Anatomy of Flowering Plants Test - 23

Secondary growth refers to the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of stems and roots, causing them to elongate and gives rise to primary tissue. 

Heartwood is also called as 'duramen'. It is dead, central wood of trees. Its cells usually contain tannins or other substances that make it dark in colour and sometimes aromatic. Heartwood is mechanically strong, resistant to decay and less easily penetrated by wood-preservative chemicals than other types of wood.

The layers of cells lying between endodermis and vascular bundle is pericycle. The pericycle is a cylinder of parenchyma or sclerenchyma cells that lies just inside the endodermis and is the outer most part of the stele of plants. Although it is composed of non-vascular parenchyma cells, it is still considered part of the vascular cylinder, because it arises from the procambium as do the vascular tissues it surrounds.

The formation of the vascular cambium is initiated when cells between the columns of vascular tissue connect the cambia inside the columns of vascular tissue to form a complete cylinder around the stem. The cells formed toward the inside are called as secondary xylem or wood and those formed toward the outside of the cambium are called as secondary phloem.
Xylem formation begins when the actively dividing cells of growing root and shoot tips give rise to primary xylem. In woody plants, secondary xylem constitutes the major part of a mature stem or root and is formed as the plant expands in girth and builds a ring of new xylem around the original primary xylem tissues.
When this happens, the primary xylem cells die and lose their conducting function, forming a hard skeleton that serves only to support the plant. Thus, in the trunk and older branches of a large tree, only the outer secondary xylem (sapwood) serves in water conduction, while the inner part (heartwood) is composed of dead but structurally strong primary xylem. So, youngest layer of secondary xylem is located just inside vascular cambium and not in center of stem, outside pith or outside vascular cambium. 
Therefore, the correct answer is option D.

As we know that dicot roots have little or no pith and thus pith rays are not the source of interfascicular cambium as in the case of dicot stem. 

Monocot stem vascular bundles are found irregularly scattered in the ground tissue. Towards the periphery, the bundles are smaller in size while towards the centre, they are larger in size. The smaller bundles are younger, while the larger ones are older. Hence, the arrangement is described as centrifugal. 

Root is marked by presence of primary xylem and phloem in different radii, separated from each other while stem have them arranged side by side in same radius.
Root xylem is exarch xylem in which xylem formed first is present towards the periphery and is termed as protoxylem while metaxylem is present near the centre. Metaxylem is present towards inner side.
Thus, the correct answer is option D.