broad band representing cells formed in the spring followed by a smaller dark band representing summer wood cells). The size of a growth ring can be representative of how well or poorly the tree grew during a particular year. The number of rings can also be counted to ascertain a trees age. Whilst the xylem continues inward, the rings towards the centre become inactive and are known as heartwood, active xylem towards the outside of the tree is known as sapwood.
 
The vascular system of a tree consists of the phloem, xylem and also rays that cross both of these in a radial sheet to transport minerals and sugars across the stem tissues. This allows starch needed for growth etc to be stored nearer to where it is required, therefore making the tree more energy efficient.
 
This system of cells runs continuously from root to the tips of branches. There formation increases the trees diameter - increase in height and spread is generated from tips of branches.
 
I understand that it can be hard getting to grips with the above, but it is imperative to gain a basic understanding of a trees vascular system. This will enable you to better understand growth problems (such as included bark), how trees fight pathogens, how wounds are healed and actions that could be fatal to a trees health.