A tree structure basically consists of three parts, namely, trunk, crown and roots. The functions of the trunk is to support the crown and to supply water and nutrients from the roots to the leaves through branches and from the leaves back to the roots. The roots are means to implant the trees in the soil, to absorb moisture and the mineral substances it contains and to supply them to the trunk.
From the visibility aspect, the structure of a tree can be divide into two categories :
The innermost central portion or core of the tree is refer as the pith or medulla. It varies in size and shape for different types of trees. It consists entirely of cellular tissues and it nourishes the plant in its young age. When the plant becomes old, the pith dies up and decays and the sap is then transmit by the woody fibers deposit round the pith. The pith of branches is nothing but merely a prolongation of the pith of stem.
The inner annual rings surrounding the pith constitute the heart wood. It is usually dark in color. As a matter of fact, it indicates dead portion of tree and as such, it does not take active part in the growth of tree. But it imparts rigidity to the tree and hence it provides strong and durable timber for various engineering purposes.
The outer annual rings between heart wood and cambium layer is refer as the sap wood. It is usually light in color and weight. It indicate recent growth and it contains sap. The annual rings of sap wood are less sharply define than those of heart wood. It takes active part in the growth of tree and the sap moves in an upward direction through it. The sap wood is also refer as the alburnum.
The thin layer of sap between sap wood and inner bark is refer as the cambium layer. It indicates sap which has yet not be convert into sap wood. If the bark is remove for any reason, the cambium layer gets expose and the cells cease to be active resulting in the death of tree.
The inner skin or layer covering the cambium layer is refer as the inner bark. It gives protection to the cambium layer from any injury.
The outer skin or cover of the tree is refer as the outer bark. It is the outermost protective layer and it sometimes contains cracks and fissures. Outer bark consists of cells of wood fiber and is also refer as the cortex.
The thin radial fibers extending from pith to cambium layer are refer as the medullary rays. The function of these rays is to hold together the annual rings of heart wood and sap wood. These rays are sometimes broken and in some varieties of trees, they are not very prominent.